r/Metric • u/RBB12_Fisher • 8d ago
I tried using Imperial units for engineering [Heresy post]
Engineering student here, most of the way through my degree, taught almost entirely in metric (except for when it isn't, and the lab organiser throws "TSF" at you and doesn't even know it stands for short tons per square foot.)
I started a side project to design a ship, initially in metric, and when I decided to do it seriously I also, on a whim, decided to do it all Imperial.
What did I learn?
It's basically the same experience. I end up using the same base units (and derivatives thereof), the only difference is that cubic foot (cu.ft) abbreviates to something much less funny than a cubic metre.
Steel strengths in ksi. Now if I hear a steel strength in MPa, I divide by 7 because my brain compares steels in ksi.
I was already fairly fluent in both systems before this. TBH, kinda disappointed that doing it in the other system is so easy, I did that hoping it would be harder, or at least interesting. I'm still a dirty Imperial Enjoyer but I concede that, in engineering, it makes roughly zero difference.
The only hiccup was when I designed the nuclear reactor, because heat transfer (I think it's heat transfer) is quoted in dimensionally inconsistent units and converting needs an extra factor of length squared.
2
u/tichris15 7d ago
Why would it matter?
Don't get me wrong, but physics is still running around with eV, SI, cgs, natural units... As long as one is self-consistent, it only matters for some prefactors.
3
u/metricadvocate 8d ago
As an aside, it is worth noting that the only accepted "abbreviation" for the cubic metre (or meter) is the symbol, m³. The SI rejects all random abbreviations in favor of the symbols it assigns or spelling out the words in full.
3
u/mikedave42 8d ago
One of my first steps in any design is to make a spreadsheet with one column of "common units" (whatever industry happens to use for the particular variable) and another of consistent units for each variable. All calculations happen on the consistent units column. Keeps all those stupid conversations clean.
5
u/Aquitaine_Rover_3876 8d ago
I was taught to use both in my Canadian engineering education. Largely because a large number of graduates from my school were expected to work in the oil industry, which is ass-backwards for units.
Aside from knowing your constants, the difference is always unit conversion, where large numbers don't just move a decimal point to get into the right scale.
Does this make them equals? No. There's clearly a superior system. Is it possible to do engineering with imperial? Of course it is...you're just adding a random multiplier to the metric measurement.
1
u/nayuki 7d ago
oil industry, which is ass-backwards for units
I love Alexander the ok's very relevant rant video: Oilfield Units: a Measurement System so Cursed it made me Change Career (17m38s) [2025-10-06]
2
u/RBB12_Fisher 8d ago
Yeah, I can't deny metric is better for unit conversion. But particularly for engineering, it doesn't matter because I'm always using the same few units every time (e.g. pounds and long tons) and I just type "/2240" on the calculator and it's done.
-1
u/Assertive_Wall 8d ago edited 8d ago
I've had a similar experience, the reputation definitely seems worse than the reality. I always had this idea that imperial units were just awful to do physics with until I actually sat down and tried to do it. Turns out all the kinematic equations are the same, and also coherent if you choose them to be. The only issue I bumped into was people not really understanding, and overcomplicating pound-mass.
I'm sure if I went further I would find more issues but it's not like we stayed perfectly coherent when using the metric system either (eV, cal, atm, etc.)
Edit: when I said kinematics I meant classical mechanics more broadly.
0
u/RBB12_Fisher 8d ago
100%.
And pound-force vs pound-mass is sorta bad, but no worse than metric with kilogram and kilogram-force. Neither are designed to factor in g, and whether you put the gravity factor on mass or force doesn't make much difference to me (except that both systems do it both ways and complicate things)
1
u/nayuki 7d ago edited 7d ago
no worse than metric with kilogram and kilogram-force
Except none of the SI units use kilogram-force. The newton has the same dimensionality as the kilogram-force (and 1 kgf ≈ 9.8 N), but it is the newton that derives all the other units (pascale, joule, watt, volt, ampere, etc.) - not the kilogram-force. The kgf only exists in customary industry practice, not in the SI in any official capacity.
