Calculators/Stair stringer
Total rise in, equal risers out — with tread count, total run, stringer length and the 2R+T comfort check, against whatever riser and tread limits your job is working to.
Finished surfaces, not subfloor. Flooring thickness at either end changes the answer.
Defaults are the residential limits most commonly cited in the US — a 7¾″ maximum riser and a 10″ minimum tread. Confirm against your local code; jurisdictions amend them, and commercial work is governed differently.
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Check the result against the job. This does the arithmetic from the values you enter. Building codes, span tables and material specifications vary by jurisdiction and by product, and the governing document is your approved drawings and your local code. Nothing you type here leaves your browser: the calculation runs on your device and we never receive it.
How this is checked. Every formula on this site runs from one shared calculation engine, and every one has a test fixture whose expected answer was worked out from the geometry or taken from a figure the trade already publishes — never from what the code returns. Some land exactly, which is the point: a 6/12 hip over a 144″ run is exactly 216″, a 45° crown in a square corner bevels at exactly 30°, and a 30° conduit bend has a multiplier of exactly 2. Where a number is a convention rather than arithmetic — a code limit, a waste allowance, bundles per square — it is an input with a stated default rather than a hidden constant. Found something wrong? Tell us at [email protected] and we will fix it and say so.
The one thing a stair cannot do is have an odd step. Every riser in a flight has to be the same height, because people climb by rhythm and a single different step is what catches a toe.
So the count comes first and the height falls out of it. Divide the total rise by your maximum riser and round up — rounding down would put every riser over the limit. Then divide the total rise by that whole number of risers, and every step is identical by construction.
A 109 inch rise against a 7¾ inch maximum gives 14.06, which rounds up to 15 risers at 7¼ inches each. Fourteen treads at 10 inches is 140 inches of run, and the stringer's line length is the diagonal of 109 and 140 — about 177½ inches.
2R + T — twice the riser plus the tread — should land near 24 to 25 inches for a stair that feels natural. It encodes something real: a taller riser needs a shallower tread, because the stride shortens as it steepens. It is a comfort rule, not a code requirement, but a stair that fails it will feel wrong to everyone who uses it and nobody will be able to say why.
For marking the stringer itself once you have these numbers — including dropping the stringer by one tread thickness so the first and last risers come out equal — see how to lay out stair stringers.
Divide the total rise by the maximum riser height your code allows and round up to the next whole number. That gives the riser count. Then divide the total rise by that count to get the actual riser height, which will be equal for every step. Rounding up is what keeps you under the limit; rounding down puts you over it.
Because people climb stairs by rhythm rather than by looking. A riser that differs from the others is a trip hazard, and codes limit the variation between the largest and smallest riser in a flight to a small tolerance. It is also why you never absorb a leftover measurement into the top or bottom step.
One fewer tread than risers, in the usual case where the top riser lands level with the upper floor and the floor itself acts as the final tread. Fifteen risers means fourteen treads, and the total run is fourteen tread depths.
A rule of thumb for whether a stair will feel comfortable: twice the riser height plus the tread depth should land near 24 to 25 inches. It captures the fact that a taller riser needs a shallower tread to keep the stride natural. It is a comfort check, not a code requirement.