Every quotation, packing list and freight calculation starts with the same question: how much does the part weigh? For a flat blank it is arithmetic. For a drawn housing, a machined fitting or a tube frame it is arithmetic that has to be done in the right order, because the finished part does not weigh what the blank weighed.
How do you calculate the weight of a metal part?
Weight equals volume multiplied by density. Work in millimetres and grams per cubic centimetre: volume in cm³ = volume in mm³ / 1,000, and weight in kg = volume in cm³ × density / 1,000. Density values: mild steel 7.85 g/cm³, stainless 304 8.00, copper 8.96, brass 8.44, aluminium 6061 2.70, zinc 7.14, titanium 4.51. For sheet and strip, volume is thickness multiplied by the developed area, so weight per square metre at 1 mm thick is simply the density in kg: 7.85 kg/m² for steel, 2.70 for aluminium. For a drawn part, calculate from the blank, because the wall thins and the flange thickens - the finished part is usually slightly lighter than the blank it came from.
The formulas for the shapes that come up in sheet metal and machining
| Shape | Volume | Note |
|---|---|---|
| Flat sheet / blank | Length × width × thickness | Use the developed length for a bent part, not the drawing outline |
| Round bar | π × (d/2)² × length | d in mm gives mm³; divide by 1,000 for cm³ |
| Tube | π × (D² − d²)/4 × length | D outside, d inside diameter |
| Drawn cup (approximate) | π × d × h × t + base | Use wall thickness t at the measuring position on the drawing |
| Hexagon bar | 2.598 × (across flats)² × length | Across flats, not across corners |
| Square / rectangular bar | width × height × length | - |
Density values to calculate with
| Material | Density (g/cm³) | kg per m² at 1 mm |
|---|---|---|
| Mild steel (DC01-DC04) | 7.85 | 7.85 |
| Stainless 304 / 316 | 8.00 | 8.00 |
| Stainless 430 / 430F | 7.70 | 7.70 |
| Aluminium 1050 / 6061 | 2.70 | 2.70 |
| Aluminium 5052 | 2.68 | 2.68 |
| Copper C11000 | 8.96 | 8.96 |
| Brass CuZn37 | 8.44 | 8.44 |
| Zinc | 7.14 | 7.14 |
| Titanium Grade 2 | 4.51 | 4.51 |
A worked example: 500 stamped brackets
A bracket is stamped from 1.5 mm DC04 with a developed blank of 140 mm × 60 mm. Area = 8,400 mm² = 84 cm². Volume = 84 × 0.15 = 12.6 cm³. Weight = 12.6 × 7.85 = 98.9 g per part. Five hundred parts weigh 49.5 kg - which decides whether the order ships as a parcel or as freight, before anyone has opened the drawing for the tolerance discussion. If the same bracket were made in 304 the same calculation gives 100.8 g per part, and the total moves by nearly a kilogram.
Why the blank, not the finished part
- Material is quoted by weight - the coil or strip you buy is priced per kilogram, so the blank weight is the number that goes into the piece price.
- Scrap has a route - the difference between blank weight and finished part weight is scrap, and on a progressive die it is the difference between a competitive price and a losing one.
- Draw parts change weight distribution - the wall thins and the flange thickens, so a drawn housing is a little lighter than its blank. Assuming the finished weight for material purchasing under-buys every time.
- Freight and handling - the packing list is calculated from the finished weight plus packaging, so the two numbers are used for two different purposes in the same quotation.
Key point
Volume times density, with density in g/cm³ and volume in cm³. For sheet parts use the blank: thickness times developed area. For machined parts use the finished geometry plus the bar you have to take it out of - the scrap between the two is part of the price.
Frequently asked questions
How do I calculate the weight of sheet metal?
Multiply the developed area by the thickness to get the volume, then multiply by the density. In practical units, sheet weight in kg = area in m² × thickness in mm × density in g/cm³.
What is the density of steel in kg/m3?
Mild steel is 7,850 kg/m³ (7.85 g/cm³). Stainless 304 is 8,000 kg/m³ and aluminium 6061 is 2,700 kg/m³.
Why is the blank weight used instead of the finished weight?
Because material is bought and priced by weight, and the scrap between blank and finished part is part of the cost. Quoting from the finished weight understates material on every part.
Does a deep drawn part weigh less than its blank?
Usually slightly, because the wall thins more than the flange thickens. The difference is small on a shallow part and significant on a deep one with a heavy reduction.