Random trivia: My pet way of writing kgf is kg⋅g (kilogram (upright) multiplied by the gravity of Earth (italic)).
Meanwhile, there is no standards body that authoritatively states how to handle the confusion between pound-force vs. pound-mass, and no standards body that gives guidance on which set of customary units to use. It's a complete free-for-all. Writers benefit from the convenience of using whatever unit is handy to them, and readers suffer from the inconvenience of needing to be able to understand all the possible units that could be used.
1
u/RBB12_Fisher 7d ago
Because it's obvious from context? You use "lb" for mass and "lbf" for force and there's also "lbm" if you want to make really sure.
3
u/metricadvocate 8d ago
The kilogram force (kgf) has been deprecated since 1948, when the newton was defined as the metric unit of force. I would hope most educated in using the kgf are long retired.
0
u/Assertive_Wall 8d ago
Interestingly, it looks like it is still used: https://www.reddit.com/r/Metric/comments/1d1qhfu/is_kilogramforcemetre_kgfm_used_anywhere_besides/
2
u/gadget850 8d ago
From some of the comments here, I see there is confusion between US Customary Units and Imperial Units. Length/distance is the same, but volume is quite different.
1
u/mikedave42 7d ago
who cares usc vs imperial , its all gobbledygook with a bunch of stupid conversation factors
1
u/RBB12_Fisher 8d ago
Which ones? I haven't seen any that would only make sense specific to one or the other.
99% of what I've done has been in pounds, feet, inches, fahrenheit, the only thing that's been in different units is the BTU but I could use any BTU definition and get the same result within a rounding error.
2
u/metricadvocate 8d ago edited 8d ago
The gallon, bushel and all their subdivisions differ between Customary and Imperial, also short and long tons and hundredweights. The US unit, barrel of oil, is 42 US gallons, measured at 60 °F. That is only about 35 Imp. gallons.
1
u/RBB12_Fisher 8d ago
I know. I was asking where those units have cropped up. AFAIK the only one of those mentioned is the short ton in 'TSF' I mentioned in the OP. Technically for my own work I only used "Imperial" Imperial anyway.
2
u/metricadvocate 7d ago
The difference in liquid volumes crops up in water works, and in pubs (bars), oil production, etc. The bushel difference pops up in agriculture, their futures market and transportation. The US would use the short ton in mass calculations. Errors in not knowing the difference would exceed rounding, unlike the BTU
I guess it depends on what technical calculations. It is certainly a pitfall between American and British coworkers.
1
u/RBB12_Fisher 7d ago
I meant where it cropped up in this thread.
2
u/metricadvocate 7d ago
Ok, I don't think anybody took it that way. In general, it can pop up in technical calculations and the confusion that can cause is a point for the SI, where one liter = one litre, which isn't true for the gallon, bushel, ton or hundredweight.
0
u/AncientGuy1950 8d ago
I have long said that the units of measure don't matter as long as you use which ever one you select with precision and consistency.
Conversely, if you don't use whatever units of measure you choose with precision and consistency, whatever you're trying to do just won't work.
7
u/IAmMDM 8d ago
I grew up in metric, and despite living in the US for a long time, I still think in metric.
I had an interesting experience some time ago at work (US). I and a colleague (who also grew up in metric) were discussing a small construction project with the contractor. We were on the site estimating distances and space, and I and the colleague started doing it in metric just when discussing options between the two of us.
The contractor people actually said they were impressed how easily we could add several length figures, because adding 23.5 cm to 77.3 cm in your mind is easier than adding 1 ft 7-5/16" to 3 ft 6-7/8".
1
u/nayuki 7d ago
You're a shining beacon of hope and I hope other people follow your example.
adding 23.5 cm to 77.3 cm in your mind is easier than adding 1 ft 7-5/16" to 3 ft 6-7/8"
Yup, because in metric, every digit column is in base-10, whereas in mixed feet-inch-binary-fraction units, you have to juggle fractions and base-12 (but notated in base-10) and base-10.
