How do I calculate stair rise and run?
Start with total rise — finished floor to finished floor, not subfloor to subfloor. Divide it by 7 to get a starting riser count, then round to the nearest whole number. Divide total rise by that count to get the actual riser height.
The riser count is fixed by the height; you do not get to choose it freely. What you choose is the tread depth, and that sets how much floor space the stair consumes.
Worked example: 105 in of total rise ÷ 7 = 15 risers exactly, each 7 in. Fifteen risers means 14 treads. At 10.5 in per tread the total run is 147 in, or 12 ft 3 in of floor. The stringer is √(105² + 147²) = 180.6 in, about 15 ft 1 in.
Note the trap: risers and treads are never equal in number. A flight always has one more riser than tread, because the top riser lands on the floor above rather than on a tread.
The code rules this checks
Maximum riser height 7¾ in and minimum tread depth 10 in, per IRC R311.7.5. Minimum stair width 36 in per R311.7.1. Minimum headroom 6 ft 8 in per R311.7.2.
Riser heights within a single flight must not vary by more than ⅜ in between the tallest and shortest. This is the most commonly failed item at inspection, and it almost always comes from measuring to the subfloor and then adding finish flooring to only one end.
Beyond code there is the Blondel rule: 2 × riser + tread should land between 24 and 25 in. A stair can pass inspection and still feel wrong, and this is the number that predicts it.
Commercial stairs under IBC are stricter — 7 in maximum rise and 11 in minimum tread. This calculator applies residential IRC limits.
Typical stair material cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| Pressure-treated 2×12 stringer stock | per linear ft | $3.50 – $6.00 |
| Pre-cut stringer (3–7 steps) | each | $25.00 – $60.00 |
| PT tread 2×12 | per 36 in tread | $12.00 – $22.00 |
| Oak stair tread (interior) | each | $35.00 – $90.00 |
| Riser board | each | $6.00 – $20.00 |
| Stringer hangers / angle brackets | per stair | $15.00 – $45.00 |
| Handrail with brackets | per linear ft | $8.00 – $30.00 |
Interior finished stairs with hardwood treads and a railing system typically run $1,500–4,000 in materials. Exterior PT deck stairs are usually $150–500.
What goes wrong on this job
- Measure total rise from finished floor to finished floor. If the upper floor is getting ¾ in hardwood and the lower is staying bare concrete, that ¾ in belongs in your total rise or every riser will be off.
- Cut one stringer, then test-fit it before cutting the others. A ⅛ in error repeated across three stringers is three wasted 2×12s.
- Account for the bottom riser losing tread thickness. When you notch a stringer, the bottom riser ends up taller than the rest by exactly the tread thickness unless you trim the stringer foot by that amount. This is the single most common stair-building mistake.
- A notched 2×12 leaves about 5 in of throat — the minimum most inspectors accept. Never notch a 2×10 for a structural stringer.
- Landings reset the count. If total rise pushes past about 12 ft, code requires an intermediate landing, and each flight is then calculated separately.
Frequently asked questions
What is the maximum stair riser height?
7¾ inches for residential stairs under the IRC. Commercial stairs under the IBC are limited to 7 inches. Riser heights within one flight cannot vary by more than ⅜ inch.
How many stairs for 8 feet?
Eight feet is 96 inches. Divided by a 7-inch target that is 13.7, so you use 14 risers at 6.86 inches each, giving 13 treads. At 10.5-inch treads the run is 136.5 inches, about 11 ft 5 in.
Why is there one more riser than tread?
The top riser lands flush with the upper floor, which acts as the final tread. So a flight of 15 risers has 14 treads.
What size lumber for stair stringers?
2×12 is the minimum for a notched stringer. After cutting the rise and run notches you need at least 5 inches of remaining material — the "throat" — and a 2×10 does not leave enough.
What is the ideal stair angle?
Between 30 and 35 degrees. Below 30 the stair eats floor space; above 37 it feels like a ladder and becomes tiring to descend.