# Are kilograms and liters equivalent?

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Just today I’ve noticed in the wrap of my liquid soap there’s a “290g/300ml” on it. Weren’t those two measures supposed to be equivalent? Or have they lied to me my whole life? Lol

In: Chemistry  1 gram of water is equal to one cubic centimeter of water which is equal to one millileter of water.

So that idea is useful in a lot of different circumstances, but the substance has to be the same density as water and soap usually doesn’t since it’s thicker. It depends on the density of whatever you are measuring. A kilogram of water is roughly a liter. But for things that float it is more then a liter and for things that sink it is less then a liter. One liter of water is equal to one kilo. So anything heavier/denser that water will not be equivalent between l/kg A kilogram was defined as the weight of 10^-3 cubic metres of water at standard atmospheric conditions (15°C, 101325 Pa Air pressure). (It’s been redifined in relation to the boltzman constant now, but that’s not important). Liquid soap, whilst it’s mainly water, is not exclusively water, and as such it’s possible for it’s desnity to not be exactly 1kg/l. They are not equivalent in general.

Kilograms are a unit of mass measuring how heavy something is while liters are a unit of volume measuring how much space is needed to hold something.

You might have thought they are equivalent as one liter of water weighs one kilogram (approximately depending on the temperature—and thus density—of the water).

Some things like gold are denser than water and thus weigh more than a kilogram if you have a liter of it (a cube with side lengths 10 centimeters or around 4 inches is one liter).

Other things like the soap you looked at are less dense and thus weigh less than a kilogram per liter. No they’re not. Kilogram is a unit of mass, liter is a unit of volume. They are related to each other by density;

density = mass/volume

Basically, the density measures how “closely packed” the mass of an object is. Bowling balls have around the same volume as basketballs, but we both know they are a lot heavier. That’s because the material used to make bowling balls is very dense (high mass per volume) than a basketball which is mostly filled with air.

The ONLY time mass and volume are numerically equivalent is when the density is 1. Water for example, has a density of ≈1 because 1g fills 1cm³, or 1kg fills 1L. So for water, we can easily calculate mass from volume and vice versa because the density is 1.

You can calculate the density of your bar of soap:

290g/300ml ≈ 0.967 g/ml

Very close to 1, but not quite.

Edit: removed 1kg/m³ because it’s not true Kilograms and Liters are *not* equivalent. Kilograms are a measurement of mass, Liters a measurement of volume. Some liquids can be lighter or heavier while still occupying the same physical space. This is called density.

A liter of water does have the mass of 1 kilogram. However anything that is not as dense as water will weight different things at 1 liter. Soap is not water (it has other stuff in it) so it doesn’t have to be 1kg at 1l. No. They measure two entirely different properties – mass and volume. These are related through a separate property – density.

The mass of a kilogram is partly defined by volume – 1000cm^3 of water at room temperature and pressure has a mass of 1kg, provided it has no impurities, this volume is equal to 1 liter.

So one liter of water has a mass of one kilogram. If something has a different density than pure water, or the temperature/pressure vary then the mass contained within one liter will no longer equal one kilogram.  It’s a generalization that is roughly true when talking about water at a certain temperature.

Basically a rule of thumb. One measures mass and the other measures volume, so no, they’re not equivalent. 300mL of water will weigh less than 300mL of gold. 290g of gold will take up less space than 290g of water.