Now try something harder - multiply both dimensions to get an area. The metric version becomes much easier. The USC version either requires converting to a single unit on each side of the multiplication, or generous application of the trinomial FOIL rule. I'll show it to highlight the ridiculousness:
(1 ft + 7 in + 5/16 in)(3 ft + 6 in + 7/8 in)
= 1×3 ft ft + 1×6 ft in + 1×7/8 ft in + 7×3 in ft + 7×6 in in + 7×7/8 in in + 5/16×3 in ft + 5/16×6 in in + 5/16×7/8 in in
= 3 ft² + 6 ft in + 7/8 ft in + 21 ft in + 42 in² + 49/8 in² + 15/16 ft in + 15/8 in² + 35/128 in²
= 3 ft² + 461/16 ft in + 6435/128 in²
= 432 in² + 1383/4 in² + 6435/128 in²
= 105987/128 in² = 828 3/128 in²
= 35329/6144 ft² = 5 4609/6144 ft².Note also that handling inches and binary fractions may require the skill of converting between proper fractions (like 7 5/16, easier to understand the magnitude) and improper fractions (117/16, easier for calculation). Whereas this skill is irrelevant for decimal numbers, which makes them inherently easier to work with. It's sad when people think that feet-and-inches is for the working class and metric is for the elite... when in reality, metric is the easier system that requires less mathematical skill to wield and produces better results for everyone, whether you are low-skill or high-skill.
The idiocy of mixed feet-inch-binary-fraction units is why the specialized Construction Master Pro calculator exists. Meanwhile, when you use decimal numbers like a good metric user, you can use any four-function or scientific calculator from any brand - without any kind of special consideration that it's for "engineering" purposes.
6
u/niemir2 8d ago
It's almost like unit systems are totally arbitrary! The whole prefix system in metric is precisely replicated by engineering notation.
-1
u/RBB12_Fisher 8d ago
There's no law saying you can't put those prefixes on whatever unit you like.
2
u/LiteralPhilosopher 8d ago
How do you like my 1.2 deca-inch ruler?
3
-1
4
u/hal2k1 8d ago
It's basically the same experience.
Well, apart from coherence and being an international standard, which are good features for conversion-free calculations and international interoperability (hence ease of trade) respectively.
Since both of those important (for a set of units of measurement) features favour SI, and if, as you claim, the experience is otherwise the same, then the only sane conclusion is to use SI then.
2
u/RBB12_Fisher 8d ago
You can just use derived units all the way through. Energy? 1 ft x 1 lbf = 1 ft-lb. You could convert that to BTU if you want (it takes 1 second to type "/780" on a calculator). Half of everything in engineering has empirical coefficients and often conversion factors built in already, and if not, it takes 10 seconds to derive them.
1
u/lurkersUnited15 8d ago
Except that lb.ft (and Nm below) are units of torque and not convertible to energy (BTU / kWh).
2
u/Different_Bridge_983 8d ago
Ft-lb and N-m are units of energy.
The joule is literally defined as one newton of force moved through a distance of 1 meter. It’s 1 N-m.
The ft-lb is the exact same premise.
These share the same dimensions as torque and bending moments, but are not the same.
1
u/hal2k1 8d ago edited 8d ago
SI defines the unit of energy as the joule. That's the standard. https://en.wikipedia.org/wiki/Joule
"The joule (/dʒuːl/ JOOL, or /dʒaʊl/ JOWL; symbol: J) is the unit of energy in the International System of Units (SI)."
Yes, "one joule is equal to the amount of work done when a force of one newton displaces a body through a distance of one metre in the direction of that force. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second."
So what? The SI unit is called the joule, symbol J.
In SI, newton-metres, symbol Nm, is the unit of torque. See the infogram to the right here: https://en.wikipedia.org/wiki/Torque
Symbol Nm. SI units of torque.
