Enter the floor-to-floor rise and the run you have available, and this works out how many risers you need, the exact riser height, the going, the stair angle and the length of stringer stock to buy. It also gives you the bottom cut for a dropped stringer — the tread-thickness deduction that catches people out — plus a dimensioned side elevation and a marking-out table you can print and take to the bench. Everything runs in your browser; nothing is uploaded and it works offline.
Stair dimensions
Divides the run you have by the tread count
Notes
What is the bottom cut, and why does my first step come out too tall?
You lay a stringer out as a row of identical triangles, so every notch is the same. But the first tread sits on top of the stringer, which makes the first step taller than all the others by the thickness of the tread. The fix is to shorten the stringer at the bottom by that same amount before you cut the seat — "dropping the stringer". If the finish floor at the bottom of the flight has not been laid yet and you are standing the stringer on bare subfloor, you take off less: drop = tread thickness − finish floor thickness. That single subtraction is the most common reason a newly built flight has one step that feels wrong underfoot.
Do I use the going or the tread depth for the layout?
The going. That is the horizontal distance from one riser face to the next, and it is what you mark on the stringer with the framing square. The tread board is wider than the going because it overhangs with a nosing — the tool shows both, and the nosing does not change the stringer layout at all. It only changes the board you buy.
Which stringer length number do I order material to?
Use the "stock length per stringer" figure, not the diagonal. The diagonal is the straight line from the bottom of the first riser to the top of the last one; the actual board has to run past that at both ends to leave a seat cut at the bottom and a plumb cut at the top, and it has to be wide enough to keep the throat you specified below the deepest notch. The tool reports the minimum board width it needs — if that number comes out above about 235 mm (9¼ in) you are past what a 2×10 will do and you need a 2×12, or a laminated/engineered stringer.
What this tool does not do
It handles a single straight flight with equal risers, on a level floor. It does not do winders, landings, curved or tapered flights, and it does not calculate headroom, handrail heights, guard spacing or fixings — measure your headroom separately before you commit to a riser count. The comfort ranges it flags (riser 150–200 mm, going 250 mm and up, 2×riser + going between 600 and 640 mm) are common rules of thumb used in the trade for a flight that walks well; they are not a code check and this tool makes no claim about compliance with any building regulation in your jurisdiction. Whatever you build, check it against the rules that apply where you are, and against the inspector who will actually look at it.