How to use the stair calculator
- Choose US (inches) or metric (centimeters).
- Enter the total rise: the height from the finished floor at the bottom to the finished floor at the top. Measure after flooring is chosen, since a thick floor changes the top step.
- Enter your preferred riser height. The calculator treats it as a maximum and divides the rise into equal risers no taller than this.
- Enter the tread depth, measured from riser face to riser face (not including the nosing).
- Optionally enter a total run limit if the stair must fit in a set length of floor, and the headroom you have over the stair.
How stairs are calculated
Every riser must be the same height, so the rise is split into equal parts. A straight stair has one tread fewer than risers because the top "tread" is the upper floor. The stringer figure is the straight-line distance from the bottom floor to the top landing, which tells you the board length to look for; buy boards at least a foot longer for the plumb and level cuts. The comfort rule, often credited to the French architect Blondel, says that two risers plus one tread should equal a normal walking stride.
Worked example
The defaults are a total rise of 108 inches (9 ft), a preferred riser of 7.5 in and treads 10.5 in deep:
- Risers: 108 ÷ 7.5 = 14.4, rounded up to 15
- Riser height: 108 ÷ 15 = 7.2 in, about 7 3/16 in
- Treads: 15 − 1 = 14, so the total run is 14 × 10.5 = 147 in (12 ft 3 in)
- Stringer: √(108² + 147²) = √33,273 = 182.4 in (15 ft 2.4 in)
- Angle: arctan(7.2 ÷ 10.5) = 34.4°
- Comfort: 2 × 7.2 + 10.5 = 24.9 in, inside the 24 to 25 in range
If you allow risers up to 7.75 in, 108 ÷ 7.75 = 13.9 rounds up to 14 risers of 7.71 in, which saves one tread and 10.5 in of run. In metric, a 270 cm rise with 18 cm risers gives exactly 15 risers and 14 treads of 26 cm, a run of 364 cm.
US residential code limits
| Rule (IRC R311.7) | Limit |
|---|---|
| Maximum riser height | 7¾ in (196 mm) |
| Minimum tread depth | 10 in (254 mm) |
| Minimum headroom | 6 ft 8 in (2,032 mm) |
| Minimum stair width | 36 in (914 mm) above the handrail height |
| Largest riser minus smallest riser in a flight | ⅜ in (9.5 mm) |
| Maximum rise between floors or landings | 151 in (3,835 mm) |
These figures come from section R311.7 of the 2021 International Residential Code, which covers one- and two-family homes in most US states. Treads under 11 in deep also need a nosing of ¾ to 1¼ in. Your city or county may use a different edition or add its own amendments, and commercial buildings follow the stricter International Building Code. Treat the checks here as a planning aid and confirm the design with your local building department before you cut stringers.
Frequently asked questions
What is a comfortable stair riser and tread?
Most people find risers of about 7 to 7½ in (18 to 19 cm) with treads of 10½ to 11 in (27 to 28 cm) comfortable. The 2R + T rule is a quick check: 2 × 7.2 + 10.5 = 24.9 in falls in the comfortable band. Steeper stairs save floor space but are harder to climb, especially for children and older adults.
Why does the calculator round the number of risers up?
Rounding up keeps every riser at or below the height you asked for. Rounding down would make each riser taller than your preferred maximum, which may break the 7¾ in limit. The actual riser is then the total rise divided equally among the risers.
How long a board do I need for a stringer?
Start with the stringer length shown, then add about 12 in (30 cm) for the cuts at the top and bottom. For the default stair that's roughly 15 ft 2 in plus a foot, so a 16 ft 2×12 is the usual choice.
Does the total rise include the flooring?
It should. Measure from the finished floor at the bottom to the finished floor at the top. If the flooring isn't down yet, add its thickness at the top and bottom, or the first and last risers will end up a different height from the rest.
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