Stair Calculator
Enter the total rise and target riser to get the full layout — riser count, actual riser height, run, stringer length — with IRC compliance and comfort checks built in.
Last updated: 2026-09-22
How the calculation works
- Riser count divides total rise by the target and rounds — the actual riser is then rise ÷ count, so it lands as close to target as whole risers allow.
- Treads are one fewer than risers: the finished floor above is the last 'tread'.
- Total run multiplies treads by tread depth; the stringer is the hypotenuse of the rise/run triangle.
- Two checks run automatically: IRC compliance (max riser 7¾ in, min tread 10 in) and the 2r+t comfort rule.
Formula
risers = round(total_rise / target_riser) actual_riser = total_rise / risers treads = risers − 1 total_run = treads × tread stringer = √(total_run² + total_rise²) comfort = 2r + t
| Symbol | Meaning | Unit |
|---|---|---|
r | Riser height (vertical) | in |
t | Tread depth (horizontal, nosing to nosing) | in |
2r+t | Comfort rule — ideal 24–25.5 in | in |
Worked example
Interpreting the result
The stringer figure is the board length before cutting — a 2×12 is 11.25 in deep, which clears risers up to about 8 in; deeper stairwells or wide stairs need engineered stringers. If the actual riser misses IRC by a hair (7.76–7.9 in), try one more riser: the riser gets shorter and the run lengthens — check that the lower landing has the extra ~10 in. The comfort rule is why some stairs feel wrong despite passing code: code is the floor, 24–25.5 in is where people don't stumble.
Assumptions
- Straight run stair — L-shapes and winders change the run and stringer layout entirely.
- Tread depth measured nosing to nosing (the IRC convention).
- Uniform risers: the first step onto the bottom tread must match the rest within 3/8 in (account for finish floor thickness).
Limitations
- No headroom check — verify 6 ft 8 in clear above every nosing against the framing above.
- No landing, winder or spiral geometry.
- Handrail and guard requirements (34–38 in height) are noted but not calculated.
Layout before cutting: the stringer math that saves lumber
A 16 ft 2×12 is expensive, and one mis-marked stringer is scrap. Carpenters lay out stringers with a framing square and stair gauges set to the exact riser and tread figures — the numbers this calculator produces to three decimals. Two practical checks before cutting: first, measure the rise from finished floor to finished floor, not framing to framing — a ¾ in hardwood overlay shortens every riser by ¾ in and pushes the bottom step out of tolerance; second, remember the bottom riser is shorter by the tread thickness (the floor itself serves as the bottom tread's top surface). Getting these two right is the difference between a stair that passes inspection and a rebuild.
Frequently asked questions
What is the maximum riser height by code?
IRC R311.7 caps residential risers at 7¾ in and requires treads at least 10 in deep. The tightest permitted stair climbs about 7.75 in per step; anything steeper is a variance or a ladder.
How do I calculate the number of steps?
Divide the total rise by a target riser (7.5 in is comfortable) and round to whole risers. A 9 ft (108 in) rise needs 14 risers at 7.71 in. Treads are always one fewer — the upper floor is the final step.
What is the 2r+t rule?
Twice the riser plus the tread should land between 24 and 25.5 in — the stride geometry a human leg expects. Steep stairs (high riser) need deeper treads to compensate; shallow treads want higher risers. It's a comfort guideline, not code, but stairs outside it feel wrong within three steps.