Deck stair calculator
Measure the total rise from the deck surface to the finished landing, and this divides it into risers that are all the same height — which is the thing the code actually cares about. Then it gives you the run the flight needs and the stringer length to cut from.
Every riser in a flight must be within a small tolerance of every other — that is what makes the division exact rather than rounded. Check your riser and tread limits against the code your jurisdiction adopted.
Why the riser height is a decimal
The rule that governs a flight is uniformity: within one flight the greatest riser may not exceed the smallest by more than ⅜ inch, and the same holds for tread depth (§ R311.7.5). People trip on stairs when one step is different, and that is what the code is preventing.
So the arithmetic runs the other way from how people expect. You do not pick a riser height and see how many fit. You divide the total rise by the smallest number of risers that keeps each one at or under 7¾ inches, and accept whatever decimal that gives — then cut every stringer to it. Tread depth is the other half: not less than 10 inches, measured between the vertical planes of the foremost projection of adjacent treads. The flight itself wants 36 inches of clear width above the handrail height, and the handrail sits between 34 and 38 inches above the nosing line.
There is always one fewer tread than riser, because the deck surface itself is the top "tread". That off-by-one is the second commonest stair mistake.
Measuring the total rise properly
From the finished deck surface to the finished landing surface — not to the joist, and not to bare dirt where a pad or pavers are going later. Get either end wrong and every riser in the flight is wrong.
Measure with a level and a straightedge run out from the deck to a stake at the landing point, not with a tape held down a slope. Ground falls away, and the tape follows it.
The bottom riser is the one that changes. The stringer's bottom is cut short by the thickness of one tread, and the landing pad's finished height is part of the geometry. Both are how a flight ends up with one odd step at the bottom.
The run is usually the surprise
A flight needs its total run in flat, usable ground, and on a raised deck that run is longer than people picture. Check it against the yard before you commit to the stair position: a flight that lands in a flower bed, on a slope or against a fence has to move.
The stringer length is the diagonal, and it decides the stock you buy. Cut stringers are traditionally taken from 2×12, and the notches leave a surprisingly slender remaining section — the throat — which is why stringer spacing matters and why a wide flight needs more than two.
What this does not cover
Landings, winders, curved flights and anything with a change of direction. Handrail geometry and graspability, guard requirements alongside the flight, and the illumination requirement some jurisdictions apply — those are in railing height code.
Code requirement
Riser, tread and uniformity limits are in IRC 2021 R311.7 as adopted locally. Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
Sources
- 2021 IRC § R311.7, Stairways (UpCodes) read 2026-09-04
- 2021 IRC § R312.1, Guards (UpCodes) read 2026-09-04