Saw kerf and trim allowances for steel cutting lists
Every cut turns a strip of steel into swarf or slag. That strip is the kerf. Leave it out of a cutting list and the last part on a bar comes up short — usually when the bar is already on the saw.
What kerf is
Kerf is the width of material removed by the cutting tool: the thickness of the saw blade plus any set or wobble, or the width of the plasma, laser or oxy-fuel cut. It is lost on every cut, so it adds up fast. Ten parts on one bar means nine or ten cuts, and at 2 mm each that is up to 20 mm of steel gone.
Typical kerf by cutting method
These are typical ranges only. Blades wear, machines are set up differently and material thickness changes the result, so the best number is always one you have measured on your own machine.
| Cutting method | Typical kerf |
|---|---|
| Bandsaw (structural sections) | about 1–2 mm |
| Cold saw / circular saw | about 2–3 mm |
| Abrasive cut-off disc | about 2–3.5 mm |
| Plasma (CNC) | about 1.5–4 mm, wider on thick plate |
| Laser | about 0.2–0.5 mm |
| Oxy-fuel | about 2–5 mm or more on heavy plate |
| Waterjet | about 1 mm |
How to measure it: cut a short offcut, measure the bar before and after with a tape or calipers, and divide the loss by the number of cuts. Do it again when you change the blade or the consumables.
A worked example
You need four parts of 3000 mm from a 12 000 mm length with a 2 mm saw. On paper 4 × 3000 = 12 000, so it looks like a perfect fit. In the workshop it is not: separating four parts takes three cuts, which is 6 mm of kerf, so the fourth part ends up 2994 mm long — and that assumes the mill end is square and usable, which it rarely is.
The fix is to plan with the kerf and a trim allowance included. With 2 mm kerf and a 5 mm trim at the start of the bar, the usable length is 11 995 mm. Three parts of 3000 mm (9000 mm plus kerf) fit comfortably, and the remainder is about 2990 mm — a useful offcut rather than a short part.
Trim allowances
Stock lengths are not always cut square or clean at the ends. Mill ends can be out of square, damaged in transport or marked with paint and stamps. A trim allowance is the amount you take off the end before the first part, so every part is cut from clean, square steel.
- Trim start: squared off before the first part. A few millimetres is common on saw-cut stock; more on rough mill ends.
- Trim end: material kept back at the far end, for example where the bar is held in the vice or where the end is damaged.
- Plate edge trim: on sheets, a strip around the edge that is not used for parts, to clean up mill edges or allow for clamping.
Setting kerf and trims in SteelCutList
In the length optimiser, enter your saw kerf (2 mm by default) and the trim at the start and end of each bar under Settings. The optimiser adds the kerf after every part and only uses the length between the trims, so every bar in the plan can actually be cut. In the plate optimiser, set the kerf for your shear, saw or cutting table (3 mm by default) and the edge trim around each sheet.
Keep the minimum usable offcut setting realistic too: anything shorter than that value is treated as scrap in the plan, and anything longer is listed as an offcut to put back on the rack.
Quick checklist
- Measure kerf on your own machine and update it when blades or consumables change.
- Always add a start trim unless your stock is already square-cut.
- Check that the longest part plus trims fits inside the stock length.
- Treat the optimised plan as a plan: check critical lengths against the shop drawings before cutting.