Enter the total rise from floor to floor to lay out your stairs: number of risers, riser height, treads, total run, stringer length and angle, checked against the IRC’s riser and tread limits.
How to use the stair calculator
- Measure the total rise: the vertical distance from the finished floor at the bottom to the finished floor at the top. For deck stairs, measure from the deck surface to the ground or landing where the stairs will end.
- Leave the maximum riser at 7¾ in and the tread at 10 in for the IRC minimums, or change them to suit your local code or a more comfortable stair.
- Read the number of risers and the exact riser height, then the total run and stringer length.
- Optionally enter the horizontal space you have, and the calculator checks the run fits.
Stair formulas
Risers = total rise ÷ maximum riser height, rounded up Riser height = total rise ÷ risers Treads = risers − 1 (the top floor acts as the last step) Total run = treads × tread depth Stringer length = √(total rise² + total run²) Angle = arctan(riser height ÷ tread depth)
Rounding the riser count up guarantees that every riser is at or under your maximum, and dividing the rise evenly keeps every riser the same height.
Worked example: a 108 in rise (9 ft floor to floor)
- Risers: 108 ÷ 7.75 = 13.94, rounded up to 14 risers.
- Riser height: 108 ÷ 14 = 7.714 in, about 7 11/16 in.
- Treads: 14 − 1 = 13. Total run: 13 × 10 = 130 in (10′ 10″).
- Stringer: √(108² + 130²) = 169 in, or 14′ 1″.
- Angle: arctan(7.714 ÷ 10) = 37.6°.
Deck stairs with a 30 in rise need 4 risers of 7 1/2 in and 3 treads, a
2′ 6″ run and a stringer about 3′ 6 7/16″ long.
IRC stair rules at a glance
| Rule | IRC requirement |
|---|---|
| Riser height (R311.7.5.1) | Not more than 7¾ in; largest may exceed smallest by 3/8 in at most |
| Tread depth (R311.7.5.2) | Not less than 10 in; same 3/8 in variation limit |
| Nosing (R311.7.5.3) | ¾ to 1¼ in projection; not required when treads are at least 11 in deep |
| Headroom (R311.7.2) | At least 6 ft 8 in |
| Width (R311.7.1) | At least 36 in clear above the handrail height |
| Handrail (R311.7.8) | 34 to 38 in high, on at least one side of flights with four or more risers |
Summarized from the City of Boise’s IRC stair handout and the Town of Timnath’s 2018 residential stair guide. Many jurisdictions amend the IRC, so confirm with your building department before you cut stringers.
Riser counts for common rises
| Total rise | Risers | Riser height | Treads | Run at 10 in treads |
|---|---|---|---|---|
| 24 in | 4 | 6 in | 3 | 2′ 6″ |
| 30 in | 4 | 7 1/2 in | 3 | 2′ 6″ |
| 36 in | 5 | 7 3/16 in | 4 | 3′ 4″ |
| 48 in | 7 | 6 7/8 in | 6 | 5′ 0″ |
| 96 in | 13 | 7 3/8 in | 12 | 10′ 0″ |
| 102 in | 14 | 7 5/16 in | 13 | 10′ 10″ |
| 108 in | 14 | 7 11/16 in | 13 | 10′ 10″ |
| 114 in | 15 | 7 5/8 in | 14 | 11′ 8″ |
| 120 in | 16 | 7 1/2 in | 15 | 12′ 6″ |
Tips for laying out stringers
- Measure after the flooring is known. Finished-floor heights change the total rise; a thick tile or hardwood floor at the top can change a riser.
- Drop the stringer. Cut the bottom of the stringer shorter by the thickness of one tread, so the first step isn’t taller than the rest once treads go on.
- Use a framing square with stair gauges set at the riser height and tread depth, and step it along the board.
- Comfort check. A traditional rule of thumb says 2 × riser + tread of about 24 to 25 in feels natural; the calculator shows yours.
Questions
How many steps do I need for a 9 ft rise?
A 108 in rise needs 14 risers of 7 11/16 in, which means 13 treads, with the upper floor as the top step.
What is the maximum riser height?
7¾ in under IRC R311.7.5.1, with no more than 3/8 in between the tallest and shortest riser in a flight.
How long should a stair stringer be?
The square root of rise² + run². For a 108 in rise and 130 in run, about 14′ 1″, so a 16 ft board.
Why is there one less tread than risers?
The top floor or deck acts as the final step, so the last riser leads straight onto it.
Sources
- IRC Section R311.7.5: Stair Treads & Risers (Handout #437), City of Boise, Feb 2022.
Used for: Maximum riser 7 3/4 in, minimum tread 10 in, 3/8 in variation, nosing rules. - 2018 Residential Stair Guide (IRC R311.7), Town of Timnath, Colorado.
Used for: 36 in minimum width, 6 ft 8 in headroom, handrail height 34–38 in.