Volume times density, in one line
A solid rectangular block weighs its volume multiplied by the material's density. Get the volume from the three sides, then multiply by density. The only thing to watch is that every unit agrees.
Keep dimensions in metres and density in kg/m³ and the answer is in kilograms. A 2 × 1 × 0.5 m steel block: volume 1 m³, times 7,850 kg/m³ = 7,850 kg.
Density of common materials
These are the numbers that drive the result. Pick the one for your material and multiply it by the block's volume.
| Material | kg/m³ | g/cm³ | lb/ft³ |
|---|---|---|---|
| Steel (mild) | 7,850 | 7.85 | 490 |
| Aluminium | 2,700 | 2.70 | 169 |
| Copper | 8,960 | 8.96 | 559 |
| Concrete | 2,400 | 2.40 | 150 |
| Oak (hardwood) | 700 | 0.70 | 44 |
| Pine (softwood) | 500 | 0.50 | 31 |
| Water (reference) | 1,000 | 1.00 | 62 |
Anything denser than water (1,000 kg/m³) sinks; anything lighter floats. Most woods float; every metal and concrete sinks.
Getting the units right
- One unit for all three sides. Convert first: 100 cm × 50 cm × 200 cm is 1 m³, the same as 1 m × 0.5 m × 2 m. Never multiply mixed units.
- Match density to dimension. Metres go with kg/m³; centimetres go with g/cm³. To swap, 1 g/cm³ = 1,000 kg/m³.
- Mass, not force. This gives mass (kg or lb). The block's weight as a force in newtons is mass × 9.81, and would be six times smaller on the Moon; the mass never changes.
- Solid only. The formula assumes a full solid block. A hollow box or a frame weighs far less than its outer dimensions suggest.
Common questions
How do I find the weight of a rectangular block?
Multiply length by width by height to get the volume, then multiply the volume by the material density. A steel block 2 m by 1 m by 0.5 m is 1 cubic metre, and at 7,850 kg per cubic metre it weighs 7,850 kg.
Why does a steel block weigh so much more than a wooden one the same size?
Density. Steel is about 7,850 kg per cubic metre and softwood about 500, so an identical block of steel is roughly 15 times heavier. The size sets the volume; the material sets how much mass fills it.
What is the most common mistake?
Mixing units. If length is in metres, width in centimetres and height in millimetres, the volume is meaningless. Put every dimension in the same unit before multiplying, and match the density unit to it (kg/m3 with metres, g/cm3 with centimetres).
