Weight is volume times density — the shape just sets the volume
Every result comes from the same rule. Pick the volume formula for your shape, multiply by the alloy density, and divide grams by 1,000 for kilograms. A plate is length × width × thickness. A round bar is πr² × length. A tube is π(R² − r²) × length, using outer and inner radius. Keep every dimension in the same unit before you multiply.
Alloy densities
The alloy changes the density only slightly, so it rarely moves the weight by more than a few percent.
| Alloy | Density (g/cm³) | Common use |
|---|---|---|
| Pure / 1100 | 2.71 | Electrical, packaging |
| 5052 | 2.68 | Marine, salt exposure |
| 6061 | 2.70 | General structural |
| 2024 | 2.78 | Aircraft, high stress |
| 7075 | 2.81 | Aerospace, racing |
7075 is only about 4% denser than 5052, so swapping alloy changes the number far less than getting a dimension wrong does.
Why aluminium wins on weight
For the same part, aluminium comes in around a third the mass of steel. That is the whole appeal in vehicles and structures where every kilogram costs fuel.
Same volume, same shape: the aluminium part weighs about 34% of the steel one.
The mistakes that throw the number off
- Radius vs diameter. A 10 mm bar has a 5 mm radius. Squaring the diameter by mistake quadruples the weight.
- Treating a tube as solid. Subtract the hollow bore, or you will double-count the empty middle.
- Mixed units. Volume in cm³ with density in g/cm³ gives grams. Convert lengths first, round last.
Common questions
What is the density of aluminium?
Pure aluminium is about 2.70 grams per cubic centimeter. Common alloys sit close to that: 6061 is 2.70, marine-grade 5052 is 2.68, and high-strength 7075 is 2.81. For most jobs 2.70 g/cm3 is accurate enough.
How do I find the weight of an aluminium tube?
A tube is a solid rod minus its hollow center. Weight = pi times (outer radius squared minus inner radius squared) times length times density. Use the wall, not the full outer circle, or you will overstate the weight badly.
How much lighter is aluminium than steel?
About one third the weight. Aluminium is roughly 2.70 g/cm3 against steel at 7.85, so an identical part in aluminium weighs about 34% as much. That weight saving is the main reason aircraft and cars use it.
