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Comment by infogulch | Hacker News Reader

You're recalling correctly, gallon is a measure of volume. I would assume they used density conversions just to make it into a "relatable" measure for US readers.

But there is no US-gram. The SI unit for mass is the kilogram, (for which "kilo" is shorthand), where kilo- and giga- are standard SI prefixes. Units and prefixes are designed to be mixed and matched, so gigagram is just as valid as kilogram, even if it's more rare.

Edit: thank you userbinator for the base unit correction. I've edited above to not be blatantly wrong.

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The SI unit for mass is the gram, (not "kilo" which is just shorthand for kilogram, which is just commonly used because it's relatable)

The SI base unit for mass is actually the kilogram. It is memorable and notable for being the only base unit that already has a prefix.

Oh, thank you for the correction, not sure why I forgot that.

> the only base unit that already has a prefix.

That is interesting. So is kg actually not designed to work with other prefixes? Is it supposed to be megakilogram vs gigagram? Or are you just supposed to convert to something else or always use scientific notation? Or am I right about gigagram being valid, but for the wrong reasons?

The kilogram the SI standard. It's used in kilogram-meter-second reasoning systems as the base of derivative units, such as Newtons (1kg * G) or torque (N*m). (Basing these off of the gram would be... weird. You might protest the gram's the wrong size, but it's too late to change it.) Prefixes are applied as normal to 'gram' despite this (so milligram, microgram, and exotically gigagram) are correct.

Gigagram just seems exotic in the same way that megameter does - it's perfectly correct and understandable, it's just rarely necessary.

Some sciences used to prefer (or still prefer?) doing it the other way and use the CGS system, where you prefix meter instead of gram.

https://en.m.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%8...

No, it's supposed to be used just like others, it just has a prefix at its 'base' unit. Milligram and Megagram would indeed be technically right, but no one uses it (edit: in the case of megagram, thanks daveguy).

I suspect this is because, sort of like length, there's many different names for essentailly different orders of magnitude and you'd first go to the largest (a tonne, 1000 kg) before adding on prefixes. I suspect this many-names factor is in turn because they're such historical things that humans have measured and quantified since prehistory, because distance and mass are the first 'quantities' we begin to encounter.

So instead of 793 gigagrams, I'd say 793 thousand tonnes, or 793 kilotonnes.

Milligram is used extensively in biochemistry when defining concentrations milligrams per milliliters or mg/ml. This is the equivalent of g/L, but in biochemistry lab quantities are often measured in mg and ml rather than larger masses and volumes.

Also common: micrograms and microliters.

Sorry you're right, I added that 'no one uses it' comment on afterwards in reference to the Mega- prefix and in my exhausted state didn't parse the milli- version which indeed people do use.

Per my latter comment, it's because we don't really deal with everyday words for masses smaller than a gram, but we do for those bigger. It still holds that as far as I can think, people will go to the closest 'common term' word and then use prefixes to bridge the remaining gap (milligram, megatonne)

No-one uses milligram?! hehe uh.. Maybe that was a typo. What do you call that amount?

Also, 'liter' is used only in the US I believe. You guys just won't play with the other children, huh. :-)

edit: ah ok sorry NamTaf. Get some sleep!

I do not have any sources, but I'm pretty sure litre (or liter) is used very extensively, across the ocidental world at least, including Australia. What country are you from?

Yes, liter or litre is used across the world. 'Liter' is used only in the US I believe.

I'm pretty sure that the only time I use 'liter' is for soda pop. I'm from the US.

To be clear, a "liter" and a "litre" are the same unit of volume, just spelled differently. (It's not like "gallons", where the US and Imperial gallon have different volumes.)

Liter is used very widely in Europe, at least in non-english languages.

Ah, such as? I assume it's litre in France, I think it's litro in Spanish..

I know about German [1], Slovak [2]

[1] https://www.duden.de/suchen/dudenonline/Liter

[2] http://slovniky.korpus.sk/?w=liter

I think it's the same in Slovenian, Hungarian, Serbian (litar?), Norwegian and Swedish.

No. The base unit with regard to notation/prefixes is definitely the gram.

This question of whether there's a meaningful send in which the base unit is the kilogram and not the gram really confuses people and I don't know what the answer is. People eagerly proclaim (a) it has something to do with French historical convention (but, they admit when pressed, this gives it no non-trivial meaning today) or (b) it has to do with the size of the actual reference weight in France (which doesn't make sense because the second was never an object, and a meter no longer is represented by an object yet we still call it a base unit).

The only non-trivial interpretation I've heard (although it is still silly) is that the kilogram is the base unit in the sense that adding "kilo" to gram makes the derived units like the Newton the simplest. (You could define a Newton in terms of grams and, say, kilometers and seconds, or meters and milliseconds, but this would end up disturbing the other derived units even worse.) But of course, this is just a statement about the derived units.

No, the SI base unit of mass is the kilogram. From the NPL: http://www.npl.co.uk/reference/measurement-units/

"The kilogram is the International System of Units (SI) unit for mass. It is the only remaining base unit to be defined by a physical object. All standards of mass must ultimately be traceable to this one object, a cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM) in France"

NIST is working on a new reference for the kilogram based on Planck's constant, so that quote may become invalid in the near future.

NPL invented the Kibble balance that is being used in the proposed redefinition[0]. They discuss it and the Avagadro project on the page. So indeed it won't be valid much longer!

[0] http://www.npl.co.uk/educate-explore/kibble-balance/

The entire point of my comment is to note that "the SI base unit is the kilogram and not the gram" appears to be a contentless statement which is repeated mindlessly by people. And you responded by just repeating the statement that someone else made.

If the President issues a statement that says "the base unit of the US is the furlong", but nothing else about the US or its system of measurements changes, this is a contentless statement. If someone else asks "but what does it actually mean for the furlong, as opposed to the inch, to be the base unit?", pointing them to the President's statement does not help.

(Remember, saying "the kilogram is the base unit" cannot refer to the fact that the mass object in France weighs a kilogram, not a gram, because people also say "the meter and the second are base units", neither of which are physically instantiated anywhere.)

It's vary rarely written.

As an mechanical engineer educated in the UK the base unit would be kg but you would use ax10^n [kg] to denote mass.

https://www.bipm.org/en/measurement-units/history-si/name-kg...

Love the last paragraph:

The decision of the Republican government may have been politically motivated; after all, these were the same people who condemned Lavoisier to the guillotine. In any case, we are now stuck with the infelicity of a base unit whose name has a "prefix".

Hehe thanks, yeah. I meant US billion (10^9 not 10^12). I'm not saying 'gigagram' isn't valid, just I'd never heard it used before so had no sense how big it is. Amounts that big are usually talked about in millions or billions of tonnes or something like that. (1 tonne = 1000kg) Hmm this is harder than I thought, USA doesn't have the "o" sound in "tonne" either, hehe—it rhymes with "pot" for non-US speakers of English, 'on' with a 't' in front.

The differing meanings of billion, trillion, etc. drive me crazy. WHY DOES IT HAVE TO BE THIS WAY???

Ah you're talking about short scale and long scale. Actually most English speaking countries use short scale, even in the UK for official sources. It's unfortunate that there are so many hiccups in communicating numbers and units even with the SI system.

US seems to use the kilogram for one and only one thing: reporting the size of drug hauls.

There is US-gram. See slug, pound-mass.

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