[GlobalFrog]

I don't understand your issue between gram and kilo gram: gram is the base unit and the prefix kilo, meaning one thousand just says that 1 kg = 1000 grams. It is exactly the same as meters and kilometers: meters is the base unit and 1 km = 1000 meters.

[DavidSJ]

In SI, kg is the base unit, and g is a derived unit.

[jillesvangurp]

There's an etymological reason for the word gram. It derives from a greek word γράμμα which roughly translates as "small weight" and made its way into French via the latin gramma to the French gramme, and the English gram. And 1kg is just very chunky. It wouldn't be right to refer to that as small.

As the name kilogram implies, gram is actually the unit here. But it was derived from the mass of a standard 1 kg chunk of metal that lives in a museum somewhere near Paris. This is the literal base unit of mass (at least historically, the definition has since been redefined using the Planck constant). A 1 gram chunk would have been tiny and be tedious to work with doing e.g. experiments with gravity.

They also have the original prototype meter in the form of a length of platinum-iridium alloy bar. And because the specific reference object for mass weighs 1kg instead of 1g, it means 1kg is the base unit in SI.

But quite obvious in the system of measurements, the gram is the logical unit here that you augment with prefixes and people commonly handle a lot of mass quantities that are in the order of grams rather than kg.

Derivations are simple. Simply apply powers of ten and their commonly used prefixes (kilo, milli, mega, micro etc.). The base unit is something physical that you can point at as the base unit. Or at least historically that was the intention.

There's also convenience. A 1l of water is about 1kg and a volume of 10x10x10cm. or 1 dm3. That's not accidental but intentional. It makes it easy to work with volumes and masses for people. Never mind that a liter of water isn't exactly a kg (because water purity, temperature, and a few other things).

[StopDisinfo910]

Kilogram is indeed the base SI unit and not gram. It’s an exception.

Every formula using SI will expect mass in kg and you will be off a factor of 1000 if you use gram as the base unit. Same with derivative units like the newton which all use mass in kg for conversion.

[selkin]

It’s an historical artifact, as it was easier to manufacture a reference kilogram than a reference gram.

Considering today we set the kilogram by fixing the Planck constant and deriving it from there, we can just divide each side of the definition by 1000 and use that as a base unit. Using kg as the base unit is completely arbitrary, as we can derive each unit of weight directly from the meter and the second, not from the base unit.

[mmooss]

Why not call the thing that weighs ~2.2 pounds a 'gram'?

[selkin]

For the same reason it was not renamed "Wug".

[mmooss]

It's not the same reason. Gram is already part of the nomenclature, wug is not. The change I asked about would shift the relation of the prefixes to the masses: kilogram would represent a mass 1,000 times larger than it does now.

[selkin]

It's exactly the same reason: gram referenced a known quantity. Changing it by a few insignificant digits because of the kilogram update wouldn't force people to realign their perception of it.

Changing it to ~1,000 times what it used to be, or giving it a new name, would force people to realign.

There's reason many people still prefer customary and imperial units, and it's not just bigotry and nationalism (even if they play a part in that preference).

[tokai]

I think they mean that the gram is defined as 1/1000 of a kilogram. With a kilogram having a definition based on physical constants.

[jabl]

The kilogram is no longer defined by a physical artifact, fwiw.

Anyway, the point is the inconsistency in the system due to the kilogram being the base unit. So derived units are defined in terms of kilogram rather than gram. Say, the unit of force, Newton (N), is defined as kgm/s^2 and not gm/s^2). Or pressure, Pascal (Pa) which is N/m^2 which inherits N being defined in terms of the kilogram). And so on. Anyway, an annoying inconsistency maybe but doesn't really affect usage of the system once you get used to it.