Stair Calculator
A comfortable stair has one non-negotiable property: every riser is exactly the same height. This stair calculator divides your total floor-to-floor rise into equal risers nearest your target height, then derives the tread depth from your available run, the stringer board length from the Pythagorean theorem, and the stair angle. It flags layouts that break the IRC limits on riser height and tread depth so you can redesign before you cut.
Floor-to-floor (or ground-to-deck) vertical height.
Horizontal space available — leave 0 to use the 11 in ideal tread.
Comfortable risers are usually 7–7.5 in.
Number of risers
14
- Riser height
- 7.5in
- Tread depth
- 11in
- Stringer lengthStraight-line cut length — buy longer stock
- 14.78ft
- Stair angle
- 36.3°
How it was calculated
- Risers = round(105.0 ÷ 7.50) = 14
- Riser height = 105.0 ÷ 14 = 7.50 in (all equal)
- Treads = 14 − 1 = 13; tread depth = 143.0 ÷ 13 = 11.00 in
- Stringer = √(105.0² + 143.0²) ÷ 12 = 14.78 ft
- Angle = arctan(105.0 ÷ 143.0) = 36.3°
How it works
The number of risers is the total rise divided by your target riser height, rounded to the nearest whole step: risers = round(total rise ÷ target riser). You then divide the total rise by that whole number, so every riser comes out identical — riser height = total rise ÷ risers.
There is always one fewer tread than risers, because the top “tread” is the upper floor or deck itself: treads = risers − 1. If you know the available horizontal run, tread depth = run ÷ treads; if not, the calculator falls back to the 11-inch ideal tread.
The stringer — the diagonal board the treads sit on — is the hypotenuse of the rise/run triangle: stringer = √(rise² + run²), converted to feet. This is the straight-line length; buy longer stock to allow for the plumb and level cuts at each end.
Stair angle = arctan(rise ÷ run). Comfortable residential stairs land around 30–37°; much steeper and the stair starts feeling like a ladder, which is exactly what the riser/tread warnings guard against.
Worked examples
8 ft rise with no fixed run
Total rise 96 in, no run given, 7 in target: risers = round(96 ÷ 7) = round(13.71) = 14. Riser height = 96 ÷ 14 = 6.86 in. Treads = 13, and with the 11 in default tread the run becomes 143 in. Stringer = √(96² + 143²) ÷ 12 = 14.35 ft, at an angle of 33.9°. No code warnings — 6.86 in risers and 11 in treads are well within limits.
105 in rise forced into too few risers
Total rise 105 in, run 132 in, 8 in target: risers = round(105 ÷ 8) = round(13.13) = 13. Riser height = 105 ÷ 13 = 8.08 in — over the 7.75 in IRC maximum, so the calculator warns you. Tread depth = 132 ÷ 12 = 11 in is fine, and the stringer is 14.06 ft at 38.5°. The fix: drop the target riser to 7.5 in for 14 comfortable risers.
How to measure
- Measure total rise from the finished lower floor to the finished upper floor — include flooring thickness on both levels, not just the subfloor.
- For deck stairs, measure from the ground (or footing top) to the top of the deck framing where the stair lands.
- Measure the available run along the floor from the stair’s starting point to any wall, door swing, or obstruction the bottom step must clear.
- Mock up the layout with a story pole: mark each riser height on a straight board and hold it in place to see the stair before you cut.
- Check headroom along the whole stair path — you need about 6 ft 8 in of clear height above every tread nosing.
- Re-measure after framing: a half-inch error in total rise spread over 14 risers is invisible, but the same error absorbed by one riser is a trip hazard.
Assumptions & limitations
Planning estimate
- All risers are assumed exactly equal — the calculator divides the total rise evenly, which is both the comfort rule and the code expectation.
- When no run is given, an 11 in tread is used as a rule of thumb for a comfortable residential stair.
- The IRC limits cited (7.75 in maximum riser, 10 in minimum tread) are the model-code reference points — your local code may differ, so verify before building.
- Stringer length is the straight-line hypotenuse; it does not include extra stock for top/bottom cuts or the thickness of tread overhang (nosing).
- Nosing projection, handrail, and landing requirements are not calculated here.
- This is a planning estimate only — not a substitute for code compliance or professional design.
Frequently asked questions
Divide your total rise by a comfortable riser height (7–7.5 in) and round to the nearest whole number. A 105 in rise at a 7.5 in target gives 14 risers. The calculator then re-divides the total rise by 14 so each riser is exactly 7.5 in — never round the riser height itself, or the last step will not match.
Muscle memory. People climb stairs without looking at their feet, expecting each step to match the last. A single riser that is even a half inch off is a trip hazard, and building codes limit the variation between the tallest and shortest riser to 3/8 in for exactly this reason.
The long-standing comfort rule is riser + tread ≈ 17–18 in: a 7.5 in riser pairs with a 10–10.5 in tread, and a 7 in riser with an 11 in tread. Codes cap the riser at 7.75 in and floor the tread at 10 in (IRC), but the most comfortable stairs sit a bit inside those limits.
The stringer is the hypotenuse of your rise and run: √(rise² + run²). For a 96 in rise and 143 in run that is about 14.35 ft — so you would buy a 16 ft 2×12. Always buy longer than the calculated straight-line length to allow for the angled cuts at the top and bottom.
The calculator flags the two headline IRC limits — 7.75 in maximum riser and 10 in minimum tread — but codes also govern nosing, handrails, guards, headroom, and landing size, and local amendments vary. Use the output as a planning layout, then verify the full stair against your local code or a professional.
Then the geometry is telling you the stair will be steep — there is no way to cheat the triangle. Options are a steeper ship’s-ladder style stair (where allowed), an alternating-tread stair, or reworking the floor plan for more run. Do not shrink treads below code minimums to make it fit.