Some of the examples go pretty far too, either calling spelling I'd never see a "common mistake" (like "gr." for grams), or by being too scientifically correct for a casual usage, for example in a press article ("(100 + 200) grams", right).

But then I saw "Set the oven to 450 K", and I realized that the examples are to be taken with a grain of salt :)

For e.g. in the UK sqm is very common, particularly when looking at property. On RightMove which is the most popular property website here, for e.g. you can see here that the default is `sq ft` with `sq m` given underneath: https://www.rightmove.co.uk/properties/140100683#/?channel=C...

In terms of things like mixture of units, I appreciate you want to give in one unit but I think the things chosen are a bit strange. I've never seen anyone write '15070 grams', it'd almost always be given as 1.507 kilograms.

But standard is kB, from kilo prefix.

As with all things, context is king. I'm not going to be confused when my colleague asks me on slack if it's OK to email me a 15mb (millibar) document - I know what they meant. Nor am I going to worry if it's actually MiB vs MB, in this context it does not matter.

Also, although it probably be nice that we all standardised on the correct prefixes, literally nobody speaks in terms of gigameters or teragrams or whatever. We carry with us an internalisation of real world measurements, and use that to compute relativity when we read/hear these numbers. It's roughly 1000km from my home town to the coast, so if if the moon is roughly 400,000 km away, then that's 400x the distance, pretty far. My car weighs roughly 1 ton, so Hafthor's 501kg deadlift record is half my car - pretty impressive!

> Avoid common non-SI units (suggested)... For serious science and engineering, SI units like metres per second should be used instead.

Yeah good luck with that... electrical power engineering speaks in kWh not MJ

> 150 pJ gamma ray instead of 938 MeV gamma ray

Particle physicists will hate you if you take electron volts from them

While I'm at it, do you know that attaching suffix to SI unit is not allowed by SI / IEC /ISO? So abbreviating megawatt of thermal energy as MWth or decibel referred to 1 milliwatt of power as dBm is technically illegal:

> "When one gives the value of a quantity, it is incorrect to attach letters or other symbols to the unit in order to provide information about the quantity or its conditions of measurement. Instead, the letters or other symbols should be attached to the quantity."

https://physics.nist.gov/cuu/pdf/sp811.pdf

Yeah... all of my spectrum analyzers, signal generators, software-defined radios, antennas and RF interface control documentation is not ISO/IEC compliant...

- "Fuel consumption of 9.4 L / 100 km" is not "the right form" is a modern take, a more classic and still valid SI one is km/l which is very similar to "Fuel consumption of 9.4 LPK";

- "25 {kW/h,kW⋅h} to boil a tank of water" are both strange forms, kWh is the common accepted way to express energy, no center dot multiplication needed;

- multiple prefix (vs power-of-10) and bare prefix are as well commonly accepted, I see no reasons to consider them wrong, while I can state formally wrong "2 kilograms of rice" because we do not measure the mass but the weight so it should be 1.961daN where deca-newtons are commonly used because 1 daN is roughly 1kgf commonly shortened to 1kg as we can commoly count 1kg[f] == 1 daN... For instance climbing equipment in the EU use daN to express maximum loads of connectors, ropes etc because of that;

- multiple quantities much depend on industry and conciseness, as we do not write units in table values but only in headers we tend not to write them three times in a row where from the context is clear what numbers means.

The biggest issue is makes habits changes. Actually we should not use km/h as well, since for SI base unites are m/s, but 3.6 is not an easy conversion like kgf/daN, so in the EU we keep using km/h, something meaningful in the past, when we go by horses and feet, but not much needed today.

Not to count software, where often recognize "°C" (two chars) BUT not ℃ (U+2103) and so on.

Oh, interesting. NIST recommends "L": https://www.nist.gov/pml/special-publication-811/nist-guide-...

Sure, if I'm doing a bit of formal writing, I'm going to do things correctly. But I don't see it as a problem for people to use odd abbreviations like "mtrs" or incorrectly pluralize as "kms" in informal writing. People will not have trouble knowing what you mean. No one is going to say things like "I drove 3 megameters over the past couple weeks". (Reading some of the longer explanations below seems to indicate that the author is not just talking about formal writing.)

