Density and Specific Gravity: Converting Between Weight and Volume
There is no universal conversion between litres and kilograms. Density is the missing variable, and it changes with temperature.
"How many kilograms in a litre?" is one of the most common conversion questions, and it has no single answer. Litres measure volume, kilograms measure mass, and the bridge between them is density — which differs for every substance and changes with temperature.
The relationship
mass = volume × density
Water at 4 °C has a density of almost exactly 1 kg/L, which is not a coincidence: the kilogram was originally defined as the mass of a litre of water. That definition has long since been replaced, but the near-equality survives.
Some practical densities:
- Water (4 °C): 1.000 kg/L
- Water (20 °C): 0.998 kg/L
- Petrol: about 0.74 kg/L
- Diesel: about 0.83 kg/L
- Olive oil: about 0.92 kg/L
- Milk: about 1.03 kg/L
- Honey: about 1.42 kg/L
- Mercury: 13.53 kg/L
A litre of honey weighs nearly twice what a litre of petrol does. This is why "a litre of anything weighs a kilogram" is a useful rough rule only for water-like liquids.
Specific gravity
Specific gravity (relative density) is the ratio of a substance's density to that of a reference — water for liquids and solids, air for gases.
Being a ratio, it is dimensionless, which is its advantage: the number is the same whether you work in kg/L, g/cm³ or lb/ft³.
A specific gravity below 1 means it floats on water. Above 1 means it sinks. Petrol at 0.74 floats, which is exactly why petrol fires cannot be extinguished with water — the fuel floats on top and keeps burning while spreading.
Temperature dependence
Most substances expand when heated, so density falls with temperature. For precise work the reference temperature matters:
- Water at 4 °C: 1.0000 kg/L
- Water at 20 °C: 0.9982 kg/L
- Water at 80 °C: 0.9718 kg/L
Water is anomalous in that its maximum density is at 4 °C, not at freezing. Ice is less dense than liquid water, which is why it floats — a property with substantial consequences for aquatic life, since lakes freeze from the top rather than the bottom.
Bulk density versus true density
For granular materials the distinction matters.
True density is the density of the material itself. Bulk density includes the air gaps between particles.
Flour has a true density near 1.5 g/cm³ but a bulk density around 0.5 g/cm³, because roughly two thirds of a cup of flour is air. This is the underlying reason volume measurement of flour is so unreliable — you are mostly measuring how compressed the air gaps are.
The same applies to sand, gravel, and any powder. Construction materials are specified by bulk density for transport and true density for structural calculation.
API gravity
The petroleum industry uses its own inverted scale:
API gravity = (141.5 / SG) − 131.5
Higher API means lighter crude. Light crude above 31.1° API is easier to refine and commands a premium; heavy crude below 22.3° API requires more processing. Water is 10° API by definition, so crude below 10 sinks.
Practical conversions
To convert volume to mass, you need the density at the relevant temperature:
- Find the density in consistent units, e.g. kg/L.
- Multiply volume by density for mass.
- Divide mass by density for volume.
Our density converter handles kg/m³, g/cm³, lb/ft³ and related units, and the specific volume converter handles the reciprocal quantity used in thermodynamics.
Measuring density without special equipment
For a liquid, the simplest method is direct: weigh a container empty, fill it with a measured volume, weigh again, and divide the mass difference by the volume. A kitchen scale and a measuring jug will get you within a couple of percent, which is enough for most practical purposes.
For an irregular solid, use displacement. Weigh the object, then submerge it in a graduated container of water and record the volume displaced. Mass divided by displaced volume gives density. This is the method attributed to Archimedes, and it still works.
A hydrometer measures liquid density directly by how deep it floats. Brewers use them to track fermentation, since sugar raises density and alcohol lowers it, so the change between readings indicates how much sugar has been converted. Battery hydrometers work the same way on electrolyte, where specific gravity indicates state of charge.
Gases behave differently
Gas density depends strongly on pressure and temperature, so a single figure is meaningless without both.
At standard conditions — 0 °C and 1 atmosphere — one mole of any ideal gas occupies about 22.4 litres regardless of which gas it is. This is Avogadro's law, and it makes gas density essentially a function of molar mass.
Air averages about 1.225 kg/m³ at sea level and 15 °C, roughly 1/800 the density of water. Helium at about 0.179 kg/m³ is far lighter, which is why a helium balloon rises: it displaces more air mass than its own total mass.
Because density falls with altitude, aircraft performance degrades at high-elevation airports on hot days — less dense air means less lift and less engine power. This is density altitude, and on a hot day a runway can effectively behave as though it sits thousands of feet higher than it does.