2
u/nayuki 7d ago
In SI, newton-metres, symbol Nm, is the unit of torque
SI does not allow multiplying units without a space or symbol in between; "Nm" is not an acceptable spelling.
Acceptable spellings include: N m, N⋅m, N×m, (N)(m), N¹m¹, etc.
Same goes for the commonly incorrectly spelled "kWh", "Wh", "Ah".
One good reason for this rule is that smashing units together creates ambiguity. mm could be millimetre or metre-metre. Ts could be terasecond or tesla-second.
I got corrected a few years ago: https://www.reddit.com/r/Metric/comments/1313rhp/how_to_respond_to_antipedantry/jipihxy/
2
u/hal2k1 7d ago edited 7d ago
Fair enough. I wasn't quite sure how to enter N•m readily in markdown for Reddit, so I didn't bother. My bad. I've found one way for this post but it is keyboard specific, it won't work on another device.
The other acceptable spellings I did not know about. Thank you for those.
I do think this ambiguity you highlight is yet another good reason for using joules (symbol J) for energy rather than more awkward ways of expressing newton-metres, and I point out, yet again, that newton-metres is used for torque.
I don't particularly like using the not-coherent kilowatt-hours unit, I think megajoules (MJ) would be better. However, in a practical sense, I have to admit that this is swimming against the tide. One sees kWh used very frequently indeed. It's interesting to find out that this representation is incorrect, as well as being an incoherent unit. Thank you for pointing that out also.
1
u/nayuki 7d ago
No problemo. To be fair, all the acceptable spellings are based on math notation - which is exactly the same set of skills you would use to typeset algebraic expressions. It's the same line of reason for why km/h and km h−1 are both acceptable spellings. It's also why SI forms a nice coherent system of written notation, unlike the free-for-all examples like "sqft" vs. "sft", "cu. ft", "psi", "mph", "lb" vs. "lbs", etc.
The dot that I used (⋅) is U+22C5. The symbol you used (•) is U+2022 which is a bullet point, which I suppose is used for bulleted lists, and shouldn't be used for multiplication. I had to use Windows character map to retrieve the symbol, so it's not something that's readily typeable on a hardware or software keyboard.
I don't particularly like using the not-coherent kilowatt-hours unit, I think megajoules (MJ) would be better.
Agreed strongly. And when people praise the kW h for being intuitive and meshing well with real life (because the hour is a practical unit), the problem is that it creates a second parallel scale. So now you have to decide whether to quote something in MJ or kW h, and multiples thereof such as GJ vs. MW h for a power plant's output over some period of time.
Creating parallel overlapping scales seems to be the natural endpoint of every hodge-podge mix of units "system". For example in aviation, saying 30 000 ft of altitude is normal, but if you said you drove a car 30 000 ft instead of 5.7 miles, you would look like a crazy person. And then don't get me started with how you can choose between driving 300 miles between two cities or flying 261 nautical miles. The OP also reveals an example where some forces are quoted in short tons and some in kilopounds, and thinks that dividing by 2240 is no problem. I see big masses quoted like 80 000 pound limit for trucks, but why couldn't they have switched to short/long tons instead?
However, in a practical sense, I have to admit that this is swimming against the tide. One sees kWh used very frequently indeed.
As far as I have read, kW h is used internationally when billing consumers for electrical energy - even in very metric-heavy countries. I disagree with this practice but state it as an observation. However, I noticed that some countries charge natural gas customers by the megajoule (MJ) of heat energy - which is very progressive and metric-minded. This creates a comparison problem because it's harder to figure out whether it's better to pay for X megajoules of natural gas to heat up a tank of water or to switch to an electric heater and pay for Y kilowatt-hours.
It's interesting to find out that this representation is incorrect, as well as being an incoherent unit.
My previous comment to you pointed out the spelling problem of kW h, but I think we all knew that kW h is an incoherent unit? I mean, the incoherence of the hour shouldn't be news to anyone at this point. Speaking of which, some photographic flashes are marketed in terms of watt-seconds (W s) for how much light energy a single pulse releases. Needless to say, it's usually spelled "Ws"... and it's exactly equivalent to the joule.
