How to use the stair calculator
- Measure the total rise from finished floor to finished floor. For a deck, measure from the top of the decking to the grade or landing where the stair will land.
- Keep the max riser at 7 3/4 in unless your local code is stricter. The calculator never makes a riser taller than this number.
- Enter the tread depth, measured nosing to nosing. This is the run of each step, not the width of the tread board.
- Add the nosing, headroom and stair width to check them against the code. Leave a field blank to skip that check.
Stair formulas
Risers = total rise ÷ max riser, rounded up Riser height = total rise ÷ risers Treads = risers − 1 Total run = treads × tread depth Stringer length = √(total rise² + total run²) Stair angle = arctan(riser height ÷ tread depth) Tread board = tread depth + nosing
The number of treads is one less than the number of risers because the upper floor serves as the top step. The stringer length is the diagonal along the nosing line. The board needs a few extra inches for the plumb cut at the top and the level cut at the bottom, so lay the cuts out on the board before you buy if the diagonal is close to a standard length.
Worked example
A basement stair with a total rise of 9 ft (108 in), a 7 3/4 in max riser, 10 in treads and a 1 in nosing.
- Risers: 108 ÷ 7.75 = 13.94, rounded up to 14 risers.
- Riser height: 108 ÷ 14 = 7.714 in, just under 7 3/4 in.
- Treads: 14 − 1 = 13 treads.
- Total run: 13 × 10 = 130 in, or 10 ft 10 in.
- Stringer length: √(108² + 130²) = 169.0 in, or 14 ft 1 in. Buy a 16 ft 2x12.
- Stair angle: arctan(7.714 ÷ 10) = 37.6°.
- Tread board: 10 + 1 = 11 in deep.
Every code check passes. The comfort rule gives 2 × 7.714 + 10 = 25.4 in, a little steep. A 7.2 in max riser adds one riser: 108 ÷ 15 = 7.2 in, and 2 × 7.2 + 10 = 24.4 in is in the comfortable range. The extra tread adds 10 in of run.
IRC stair code limits
The checks follow IRC section R311.7 for one- and two-family dwellings, as written in the 2018 and 2021 editions.
| Item | IRC limit | Section |
|---|---|---|
| Riser height | 7 3/4 in max | R311.7.5.1 |
| Tread depth, nosing to nosing | 10 in min | R311.7.5.2 |
| Variation in risers or treads in a flight | 3/8 in max | R311.7.5.1, R311.7.5.2 |
| Nosing projection | 3/4 to 1 1/4 in, not required with treads of 11 in or more | R311.7.5.3 |
| Headroom | 6 ft 8 in min | R311.7.2 |
| Width | 36 in min | R311.7.1 |
| Rise of one flight | 151 in max between floors or landings | R311.7.3 |
| Handrail | Required with 4 or more risers, 34 to 38 in high | R311.7.8 |
Your city or county may adopt a different edition or amend these numbers, so confirm with the local building department. Commercial stairs under the IBC are stricter: risers of 4 to 7 in and treads of at least 11 in. The comfort rule (2 risers + 1 tread at about 24 to 25 in) is guidance, not code.
Deck stairs
Choose Deck stairs above and enter the rise from the top of the decking to the grade or landing below. Deck stairs follow the same IRC R311.7 limits as indoor stairs. The American Wood Council guide DCA 6, Prescriptive Residential Wood Deck Construction Guide (2015 IRC basis), adds these requirements:
- Stringers are 2x12 at minimum.
- With cut stringers only, use at least three. Stringers are spaced no more than 18 in on center, so a 48 in stair needs four.
- Stringer spans are limited (DCA 6 Figure 28). A longer stair needs a 4x4 post on a footing under the stringer, or an intermediate landing.
- A stair taller than 12 ft in total needs an intermediate landing. Each landing is at least as wide as the stair and at least 36 in deep in the direction of travel.
- At grade, stringers bear on a footing at least 12 in below undisturbed ground or below the frost line, whichever is deeper. Do not bear them on a patio or pad that is not founded that deep.
- DCA 6 sized cut stringer spans for a 5.1 in throat, the wood left under the notches of a 2x12 at a 7 3/4 in rise and 10 in run. Larger steps leave less throat, which the calculator flags.
Tips for laying out stairs
- Measure rise to finished floors. If the flooring is not in yet, add its thickness, or the top and bottom risers will not match the rest.
- Mark risers by their running height (7.714, 15.429, 23.143 in and so on), not by stepping off a rounded 7 11/16 in. Small rounding errors add up across a flight and can break the 3/8 in variation limit.
- Measure headroom straight up from the line through the tread nosings to the lowest point above, such as a beam or soffit.
- Use the stair angle to set a framing square or stair gauge, and cut one stringer as a pattern before cutting the rest.