How many sheets do I need?

Area arithmetic assumes parts can be poured into a sheet like water. They can’t. Three things break it:

1. Parts don’t tessellate. Two 1400×700 desk tops on a 2440×1220 sheet need 1404mm of width side by side and only 1220mm is there, so each top uses most of a sheet on its own. In the calculator above, a small desk job has an area sum of 2 sheets and really needs 3.
2. Every cut removes material. The saw blade or cutter takes its width out of every cut. On a row of parts that is a few millimetres each time, and a few millimetres is sometimes the difference between four parts in a row and three. The kerf guide has a worked example.
3. Grain and finish limit rotation. Veneered board, woodgrain melamine and brushed stainless can’t be turned 90° to fill a gap, so fewer arrangements are possible.

A “10% for waste” allowance is a guess on top of a guess: too little on a job of big parts, too much on a job of small ones.

1. List every part with its finished size and quantity, and note which ones must keep the grain direction.
2. Use the sheet sizes you can actually buy, and the kerf of the tool that will cut them.
3. Nest the parts — by hand on paper for a few, or with a cut list optimiser for anything more.
4. Count the sheets, then check the leftover: a big offcut is stock for the next job, not waste.

That is what the calculator above does with CutNest’s engine. The full app adds several sheet sizes with prices (it picks the cheapest mix), grain per part, cut sheets for the saw, and a quote.