Calculating steel weight from a drawing

The weight of a metal part comes from one rule: weight = volume × density. Steel has a density of 7.85 g/cm³ (7850 kg/m³), so once you know the sizes from a drawing you can estimate weight to quote a price, work out shipping, and pick lifting gear. This guide gives shortcut formulas for plate, round bar and tube, with a metal density table and worked examples.

The principle: volume × density

Every formula below comes from the same rule — find the part's volume, then multiply by the material density. To avoid unit confusion, work in millimetres and use this shortcut.

Rectangular plate

Plate is the simplest case: volume = length × width × thickness.

Example: an SS400 steel plate 300 × 200 mm, 6 mm thick.

weight = 300 × 200 × 6 × 7.85 ÷ 1,000,000 = 2.83 kg

A handy shop shortcut is weight per square metre = thickness (mm) × 7.85 kg/m². So 1 mm plate is 7.85 kg/m² and 6 mm plate is 47.1 kg/m² — multiply by the area in square metres for an instant figure.

Round bar and shafts

Round bar uses the circular cross-section area (π/4 × D²) times length. The shortcut per metre of length is:

Example: a Ø20 mm steel shaft, 1.5 m long → 0.006165 × 20² = 2.466 kg/m × 1.5 = 3.70 kg

Tube and hollow sections

A hollow part uses the cross-section area of the material only — outer area minus inner area — times length and density.

Tube cross-section = (π/4)(OD² − ID²), where OD is the outer diameter and ID is the inner diameter (ID = OD − 2 × wall thickness).

Example: a steel tube OD 48.6 mm, 3.2 mm wall (ID = 42.2 mm), 1 m long.
area = 0.7854 × (48.6² − 42.2²) = 0.7854 × (2362 − 1781) = 456 mm²
weight = 456 × 1000 × 7.85 ÷ 1,000,000 = 3.58 kg/m

Density of common metals

To change material, just swap the 7.85 in the formula for that material's density — an aluminium part weighs about a third of the same part in steel.

Getting a free-form part's area from the drawing

A laser-cut part is rarely a plain rectangle — it has holes and cut corners. To estimate its weight, find the real cross-section area first, then multiply by thickness and density.

1. Draw the part at real size in DXCAD2D Online (press R and type coordinates in millimetres, e.g. 300,200).
2. Break the area into basic shapes — a big rectangle, minus the hole areas (π/4 × Ø²), minus cut corners. Use the M measure command to check each size.
3. Add up the net area (mm²) and put it in the formula: weight (kg) = area × thickness × density ÷ 1,000,000.

Example: a flange plate 300 × 200 mm with four Ø20 holes, 6 mm thick.
area = (300 × 200) − 4 × (0.7854 × 20²) = 60,000 − 1,257 = 58,743 mm²
weight = 58,743 × 6 × 7.85 ÷ 1,000,000 = 2.77 kg (lighter than the solid 2.83 kg because the holes are removed).

Frequently asked questions

How much does 1 square metre of 1 mm steel plate weigh

7.85 kg, because steel plate weighs thickness (mm) × 7.85 per square metre. So 3 mm plate is 23.55 kg/m² and 6 mm plate is 47.1 kg/m².

How much does a Ø12 steel round bar weigh per metre

Use 0.006165 × D² = 0.006165 × 12² = 0.89 kg/m (a Ø20 bar is 2.47 kg/m and a Ø25 bar is 3.85 kg/m).

What is the density of steel

Carbon and mild steel have a density of 7.85 g/cm³ (7850 kg/m³). Stainless is about 7.9–8.0 and aluminium is 2.70.

Is calculating stainless or aluminium weight different from steel

The formula is the same — only the density changes, e.g. use 2.70 for aluminium instead of 7.85. An aluminium part weighs about a third of the same part in steel.

Summary

The weight of any metal part comes from one formula: volume × density. Two shortcuts cover most work — plate = thickness × 7.85 kg/m², and round bar = 0.006165 × D² kg/m. For complex parts, find the net cross-section area from the drawing and multiply by thickness and density. To change material, change only the density value from the table.