### Simple and practical (popular) answer:

*1 liter of water "weighs" 1 kg.*

### In depth answer:

*1 liter of water is ***rarely** iqual to 1 kg.

To understand why, we should remember the concept os density.

**Density** is the ratio of mass of a body divided by its volume., or

**Density** is the mass per unit volume of a substance.

`Density = Mass / Volume`

or `Mass = Density x Volume`

There are issues in the simple answer above:

1 - Kg measures mass not weight, it actually measures mass. But, On Earth, people assumes mass is the same as weight, if that makes it easier (but it is not strictly correct).

2 - The density of water varies with temperature and pressure. (see table below)

That is why we said above that *1 liter of water is ***rarely** equal to 1 kg. When temperature and/or pressure fluctuates, 1 liter of water can contain more or less water molecules (more or less mass). The density varies as mass varies because 'Density = Mass/Volume'.

Why exactly 1?

Water was set as the **standard** because water is the most important of all liquids and the most abundant substance on the earth. So, it was logical to let 1 kg be the weight of 1l of water.

Table: density and weight (mass) of water, at standard sea-level atmospheric pressure

(source: http://water.usgs.gov/edu/density.html)

References:

https://www.quora.com/Why-is-it-that-1-litre-of-water-weighs-1-kg-while-this-doesnt-apply-for-other-liquids

http://water.usgs.gov/edu/density.html