We have users from all over the globe. Many of these people probably shake their heads at the US's use of Imperial units of measure. Many in the US feel confused when SI units are used.

Do we need an official policy on this?

  • Should we have a policy that units of measure include both forms of units?
  • Should we default to SI units?
  • Do we leave it up to the questioner and encourage edits that add the other units?


  1. I have a bar that is 1 inch (25 mm) thick...
  2. I am pumping water that is at 100 C (212 F)...

4 Answers 4


I think it's fine for the person asking the question to post in their native unit system. It will be easier for them to follow any equations in the answers using the units and constants formatted for their unit system, so answerers who are able would know their preferred units (not that an answer in another unit system would be inherently bad.)

More importantly, it provides a context clue of the kind of work and geographic area that is being asked about. This would help us with things like codes that may apply, or material sizes that may be commonly available. Even if we converted standard units, we'd still end up with all sorts of things like screw threads and material grades that are location/system specific, and don't always have 1-to-1 equivalents in other systems (or for which there are many different systems.)

Most importantly, I don't think it should be the burden of the person asking the question to translate all of their values into other unit systems. If someone wants to go back in an edit and add conversions in parenthesis, there's no harm in it, as long as the original units are still the primary ones.

  • 1
    $\begingroup$ Ethan48 is correct in stating that some things such as screw threads and nut & bolt sizes don't convert easily. It's why some mechanics have two sets of spanners: one metric & the other imperial. Sometimes near enough isn't good enough. $\endgroup$
    – Fred
    Commented Feb 14, 2015 at 15:37
  • $\begingroup$ @Fred: That's a different question altogether. Items manufactured in one system or the other need to be used with other items and tools that match them. $\endgroup$
    – Dave Tweed
    Commented Feb 14, 2015 at 19:24
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    $\begingroup$ Yeah, but the point stands that just because we can convert inches to millimeters doesn't mean we've helped everyone's understanding. I think Fred's point is that if someone asked how to tighten a 1/4" bolt, we'd end up with someone calling for a 11.113mm wrench, which doesn't exist. $\endgroup$
    – Ethan48
    Commented Feb 14, 2015 at 19:27
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    $\begingroup$ When we already force people to use a specific language, which is much harder to learn, we should also at least expect people to be able to use metric units. Especially since most people who don't want to use metric units for some stupid reason are the ones who didn't need to learn it as a foregin language. $\endgroup$ Commented Nov 30, 2023 at 13:42

As much as I'd love to say "Everyone use SI for everything," that's just not realistic. First, though the trained engineers may be familiar with the SI units, not everyone who comes here will. The big one off the bat is Kelvin. Plenty of people know what Celsius is, even if they have to look up the conversion. Someone who isn't involved in physics, chemistry, or engineering probably won't know what the Kelvin scale is, why their numbers got switched to it, and how to get to it, unless they go look it up. On the surface, that doesn't seem horrible, but I don't think someone should need to translate an answer they get to their own question.

During my time at school (in the US), we were expected to switch pretty deftly between different sets of units, which was fantastic practice for working as an engineer in the US, because a lot of things still rely on imperial units, even if most companies are trying to switch to SI or metric. The general rule of thumb was work with the units you are given, and I think that's a good general policy to adopt here. If someone asks a question using a certain unit system, it's probably because that's the system they're using in this project, that they feel most comfortable with, and that's the most practical for them.

If you are answering a question and you prefer a different set of units or there's a very good reason to switch, convert at the beginning, but then convert back at the end. All the different units that are used is an unfortunate reality of our world, but I think it's a little rude to switch someone's units out of hand with no explanation, simply because you like a different system better.

To condense my thoughts into a couple proposed guidelines.

  1. Defer to the OP's units whenever possible.

  2. Convert to do the calculations if you wish, but convert back when you give the final answer or any critical values along the way. If there are scientific, non-preferential reasons for switching (needing an absolute temperature scale such as Kelvin or Rankine instead of Celsius or Fahrenheit), explain that reason.

