Anyone who works steel knows that the quantity of material bought never matches the quantity that ends up in the product. Cuts, shaping, drilling: every process generates scrap. That difference is waste, and it is one of the most underestimated variables in working out what a product costs.
Why waste is not a single number
Five per cent waste on a straightforward cut of rectangular sheet has nothing in common with the waste from complex shaping full of curved cuts, where scrap can easily exceed 15-20%. Treating waste as one percentage for every material and every process is a simplification that almost always understates the real cost.
Stock lengths and yield
Another factor often ignored is the commercial format in which the material is bought. A section sold in 6-metre lengths, on a product needing 1.80-metre cuts, leaves a 0.60-metre remainder that may or may not be usable depending on how the stores are run. Working out net requirements without allowing for the stock format produces an optimistic estimate that the supplier's invoice reliably contradicts.
The basic formula
The gross quantity to buy comes from the net quantity the product needs, multiplied by (1 + the waste percentage). If a section requires 4 net metres and the waste on that process is 12%, the quantity to buy and to cost is 4.48 metres, not 4. On one job the difference looks small; across a hundred jobs a year, on margins that are already tight, it becomes a cost line you cannot ignore.
Calibrating waste against real data
The most reliable way to arrive at correct waste percentages is not to estimate them once at a desk, but to observe the real scrap at final costing, job after job, and gradually update the figure used in later quotes. A system that records actual waste and compares it against what was quoted lets you refine the estimate over time, material by material and process by process — arriving at an expected cost ever closer to the real one.