Stair Calculator
The riser count is the total rise divided by the target rise, rounded to the nearest whole number, and at least one. Actual rise is the total rise divided by that count, so the steps are equal. Treads are one fewer than the risers. The run is the treads times the tread depth. The stringer is the straight-line hypotenuse of the total rise and the total run. The warning lines mark a rise over 7¾ in or a tread under 10 in. Those are limits commonly quoted from IRC R311.7.5. This page has not opened the IRC PDF. Confirm the edition your building department enforces. The warning is not a compliance certificate.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Risers = round(total rise ÷ target rise), minimum 1. Actual rise = total rise ÷ risers. Treads = risers − 1. Run = treads × tread depth. Stringer = hypotenuse of total rise and run. Rise warning = 1 when actual rise is over 7¾ in. Tread warning = 1 when tread depth is under 254 mm.
Worked example
A 105 in total rise with a 7.5 in target gives 14 risers and an actual rise of 7.5 in. The tread starts at 10 in. The run is 10.83 ft and the stringer is 13.91 ft.
A straight flight from deck to yard
A straight flight from a deck to the yard is a total rise, a target rise, and a tread. Enter the finished floor to finished floor height and a 7.5 inch target if that is the rise you want to try. The page splits the rise into equal risers and reports the stringer length. It does not draw winders, and it does not approve the stair. Headroom and a landing are still yours to check.
Measuring a straight stair
Stair maths starts from the finished floors, not from the joists you can see today. Total rise is that floor-to-floor distance. The target rise is a wish. The count is the total divided by the wish, rounded to the nearest whole riser, and then the actual rise is the total divided by the count so every riser matches. 2,600 divided by 180 is 14.444, which rounds to 14. 2,600 divided by 14 is 185.714 mm, shown as 185.71. That is under 7¾ in, so the rise warning stays 0. If you aim at a taller riser and the equal split goes over 7¾ in, the warning becomes 1. Change the target or the design. Do not hide the flag.
How total rise becomes risers
Treads are one fewer than risers because the top riser lands on the upper floor and does not need a tread in the run. Fourteen risers give thirteen treads. At 254 mm each, the run is 3.302 m. At 250 mm the run is 3.25 m and the tread warning is 1, because 250 is under 254. The stringer length is the hypotenuse, sqrt(rise² + run²). For a 2.6 m rise and a 3.25 m run that is 4.16 m. It is the theoretical line from the bottom nosing line to the top. It is not the timber you buy, which needs a cut at each end and any nosing the detail adds. Measure the stock after you have drawn the stringer.
Ordering stringers from the run
The 7¾ in and 254 mm checks are the millimetre form of 7¾ inches and 10 inches, limits commonly quoted from IRC R311.7.5. The IRC PDF was not opened for this page. Building departments adopt editions, and they amend them. A private stair, a deck stair, and a commercial stair are not the same rule set. A result of 0 means this comparison did not trip. It does not mean the stair is legal. Headroom, width, handrails, landings, and uniformity tolerances are not in the formula. If the actual rises are not equal because the floors are out, the formula's equal split is the thing to fix on site, not a reason to ignore the last riser.
Checking a stair layout
Carpet, paint, and a handrail are other calculators. Concrete steps that are solid blocks belong on the concrete calculator's step shape, which multiplies count by tread by width by rise and does not check these limits. Use both pages if you are pouring steps and also checking the going. There is no cost per step here.
Stair fields and starting values
Total rise starts at 105 in. Finished floor to finished floor in inches, or millimetres on the metric toggle. Target rise starts at 7.5 in. The rise you would like before the equal split in inches, or millimetres on the metric toggle. Tread depth starts at 10 in. Tread depth. 10 in is the commonly quoted IRC minimum in inches, or millimetres on the metric toggle.
Stair risers for a 7.5 in target rise
Total rise is finished floor to finished floor. The target rise is 7.5 in and the tread is 10 in. The stringer is the straight-line hypotenuse.
| Total rise | Risers | Actual rise | Stringer |
|---|---|---|---|
| 96 in | 13 | 188 mm | 3.9 m |
| 105 in | 14 | 191 mm | 4.2 m |
| 108 in | 14 | 196 mm | 4.3 m |
| 120 in | 16 | 191 mm | 4.9 m |
A result inside the commonly quoted 7.75 in and 10 in limits is not a permit.
Common mistakes
- Treating a 0 warning as a permit.
- Using the target rise as the built rise. The actual rise is the equal split.
- Counting treads equal to risers. There is one fewer tread.
- Entering rise in feet, or metres on the metric toggle in a millimetre field.
- Reading the stringer as a cut length with waste and nosings added. It is the hypotenuse only.
- Ignoring a local rule that is stricter than the commonly quoted IRC figures.
Frequently asked questions
How many risers do I need?
Divide the total rise by the target rise and round to the nearest count. 2,600 mm at a 180 mm target is 14 risers.
What is the stringer length?
The straight hypotenuse of the total rise and the total run. It does not add a nosing or a waste cut.
What do the warning lines mean?
1 means the actual rise is over 7¾ in or the tread is under 254 mm. Those figures are commonly quoted from IRC R311.7.5. Confirm the code edition locally. This is not a certificate.
Why is 7¾ inches 7¾ in?
7.75 inches times 25.4 mm is 7¾ in. Ten inches is 254 mm. The page stores the millimetre form of those commonly quoted limits.
Does this price the stairs?
No.
Can I carpet the stairs?
The carpet calculator is a floor and a simple stair allowance is described there only if you measure the treads and risers yourself. This page gives you the count and the run.