  3. NEVER change the units in someone's original question. That's a good way to drive away a new user.

  • $\begingroup$ Should edits that change the units in a post be rejected if they don't add anything else of importance? (I'd support rejecting them.) $\endgroup$
    – HDE 226868
    Commented Feb 14, 2015 at 15:37
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    $\begingroup$ I absolutely think they should. At that point, all you've done is made it harder for the asker to understand their own question. $\endgroup$ Commented Feb 14, 2015 at 15:48
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    $\begingroup$ @HDE226868 I would say that changing units is not ok, but adding units is ok. There are some units that I can convert in my head, but some units I have no clue about (psi to Pa). Even if the other units aren't used in the answer, it helps to get a relative idea of the magnitude. $\endgroup$
    – hazzey Mod
    Commented Feb 14, 2015 at 16:20
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    $\begingroup$ @HDE226868 An edit that adds the converted units alongside the original units would be appropriate. But replacing the existing units is unnecessary and ought to be rejected as an edit. Better yet, select "improve edit" and merge in the suggested converted units. $\endgroup$
    – user16
    Commented Feb 14, 2015 at 18:09
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    $\begingroup$ It's also worth pointing out that some fields and associated literature use predominantly non-SI units. Requiring everything to be in SI would likely be off-putting to practitioners in those fields. $\endgroup$
    – user16
    Commented Feb 14, 2015 at 18:10
  • $\begingroup$ "not everyone who comes here will" But everyone SHOULD be. If someone does not understand the metric system he should go to school learn it before doing any engineering. Be the way, degree Celsius is a SI unit. $\endgroup$ Commented Nov 30, 2023 at 13:44

I'm an American, a science enthusiast, and one of those people who shake their heads at America's refusal to adopt SI units. I could go on about simplicity, but the real reason that I would support SI units in some cases is when we're working with equations.

Take, for example, the equation for heat capacity: $$C=Q/\Delta T$$ $C$ is in Joules/Kelvin, Q is in Joules, and $T$ is in Kelvin. A Joule is actually a derived unit, Newton$\cdot$meter, so we're working in units of Newtons$\cdot$Meters/Kelvin.

In this case, it's much simpler to work in SI units.

Most of the time, though, we won't be using equations. It's unfair to say to someone that they should change the measurement of the diameter of a hole from inches to centimeters. When the end result won't matter, we should just go with whatever the person asking the question wants.


TL;DR: All values should be given primarily in metric and optionally, in non-metric units secondary.

When i find a question or answer, i want to be able to understand it. In order that everyone understands the question, it should use metric units.

We already enforce a language, which much harder to learn and where it is very subjective to determine which one is better. So we should absolutely enforce the metric system which is much easier to learn and objectively the better system. It would make engineering also easier and more of a joy than a pain when people would use the metric system everywhere.

This site is about engineering, it is expected to use the metric system in engineering. Everything else is unprofessional (I would call it Gebastel oder Basteln in German, basically means crafting like a young child would do). There are many places where the metric system is not used for some stupid reason, but that does not make it less unprofessional.

Why we should use the metric system

This part is maybe a bit of a rant, but since i see how many refuse to use the metric system and that there are people defending it, i think it is somewhat necessary.

  • The metric system is just far superior compared to the historic units
  • The metric system has well defined units
  • The metric system has clearly defined unit symbols.
  • Other proper system can make sense but they are not used in engineering.
  • For complexer dimensions, like voltage, all people use metric units.
  • It would simplify a lot if we would use the same system everywhere.
  • Everyone knows the metric system.

The metric system is better

The metric system, especially the SI subset, has prefixes and they are of factors 10^N between them. Makes it really easy to convert and compare values of different magnitudes. The second advantage of the prefixes is that they work on all SI units, except degree Celsius, the same. 1 km = 1000 m, 1 kV = 1000 V, 1 kΩ = 1000 Ω, ... So you only have to learn the prefixes ones and can use them for all dimensions.

Compare that to the British imperial units that have factors between them of 1760 (British land mile vs British yard), which is hard to convert in your head. Even worse, some British units do not have whole numbers as factors between them but ratios. Take a look at this:


And that is only for the length, you have to learn a similar complex system for time, weight and speed. You may argue that we don't use most of this, but still, it is way more complicated than the metric system even when we limit the units to mil (which isn't even in the diagram), inch, foot, yard and mile.


Take a look at units for stranded wire size. When we use metric units we just use square mm, for the area. Easy, the value is proportional to the amount of copper needed and inverse proportional to the resistance. It also gives you immediately an idea about the current capacity (assuming copper under somewhat room conditions, lets exclude exotic stuff like superconductors).

Compare that to AWG, probably the worst unit. It does not give a size, instead it gives a value where you need the logarithm to calculate it from the diameter: n = -39*log(d/0.005 in,92) + 36 Or to do it vice versa: 0.005 in * 92^((36-n)/39)

Nobody can do that in their head and most engineers can't do that on a calculator without looking up this formula. And yes, the area of a stranded wire is important for things like resistance. And it has the bad sideeffect of a larger value the smaller the wire. Most people not familiar with AWG will assume a larger number means a larger wire, making it easy for someone to use a wire that is not large enough.

The AWG is even worse by not using negative numbers if the value goes below 0. It uses 00 for -1 and making 00 different from 0. Which makes it horrible for programmers. What do they use if the formula results in -0.5 ? 00.5 ? I don't know.


