Three 1200s do not come out of a 3600 board
This is the whole tool, and it is the sum every other cutting list calculator gets wrong.
A saw blade removes material. A circular saw or a chop saw takes roughly 3mm a cut. So three 1200mm pieces need 3600mm of timber plus two cuts of sawdust: 3,606mm, and the third piece comes out 6mm short.
Six millimetres does not sound like much. Over a stud wall with twenty cuts in it, it is 60mm of timber that does not exist, and it is always the last piece that discovers this.
Set the kerf to zero on the tool and watch the answer change. That is what every other calculator is telling you.
Length divided by length is not the answer either
The other half of the problem, and it is bigger.
Take five pieces at 2000mm from 3600mm stock. Total timber needed is 10 metres, and 10 divided by 3.6 is 2.8, so three boards. That is what a naive calculation says and it is what most tools return.
The real answer is five. Two 2000s will never fit on one 3600 board, so every board yields exactly one piece and wastes 1600mm. The naive sum assumed a piece could span two boards, and pieces do not span boards.
The tool shows you both numbers when they differ, because the gap is the point.
The cutting plan, not just a count
A board count tells you what to buy. It does not tell you what to do at the saw, and that is where the money gets lost.
Cut in the wrong order and the long pieces end up with nowhere to come from. Wrong order means working down the list as written, or shortest first, or whatever is nearest. The list added up on paper and you are still short at the end, with a pile of offcuts too short for anything.
So this prints board by board, in cutting order: what comes off each one and what is left. Long pieces first, because they are the constraint, and the short ones fill the gaps around them.
Why longest-first is the whole algorithm
It is called first fit decreasing. Sort the pieces longest first, then put each one on the first board it fits.
Sorting is what makes it work. Long pieces define the boards; place them first and the short ones slot into what is left. Do it the other way and the short pieces scatter across every board, and nothing long fits anywhere.
It is not provably optimal: exact one-dimensional cutting stock is a genuinely hard problem and the true best answer needs an exhaustive search. First fit decreasing lands within a few per cent, instantly, and gives the same answer every time. That last part matters more than the few per cent: a cutting plan that changes when you reload the page is no use to anybody standing at a bench with a pencil.
Every stock length, ranked
UK timber comes in 300mm steps from about 1.8m to 6.0m. The tool packs your list onto each of them and ranks the results by waste.
It shows the whole table rather than only the winner, because "second best but it fits in the van" is a real reason, and so is "the merchant only had 4.8s on the rack". Longer lengths are often cheaper per metre even when the waste is worse, which is a trade-off only you can price.
Anything longer than the longest stock gets flagged rather than silently dropped: that piece needs a joint, or a length ordered in specially, and it is better to find out now.
Section, grade and span are decided before you get here
It does not choose a section size, a grade or a span. Those are design questions and they are not answered on this site. It works out how to cut the lengths you have decided on.
Next to it: what C16 and C24 actually mean, stud wall quantities which feed straight into this, and decking.
Common questions
Why can I not get three 1200mm pieces out of a 3600mm board?
Because the saw removes material. A circular saw or chop saw takes about 3mm a cut, so three 1200s need 3600 of timber plus two cuts of sawdust: 3606mm. The third piece comes out 6mm short. Over twenty cuts that is 60mm of missing timber, and it is always the last piece that finds out.
How many boards do I need?
Not what length-divided-by-length says. That assumes a piece can span two boards, which it cannot, and that the saw removes nothing, which it does not. Five 2000mm pieces from 3600mm stock: the naive sum says three boards, and the real answer is five, because two 2000s will never fit on one 3600.
What lengths does timber come in?
UK sawn and CLS comes in 300mm steps, generally from 1.8m up to 6.0m, so 2.4, 3.0, 3.6, 4.2, 4.8 and so on. Not every merchant stocks every length, and the price per metre is often better on the longer ones even if the waste is worse.
What is a kerf?
The width of the cut: the material the blade turns into sawdust. About 3mm for a circular or chop saw, less for a bandsaw, more for a rough rip. Set it to zero in the tool and you will see exactly what every other cutting list calculator is telling you.
Is the cutting plan the best possible one?
Not provably. Optimal one-dimensional cutting is a genuinely hard problem and the exact answer needs an exhaustive search. This uses first-fit decreasing, which lands within a few per cent of optimal, runs instantly, and gives the same answer every time, and that last part matters more, because a plan that changes when you reload the page is no use on a bench.
Why does it show every stock length rather than just the best?
Because "second best but it fits in the van" is a real reason, and so is "the merchant only had 4.8s". The table is ranked by waste and you pick. A calculator cannot know what is on the rack or what will go in the roof bars.