How to Calculate Weight From Volume (With a Density Table)

If you know the volume of an object and what it is made of, you can find its weight with one multiplication. This guide explains the formula, the unit traps that cause most mistakes, and gives a table of common densities so you can check a quick estimate.

The formula

Density is how much mass fits in one unit of volume. Strictly, this gives mass, which is measured in kilograms or pounds; in everyday talk it is called weight, and for objects on Earth the two are used interchangeably.

Keep the units consistent

Most errors come from mixing units. Two combinations always work:

• Cubic metres × kg/m³ = kilograms. Steel at 7,850 kg/m³ and a volume of 0.002 m³ gives 15.7 kg.
• Cubic centimetres × g/cm³ = grams. The same steel is 7.85 g/cm³, so 2,000 cm³ is 15,700 g, the same 15.7 kg.

Note that 1 g/cm³ equals 1,000 kg/m³, which is why water is written as 1 in one system and 1,000 in the other. To use pounds, 1 kg is about 2.205 lb, and 1 lb/ft³ is about 16.02 kg/m³.

Typical densities

These are typical values. Alloys, wood species, moisture and compaction all change the real figure, so use the supplier’s number when it matters.

Worked examples

A concrete slab. A slab 2 m long, 1 m wide and 0.1 m thick has a volume of 0.2 m³. At 2,400 kg/m³ it weighs 480 kg.

An aluminium cube. A cube with 10 cm sides has a volume of 1,000 cm³. At 2.7 g/cm³ it weighs 2,700 g, or 2.7 kg.

A steel pipe. A pipe with an outer radius of 5 cm, an inner radius of 4 cm and a length of 12 cm has a wall volume of π × 12 × (25 − 16) ≈ 339.3 cm³. At 7.85 g/cm³ the pipe weighs about 2.66 kg. Only the wall counts here, not the empty space inside.

Hollow and irregular shapes

For hollow objects such as pipes, tubes and shells, use the volume of the material, not the outer volume. A shape that is partly filled, like a half-full tank, needs the volume of the liquid, not of the tank. For irregular objects, you can break them into simple shapes, calculate each and add them, or measure the volume by water displacement.

Letting the calculator do it

In the 3D Shape Calculator, open More options, pick a material or enter your own density, and the weight appears next to the volume for any of the 24 shapes. Add a price per kilogram to estimate the cost of the material. The Density Calculator works the other way: it finds the density from a known mass and volume.

Frequently asked questions

How do I convert volume to weight? Multiply the volume by the density of the material, using matching units: cubic metres with kg/m³ gives kilograms, and cubic centimetres with g/cm³ gives grams.

What is the density of steel? About 7,850 kg/m³, or 7.85 g/cm³. Stainless steel is slightly higher at roughly 8,000 kg/m³.

Why does a litre of water weigh one kilogram? The kilogram was originally defined from the mass of a litre of water, so water has a density of about 1 kg per litre, or 1,000 kg/m³.

Is mass the same as weight? Strictly no: mass is the amount of matter and weight is the force of gravity on it. On Earth the numbers are used interchangeably in everyday life.