We have one body, the BIPM, that is responsible for coordinating the definitions of the SI units. Which resulted in a system where 1 m is 1 m and everyone knows how long it is and there do not exist competing, incompatible definitions. There where some changes through history and very small changes, like the prototype metre which was 0.02% shorter than the previous definition, but that is miniscule and not relevant in engineering.

Many non-metric units have different incompatible definitions. Some of them only have small differences. For example the German Pferdestärke, which literally means horseforce, which basically every dictionary will translate as Horsepower, is 735.49875 W. On the other hand, the British Horsepower is about 745.7 W, about 1.4% more. When parts between a German and a English speaking country get exchanged, they often confuse this units, which many don't notice since the difference is small. A value in horsepower can often mean either one of this and without more information you can not know which one is used.

You, as an English speaker, may assume a inch is 25.4 mm. In Taiwan, a 寸 is 1/30 m ( about 33.3 mm ). When google translates 30 寸 (Which is 1m ) to English you get 30 inches (which is only 762 mm). https://translate.google.com/?hl=de&sl=zh-CN&tl=en&text=30%20%E5%AF%B8&op=translate You can be more specific and use the term international Inch or British inch and 英寸, but how often do you specify which unit version you use?

If you talk about ounces, you not only have a bunch of different units of mass, all named ounce, but there are also units called ounce that measure volume and some that measure force. In PCB design, 1 ounce of copper does not mean a mass, weight nor volume but a thickness of 35 µm. Now we have a unit name that is used for 4 different dimensions, 3 of them have have more than one definition.

Compare that to the metric system, where you know 1 metre is 1 metre and is not suddenly 0.95 m or 12 kg when you go to a different country.


There are 29 SI-Unit names and 24 Prefixes**. With them, all SI-Units can be written. All having a clearly defined symbol that are distinct. This makes it easy to write them and it is always clear which unit is used. Assuming people write it properly, which is sadly often not the case.

Do you know if 1 mil is: 0.001 inch, 1 international mile, a Swedish mile, someone who meant mm but didn't know how to write it, or a Biblical mile? The context may gives you some clue, but it would make things easier when you can look at the unit and know exactly what it means without having to understand everything else first.

Is 1 lbs = 1 pound or 1lb = 1 pound-second ?

Other systems

There are other systems, besides the metric system and besides historic units. One example would be the Planck system, that is only based on natural constants, without using made up factors. The Planck system makes more sense than any other system on paper. But since neither the Planck system nor other systems are used for engineering, we should not start to use them and making things even more complicated as they already are.

Complex Dimensions

As soon as the Dimension gets more complex, like voltage or electric resistance, everyone uses metric units. For voltage, everyone uses Volt, wich is 1 V = 1 kg*m^2 / (s^3 * A) Nodoy uses a unit like lb * mile * inch / (s^3 * A)

If we already agree that we use the metric system for this dimensions, why not use it for all?

One system for everything

Converting between unit system is bad, it is a source for failures and errors. It can also make things incompatible. I don't know how much resources we waste because we have to convert between units, have more failure, and less compatibility, but i would not be surprised if it costs humans 100s of Millions of euros per year.

So using a single system for everything should be a goal. We should make it easier for the future Generations, not harder. Since the Metric system is better than non-metric units, everyone should use metric units wherever possible.

Most of the world switched to the metric system about 150 years ago, because it is better what they had before. What stops the rest from also switching?

The unit system everyone knows

Everyone learns the metric system in School (hopefully). Ok, there are some that don't, but we should expect some level of knowledge when using this site. So we should expect everyone that uses this site to be familiar with the metric system. On the other hand, most schools probably don't teach how non-metric units work. We should not expect that everyone who uses this site to be familiar with non-metric units.

It makes it understandable for everyone when we use the metric system. When we use non-metric units many people will not understand it.

Comparing it to the natural Language

We enforce that all users use English. Understandable that we want a single lingua franca on this site. Not arguing against that.

But then we can also enforce a common unit system.

Opinion part:

Learning English takes years, learning the metric system does not take long. What bothers me is that often the ones where English is there native language, so they didn't had to learn it as second language, are the ones who often refuse to use the metric system. It is disrespectful when we use years to learn someone else's language, because it is the only language they speak, in order to communicate with them (again, not against this, since someone of us had to learn a foreign language), and they don't even bother to use the metric system, which is very easy to learn and objectively better.

That is a rude way of saying F**** You and your effort trying to communicate with me.

**Most of this Prefixes and many of this Unit names do most people not need to know. Average Joy doesn't need to know what Henry or Tesla means, or how many Watt a Yottawatt is.


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