Steel cutting waste: how to calculate it and reduce it

Purchase weight, net weight and scrap, with worked examples

Cutting waste is the steel you pay for but do not use. On most jobs it is decided before anything is cut: by the stock length, the sheet size and the way the parts are grouped.

The three numbers

Purchase (gross) weight	The full bars or sheets you buy
Net weight	The parts you need
Scrap	Purchase weight − net weight

Waste % = scrap ÷ purchase weight × 100

Example: bars

Six parts of 3,800 mm cut from 12 m bars: each bar gives 3 parts, so 2 bars are bought. 24 m of steel for 22.8 m of parts is 5 % waste. The same parts from 6 m bars need 6 bars, 36 m of steel, and the waste jumps to 36.7 %.

Example: plates

A 1220 × 2440 mm sheet of 10 mm plate weighs 233.7 kg. If the parts on it cover 2.38 m², the net weight is 2.38 × 78.5 = 186.8 kg and the scrap is 46.9 kg, 20 % of the sheet. If 1.0 × 0.5 m of that leftover comes off as one reusable offcut (39.3 kg), only 7.6 kg is real scrap.

Where the waste comes from

• A stock length that does not suit the part lengths.
• A sheet size that does not suit the part sizes.
• The kerf of every cut and the trimmed edges.
• Cutting drawing by drawing instead of the whole list at once.
• Parts that are not allowed to rotate when they could.
• Offcuts thrown away instead of used.

How to reduce it

1. Optimize the whole package at once, not drawing by drawing.
2. Compare the stock your supplier really has: 6 m or 12 m bars, 1220 × 2440 or 1500 × 3000 mm sheets.
3. Allow rotation where the rolling direction does not matter.
4. Keep offcuts and cut the next job from them first.
5. Use the part length, not the member length (how to check).
6. Enter the real kerf of your saw or torch.
7. Keep the leftover as one rectangle, so it is worth storing.

Related: Plate nesting software · Beam cutting list optimizer · Steel plate weight chart