And even in formal writing, a few of these are bit much, like good luck getting people to stop using electron-volts in favor of picojoules.

This just seems silly; it's hard to take this piece seriously, and all of the nitpicky arguments seriously undermine the (few) good arguments listed.

> Mass/weight distinction

> Incorrect: He weighs 70 kg; Correct: His mass is 70 kg

> Incorrect: She weighs 50 kg; Correct: She weighs 490 N on Earth

Really?! We’re not supposed to use kilograms to describe a person’s weight? And to describe someone’s weight, we have to use newtons and specify what planet they’re on? Good luck enforcing those rules.

Why not 6 Mg of wheat?

That's an excellent question. As an analogy, I would say that it's similar (but not identical) to making mistakes in natural-language spelling and grammar.

For many cases, you are correct that the meaning can be repaired with context. dis iz da kase 2 wif nglish, as u kan sea w/ this sntnce.

Likewise, there will be some cases where a single letter or word can make all the difference. "He hit her" conveys a different meaning than "He hit on her"; an ESL student might only learn the verb and not the important subsequent preposition. And then there are pronunciations: https://www.youtube.com/watch?v=3Q44_A4NzjI .

Finally, by excusing the writer of all their mistakes, it puts the onus on the reader to grasp the right meaning. And if causes the reader to be confused or misunderstand, the reader has to go back and ask questions. That's a selfish shifting of effort.

> email me a 15mb (millibar) document

It looks like the symbol for bar (unit) is "bar", not "b". It is not part of SI. https://en.wikipedia.org/wiki/Bar_(unit)

You might not appreciate this, but there are fields where mb meaning millibit actually matters. Look at information theory such as data compression and error correction codes. We might say that JPEG encodes each RGB pixel with 3.0 bits, and WebP uses 2.8 bits/pixel, so WebP is 200 millibits per pixel more efficient than JPEG. There are many codec competitions where improvements are in the range of millibits per symbol (though they're written rather normally as 0.123 bits to not scare people).

By "autocorrecting" "mb" to "megabyte(s)", we lose out on the ability to express "millibit(s)".

> literally nobody speaks in terms of gigameters or teragrams or whatever

So why do we use megabytes, gigahertz, megapascals (for some industrial chemical processes), megawatts (power plants)? It's just a matter of habit. We collectively decided that kilometre is the biggest unit we'll tolerate, even though there is nothing technically wrong with megametre, gigametre, etc. We have the same problem with avoiding megagrams, kilolitres, kiloseconds, etc.

> It's roughly 1000km from my home town to the coast, so if if the moon is roughly 400,000 km away, then that's 400x the distance, pretty far.

That's nice, and it is true that when drawing attention to a comparison, the same unit should be used (Pat Naughtin and The Metric Maven both advocate for this idea). But it's also okay to write that the moon is 400 Mm away in the context of astronomy, because very few distances between bodies in space are less than 1 Mm.

The big problem with how writers treat the kilometre is that we always end up with things like "the nearest star is 4.35 light-years or 40 trillion km away". Instead of using SI prefixes as designed, we end up jamming big number words in front of the unit, and that doesn't make things any clearer. Would you like it if I said my CPU is 3.5 billion hertz, and ban all the mega-, giga-, etc.? I could even make the case that this isn't theoretical, because for example my monitor runs at 60 Hz, and we certainly must not call it 0.000 000 06 GHz with any sense of normalcy.

> My car weighs roughly 1 ton, so Hafthor's 501kg deadlift record is half my car

And you contradicted yourself because now you're using different units. It would be better to say that your car is 1000 kg and the deadlift record is 501 kg. So it's like sometimes you accept changing units, sometimes you don't.

Then it's industry-specific jargon. Note that some places bill your natural gas consumption in megajoules. Food energy is quoted in kilojoules in SI, or calorie/Calorie/kilocalorie otherwise.

The problem is that if you want to make any comparisons, you need to use the same units.

An athlete eats 100 Calories, gets on an exercise bike, and powers a 100-watt light bulb for 3000 seconds. What is her thermodynamic efficiency?

To make the 500 km journey, the old car consumed 1 GJ worth of heat energy in gasoline. The new electric car consumed 300 kW⋅h. Which one consumed less energy?