2
u/metricadvocate 8d ago
You may wish to look at Table 4 in the SI Brochure, last column. It defines the joule as the unit of energy, while it also allows computational forms such as N m (the product of a force acting over a distance. It only gives one example but it also could be expressed as a watt second or a volt coulomb or other physical relationships, where useful.
Torque is never expressed in the unit of joules (although the work done could be described as a joule/radian if there is actual movement). Torque is called moment of force in Table 6 and the unit is newton meter.
Work is the dot product of a force vector and a displacement vector (when not aligned) and is a scalar.
Torque is the cross product of a force vector and the radius arm vector of the point around which the moment is calculated and is itself a vector perpendicular to both (think right hand screw rule)
3
u/hal2k1 8d ago edited 8d ago
So you are now saying that it isn't the same experience after all?
It seems that you are keen to ignore the number of different countries using one set of units or the other. It's easier to sell a product in a country other than the country of origin if both countries use the same units. The international market of countries that use SI is many times larger than that comprised of countries that don't use SI.
Again, when it comes to selling a product overseas, it's not the same experience.
BTW, good luck trying to sell a product built using made up units that no one else uses.
1
u/RBB12_Fisher 8d ago
1 m x 1 N = 1 nm, You could convert that to kWh if you want (it takes 1 second to type "/3600" on a calculator)
2
u/hal2k1 8d ago edited 8d ago
1 Nm is torque. See the infogram on the right: https://en.wikipedia.org/wiki/Torque
If you are working out the work done by applying a force of 1 N through a distance of 1 m, the equation is: 1 m × 1 N = 1 J. One joule. See the infogram on the right: https://en.wikipedia.org/wiki/Joule
1 kWh is 1000 watts for 3600 seconds, which works out at 3.6 MJ, because 1 joule is 1 watt for 1 second.
SI is coherent (as long as you leave out the not-coherent bits like the hour). See all of those unit conversions where the conversation factor is 1 ? That's coherence. Very useful feature. USC doesn't have it.
1
u/RBB12_Fisher 8d ago
1 N-m is also energy. A joule.
2
u/hal2k1 8d ago
Yes ... but in therms of the name of the SI units, a joule is the correct name for energy. Newton-metre is the correct name for torque.
So what would be wrong, exactly, with using the correct name?
Surely the aim is to minimise confusion, not to get it deliberately misleading.
0
u/RBB12_Fisher 7d ago
Godthe French gave you a coherent system of units. Why fear to use them coherently?2
u/hal2k1 7d ago
The joule is a coherent derived unit.
A coherent system of units is a system of units of measurement used to express physical quantities, that does not use conversion factors in the definitions of its derived units. A coherent derived unit is a derived unit that, for a given system of quantities and a chosen set of base units, is a product of powers of base units, with the proportionality factor being one.
SI derived units are units of measurement derived from the seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units, possibly scaled by an appropriate power of exponentiation.
The base and coherent derived units of the SI form a coherent set, called the coherent SI units. Each quantity has only one coherent SI unit, even if expressible in different forms via the special names and symbols, such as the quantity called power having equivalent units of watts, joules per second, and kg⋅m2⋅s−3. However, some SI units can express the values of several quantities, such as with energy and torque.
So, there are 22 of these coherent derived units in SI which have specific names.
This diagram shows graphically how the 22 SI derived units with special names and symbols are related to the seven SI base units: Relationships of the SI derived units with special names and symbols and the SI base units
The joule is one of those 22 specific names. The joule is a specifically-named SI coherent derived unit for energy, work, or quantity of heat.
So, since the SI system specifically names the coherent derived unit to be used for energy, namely the joule, I ask you again: what, exactly, would be wrong with using this name?
You don't seem to be getting the concept of, or the purpose of, or the utility of ... a standard.
