See, 1 liter isn't 1kg , 1 litre of water weighs 1kg. There's the difference, and as such there is no reasin for 1 litre of water to weigh 1 gram.

The International System of Units (SI) uses the kilogram as its base unit of mass, so it is 1 thing in SI units = 1 liter, and SI is what most people think that they mean by "metric system" these days. Even though the SI isn't entirely metric. (The Celsius scale isn't, for example.) Also, liters are not SI units; the m^3^ is the SI unit of volume, and the liter was defined in terms of that.

To put it another way, the base unit of mass is a kilo- of something, and it (not quite) equals a metric non-metric-system unit of volume of water. I dunno, the metric system seems all kinds of messed up to me!

(/j)

The gram was originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm3], and at the temperature of melting ice".

Why? Who knows. If they'd already decided on the meter, then the most logical definition of a litre is pretty awkward. You'd think it would be 1 cubic meter of water, because then the unit of volume is 1 unit of length cubed. Then one gram would be one litre of water, which is today 1000 kg, or one metric ton.

But, then for everyday things people would be using microlitres and micrograms.

Really, the metre should have been the replacement for the inch (i.e. the centimeter). Then you could have 1 litre being 1 cubic meter (what is today 1 mL) and then the unit of mass would be the mass of 1 L of water, which would be 1 gram. A person's height would be in hm, and long travel distances would be in Mm.

It's like the reason 0 is freezing water temp and 100 is boiling water.

They used water as a standard when deciding what things ought to mean.