> Particle physicists will hate you if you take electron volts from them

I can understand why they love their traditional unit of measurement.

The problem is that their unit measures the same type of quantity (energy) as an existing SI unit - the joule.

We've seen this play out in countless industries already, where they have their own specific units and refuse to interoperate.

Heating in BTUs, cooling in tons, explosions in kilotons of TNT, barrels of oil.

Astronomy in light-years or parsecs or astronomical units, typesetting in points, football in yards (instead of feet), microscopic things constantly compared to the width of a human hair, ~5 different scales of shoe sizes across the world.

> it is incorrect to attach letters or other symbols to the unit in order to provide information about the quantity or its conditions of measurement

I agree with this rule. I guess if you write it out in words, it would be:

* Incorrect: "Power: 480 volt-amps-reactive (VAR)"

* Correct: "Power (reactive): 480 volt-amps (V⋅A)"

* Correct: "Reactive power: 480 volt-amps (V⋅A)"

By the way, you can link to a specific page like this: https://physics.nist.gov/cuu/pdf/sp811.pdf#page=16

No, it is the right form.

> a more classic and still valid SI one is km/l

The SI one has always been L/km. Any use of km/L is a translation over from mpg (miles per gallon).

> km/l which is very similar to "Fuel consumption of 9.4 LPK"

km/l is the reciprocal of LPK. These are not remotely the same unit.

> kWh is the common accepted way to express energy, no center dot multiplication needed

In SI, it is unacceptable to multiply units together without a space, dot, or cross. This can create ambiguities - for example, "mm" could mean "millimetre" or "metre × metre". "ms" could mean "millisecond" or "metre × second".

I had a discussion here a while ago where I changed from "kWh" to "kW⋅h": https://www.reddit.com/r/Metric/comments/1313rhp/how_to_resp...

> multiple prefix ... well commonly accepted

Not really. Other than saying "I drove 30 k km", it is rare to stack prefixes. We don't say that CPU features are "5 millimicrometres".

> bare prefix are as well commonly accepted

And this is wrong. So, I have this electric scooter that goes 80 k at 30 k and weighs 40 k. How do you like that?

> we do not measure the mass but the weight so it should be 1.961daN

Actually, it's possible to measure mass. If the scale has an internal reference with a known mass, then it can adjust to the local gravitational constant.

What I mean is, let's say an electronic scale has an internal 10 g mass. When switching on the scale, it measures the internal mass and sees 9.8123 mN (millinewtons). Now it will use that scaling factor for anything else measured during the session. Then you move the scale to the south pole, and it measures the internal mass to have a weight of 9.9765 mN, so it compensates correspondingly.

Likewise, if you use a balance scale, then you are comparing against known masses (not weights). (We'll ignore buoyancy for the vast majority of commercial applications.)

> For instance climbing equipment in the EU use daN to express maximum loads of connectors, ropes etc

I haven't been exposed to that before, so thanks for that. That's just about the only example I have ever heard of involving the deca- prefix. I would much prefer all the non-power-of-1000 prefixes to die (goodbye to centi-, deci-, deca-, hecto-) for many reasons.

> we should not use km/h as well, since for SI base unites are m/s, but 3.6 is not an easy conversion

Correct. I have used both km/h (because obviously society forces it) and m/s in various applications and calculations. Using m/s is much more helpful for visualizing distances traveled in a short span of time, and also calculating acceleration (in m/s^2, never km/h/s), kinetic energy, etc.

This agrees with all SI units named after a person. The unit's name is lowercase (unless at the beginning of a sentence), and the symbol has its first letter in uppercase. Examples: newton (N), tesla (T), hertz (Hz), pascal (Pa), joule (J), watt (W), volt (V), ampere (A).

Note that "degrees Celsius" is capitalized in a special way.

See also: "I bought one bitcoin (1 BTC) on the Bitcoin network."

This is a very important distinction to make in scientific papers. Completely meaningless for most writing. If anything, physicists got the definition for the word "weight" wrong and patched up their mistake by adding the concept of "mass". Most mass is measured standing still on earth, so "weight" is fine unless you need to be precise.