Material weight calculator: volume times density, done properly.
Quoting a job without knowing the part weight is guessing. Material is priced per kilogram, so a 5% weight error is a 5% margin error on every piece you ship. This guide gives you the weight formula, ready-to-use shape equations, a metal density chart, and the costing logic behind them.
The one formula everything reduces to
Every material weight calculation is the same idea: weight = volume × density. Everything else is just working out the volume for a given shape and picking the right density for the material. Keep the units consistent and the answer is exact.
In SI units, volume in cubic metres (m³) times density in kilograms per cubic metre (kg/m³) gives mass in kilograms. Most shop drawings are in millimetres, so the practical trick is to work the volume in cubic centimetres (cm³) and use density in grams per cubic centimetre (g/cm³) — for steel that is 7.85 — which gives grams directly. Divide by 1000 for kilograms.
Weight equations by shape
Work the volume from the geometry, then multiply by density. Here are the shapes engineers price most often, with dimensions in mm and density ρ in g/cm³ (divide the volume by 1000 to convert mm³ to cm³).
| Shape | Volume | Notes |
|---|---|---|
| Round bar | (π/4) × D² × L | D = diameter, L = length |
| Square / flat bar | W × T × L | Width × thickness × length |
| Plate / sheet | L × W × T | Weight per m² = T(mm) × ρ × 0.001 |
| Hollow tube | (π/4) × (D² − d²) × L | D = outer, d = inner diameter |
| Hexagon bar | 0.866 × AF² × L | AF = across flats |
For a real machined part with pockets, holes and chamfers, you cannot reduce it to one shape. Either subtract the removed volume feature by feature, or take the volume straight from the 3D model, which is what a CAD-based weight tool does.
Worked example: a steel shaft
Price the raw stock for a turned shaft from EN8 steel, machined from ⌀60 mm bar, 320 mm long. Density of carbon steel is 7.85 g/cm³.
- Volume = (π/4) × 60² × 320 = 904,779 mm³ = 904.8 cm³
- Weight = 904.8 × 7.85 = 7102 g = 7.10 kg
At a steel rate of ₹70/kg, the raw bar costs about ₹497 per piece — before you account for the chips. If the finished shaft weighs 5.4 kg, you are paying for 1.7 kg of material that becomes swarf, so the real material cost per good part includes that yield loss. The free material weight calculator returns this in one step for bars, plates and tubes across a dozen materials.
Metal density chart
Use the specific grade where you have it; these are representative values for quick work.
| Material | Density (g/cm³) | Density (kg/m³) |
|---|---|---|
| Carbon / mild steel | 7.85 | 7850 |
| Stainless steel 304 | 8.00 | 8000 |
| Cast iron (grey) | 7.20 | 7200 |
| Aluminium 6061 | 2.70 | 2700 |
| Brass | 8.50 | 8500 |
| Copper | 8.96 | 8960 |
| Titanium (Ti-6Al-4V) | 4.43 | 4430 |
Note how far apart these sit. Switching a bracket from steel to aluminium cuts weight by roughly two-thirds for the same volume, which is the whole reason aluminium dominates aerospace and EV structures despite costing more per kilogram.
Why weight is the cost
Material is almost always bought per kilogram, so part weight is the spine of any cost estimate. Three numbers flow from it: raw material cost (weight × rate), scrap recovery (chip weight × scrap rate), and freight (often weight-based for metal parts). Get the weight wrong and all three are wrong.
The subtle one is yield. A part machined from bar might finish at 60% of the starting weight, so 40% is sold as scrap at a fraction of the buying price. A net-shape casting or forging starts much closer to final weight, which is why high-volume parts move to those processes even with higher tooling cost.
Common material-weight mistakes
- Mixing units. The classic error is mm³ with g/cm³ without the ÷1000. Always reconcile units before trusting the number.
- Using a generic density. "Steel" spans 7.75 to 8.05 g/cm³ across grades; stainless is not the same as mild steel.
- Forgetting the bore on tubes. Pricing a tube as solid bar massively over-estimates weight and cost.
- Ignoring yield loss. The finished-part weight is not the material you buy; cost the starting stock.
- Rounding density too hard. Using 8 for everything inflates aluminium parts threefold — pick the right value.
Weight feeds straight into costing and procurement decisions. Pair it with the worked guides on OEE calculation for machine-time cost and ISO 286 fits and tolerances for the stock allowance you machine away. For ready-to-use estimating forms, browse the MetricMech templates library.