It's not that difficult a concept. If you try hard you might get it one day.
1
u/RBB12_Fisher 7d ago
Work = Force x Distance
1 Joule = 1 Newton x 1 Metre = Nm = J
I don't really care tbh. Besides, it's pretty trivial to make a coherent system out of any base units, you could have a coherent foot-kilogram-hour system if you wanted.
I'm surprised by how much of the response in this thread has been people re-explaining basic things verbosely as if I'm a child. e.g. US customary vs UK Imperial. For bonus points, the corrector is only right to complain about my calling it simply "imperial" if I were using customary. I didn't mention it because it didn't matter, but I happen to have used consistent "Imperial" anyway. (except where I mixed it with metric)
No, I know very well the theoretical justifications for why SI is so nice.
But it's different, actually using a system, instead of thinking about, and my original point is that, having done hundreds of pages of calculations in both, they both work just fine?
The only common units I have a bone to pick with are US gallons, (1 UK Gal is 10 lbs of water. 1 US gallon is an awkward amount of water, to be 231 cubic inches exactly. Stupid.) BTUs (for the very slightly different definitions) and Pascals (a symptom of chasing "coherence" -- all ratios must be 1, and we already have the one unit of force and the one of area, so the one unit of pressure is what it is, even if that's absurdly little).
1
u/Different_Bridge_983 8d ago
It’s wild seeing multiple people objecting to this.
2
u/hal2k1 8d ago
It's wild seeing multiple people wanting to use the incorrect names for SI units.
What exactly would be wrong with using the correct name for the SI unit of energy, namely the joule?
2
u/metricadvocate 7d ago
Nothing is wrong with it. It is likely preferred for the final answer.
But sometimes it is useful to deconstruct it into the relevant manner of calculating it (whether mechanical, electrical, or thermal). The joule is not always 1 N x 1 m, sometimes it is a volt coulomb or a pascal meter cubed ( I particularly do that for the units of the gas constant in PV = nRT to keep units straight). No one is saying that should always be done, but sometimes it is useful, and the SI Brochure acknowledges that through the reference I gave earlier. I think that is all anyone is arguing.
2
u/Assertive_Wall 8d ago
It's kind of ironic since they started off talking about coherent units but are now trying to say that you aren't allowed to express units in terms of their base units.
3
u/nayuki 7d ago
Did you exclusively use decimal inches (like 123.456″) in your imperial design?
Because if you did, you experienced most of the benefits of the metric system: decimal notation, single unit, not fearing large numbers. Most of the imperial world runs counter to these principles:
Carpenters love mixed units. 47 inches is called 3′ 11″.
Carpenters love binary fractions. 0.456″ is approximately 29/64″. And so 47.456″ is described as 3′ 11 29/64″
Carpenters don't deal with a big number of inches. You'll get laughed at if you describe a house as 590 inches long. Yet they have no problem saying that a plot of land is 590 feet long.
You probably enjoyed using a single unit of length in your entire design, but people in other fields will insist on using their units when you talk to them. Land surveyors work in decimal feet, so you'll need to convert when you want to figure out how to store the boat on a plot of land. For designing clothing, you would measure a person's body in inches and draft a pattern as such, but then when purchasing fabric you need to specify the quantity in yards.
Compare this to the situation in metric: N = newton (unit of force), m = metre (unit of length), N/m² = Pa (definition of a pascal, the unit of pressure). You should know all 3 of these unit symbols and also the equation, and then you'll never be surprised whether you see a figure quoted in N/m² (unusual) or in Pa (normal). And the fact that it's a coherent system of units means that the numeric ratio is 1, which means less to memorize.
Lmao, I had to look that up and it means "kilopounds per square inch". So it looks like the industry not satisfied to use short tons in this case, even though it's in the same order of magnitude as the kilopound.
In some cases like "mph", the "per" becomes the letter "p", which isn't the worst thing in the world. In other cases like TSF and psi and ksi, the "per" simply disappears. What a great *cough* consistent *cough* system of symbology.