Stair Calculator
Stair layout from the floor-to-floor rise: risers and treads, exact riser height, total run, stringer length and angle, with the IRC limits flagged. How it works
Use the Stair Calculator
About the Stair Calculator
A 108 in floor-to-floor rise with a 7 in target riser takes 15 risers of 7.200 in and 14 treads; with 10 in treads the total run is 140 in, the stringer about 176.8 in and the angle 37.6°. The stair calculator divides the rise by the target riser and rounds to the nearest whole number, so every riser is the same height. It warns when a riser is above 7 3/4 in or a tread is under 10 in, the limits in the IRC model code, and lists the layouts with one riser fewer and one more. Optional nosing, headroom and floor thickness give the tread board depth and the length of the stairwell opening.
Stair layout for common floor-to-floor heights
| Total rise | Risers | Exact riser | Treads | Total run | Stringer | Angle |
|---|---|---|---|---|---|---|
| 36 in | 5 | 7.200 in | 4 | 40 in | 53.8 in | 42° |
| 48 in | 7 | 6.857 in | 6 | 60 in | 76.8 in | 38.7° |
| 96 in | 14 | 6.857 in | 13 | 130 in | 161.6 in | 36.4° |
| 100 in | 14 | 7.143 in | 13 | 130 in | 164 in | 37.6° |
| 104 in | 15 | 6.933 in | 14 | 140 in | 174.4 in | 36.6° |
| 108 in | 15 | 7.200 in | 14 | 140 in | 176.8 in | 37.6° |
| 112 in | 16 | 7.000 in | 15 | 150 in | 187.2 in | 36.7° |
| 120 in | 17 | 7.059 in | 16 | 160 in | 200 in | 36.9° |
| 126 in | 18 | 7.000 in | 17 | 170 in | 211.6 in | 36.5° |
Target riser 7 in and tread depth 10 in, the calculator’s defaults. No riser in this table is above 7 3/4 in. The stringer is the straight-line length from floor to floor, before nosing and notches.
IRC R311.7.5 riser and tread limits (2018 model code)
| Item | Limit |
|---|---|
| Riser height | 7 3/4 in (196 mm) maximum |
| Riser variation in a flight | 3/8 in (9.5 mm) between the tallest and the shortest riser |
| Tread depth | 10 in (254 mm) minimum |
| Tread variation in a flight | 3/8 in (9.5 mm) between the deepest and the shallowest tread |
Section R311.7.5 of the 2018 International Residential Code, as reproduced in the Town of Timnath 2018 Residential Stair Guide. Local codes differ: the edition your building department has adopted and its amendments decide what applies.
Frequently asked questions
- How many stairs do I need for a 108 inch rise?
- 15 risers of 7.200 in with a 7 in target, and 14 treads, because the floor above is the last tread. At 10 in per tread that is 140 in of run. A 36 in rise takes 5 risers of 7.200 in and 4 treads; with 10 in treads the run is 40 in and the stringer about 53.8 in.
- How long does the stairwell opening need to be?
- Long enough to keep your headroom under its far edge. The calculator measures it along the flight from the edge of the upper floor: (headroom + floor thickness) × tread run ÷ exact riser. A 108 in rise with 7.200 in risers and 10 in treads, 80 in of headroom and a 10 in floor needs a 125 in opening. It uses the headroom you type; check what your local code requires.
- How do you calculate stair rise and run?
- Measure the total rise from finished floor to finished floor, divide by the riser height you want and round to the nearest whole number: that is the number of risers, so the exact riser can be a little above or below your target. The exact riser is the rise divided by that number, the treads are one fewer than the risers, and the run is treads × tread depth. 100 in gives 14 risers of 7.143 in, 13 treads and 130 in of run.
- What is the maximum riser height for stairs?
- 7 3/4 in in the IRC model code (R311.7.5.1), with at most 3/8 in between the tallest and shortest riser in a flight, and a tread of at least 10 in (R311.7.5.2). The calculator warns when it goes past them: a 9 in target on a 108 in rise gives 12 risers of 9.000 in. Local codes differ, and the calculator does not say a stair is approved.
- How long is a stair stringer?
- The straight-line length is √(rise² + run²): 108 in of rise and 140 in of run make about 176.8 in. Nosing, the cuts at the top and bottom and the notches are not included, so the board you cut it from has to be longer.
- What changes with a deeper tread?
- Only the run, the stringer and the angle. A 7.5 in target with 11 in treads on a 108 in rise gives 14 risers of 7.714 in, 13 treads, 143 in of run, a 179.2 in stringer and an angle of 37.1°.
Related Tools
View all toolsRoof Pitch Angle Calculator
Roof pitch from rise and run or an angle: degrees, x:12 pitch and slope percent, with a slope class. A 6-in rise over a 12-in run is 26.57°.
Slope Calculator
Enter two points for the slope (also as a fraction), y-intercept, distance, angle and line equation, or find a missing coordinate from a point and a slope.
Sales Tax Calculator
Add sales tax to a price to get the total, or remove it from a total to find the price before tax. Enter the amount and your rate; tax is rounded to the cent.
Percentage Calculator
Percentage calculator for seven questions: X% of Y, X is what % of Y, percent change, percent difference, X is Y% of what, and a percent increase or decrease.
Cubic Yard Calculator
Cubic yard calculator for rectangles, circles, triangles, cones, pyramids, hemispheres and tubes, in feet, inches, yards, meters or centimeters.
Matrix Calculator
Add, subtract and multiply matrices up to 10 × 10, or get the determinant, inverse, transpose, rank and row-reduced form. Answers are exact fractions.