Free stair calculator
Enter your floor-to-floor rise, target riser height, and tread depth — this stair calculator finds the step count, actual riser height, total run, and stringer length with an IRC code check, updated live, as you type.
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IRC R311.7.5: max riser 7.75 in, min tread 10 in. Consult local codes before building.
Results are estimates. Consult a professional.
How the stair calculator works
A staircase is defined by two key dimensions: rise (the vertical height of each step) and run (the horizontal depth of each tread). The calculator divides the total floor-to-floor height by a target riser height to find the number of steps, then adjusts each riser so all steps divide the height evenly. From there it calculates the total horizontal run and the stringer length — the diagonal board measurement you need to cut your framing lumber.
Worked example: basement stair at 9 ft floor height
A homeowner is framing a new basement stair with a 9 ft (108 in) floor-to-floor height. He targets a comfortable riser near 7.5 inches and plans 10.5-inch tread depth — a standard 2×12 with a 1.5 in nosing overhang. He needs the step count, actual riser height, total run, and stringer length to cut his 2×12 stringer stock.
Stair configurations for common floor heights
These configurations use a 7.5-inch target riser and a 10.5-inch tread depth — the standard for comfortable residential interior stairs. All actual riser heights stay within the IRC R311.7.5 maximum of 7¾ inches. Allow for landing space at the top and bottom beyond the total run.
| Floor height | Risers | Riser height | Treads | Total run (10.5 in treads) |
|---|---|---|---|---|
| 8 ft (96 in) | 13 | 7.38 in | 12 | 10 ft 6 in |
| 9 ft (108 in) | 14 | 7.71 in | 13 | 11 ft 4½ in |
| 10 ft (120 in) | 16 | 7.50 in | 15 | 13 ft 1½ in |
Target riser: 7.5 in; tread depth: 10.5 in. Source: 2021 IRC R311.7.5; standard residential stair geometry.
Tips for planning and building stairs
Stair geometry is unforgiving — a riser that is even ⅜ inch off uniform height is a trip hazard and a code violation. These five points cover the planning decisions that matter most before a single cut is made.
- Measure total rise from finished floor to finished floor — not subfloor to subfloor. Flooring thickness at top and bottom changes your actual rise, and an error of even ½ inch shifts every riser in the flight. Measure twice before cutting stringers.
- All risers in a flight must be within ⅜ in of each other — IRC R311.7.5.1 limits riser height variance to ⅜ inch within a single flight. This is why the actual riser must be computed by dividing total rise evenly across all steps rather than rounding each one individually.
- Use the rise + run comfort check — add your riser height to your tread depth. A sum of 17–19 inches produces a natural, comfortable stride. Below 17 feels steep and choppy; above 19 feels overly stretched.
- Cut stringers from 2×12 lumber — notches for risers and treads must leave at least 3.5 inches of net depth remaining in the stringer after cutting (IRC R311.7.10). A 2×12 (11.25 in actual width) is the standard residential choice; two or three stringers support a standard 36-inch-wide stair.
- Confirm 6 ft 8 in headroom before framing the rough opening — measured vertically from the stair nosing to any obstruction above. This is especially critical for basement stairs tucked under a floor beam or duct; plan the opening first, then verify headroom clears before the concrete is poured or framing is complete.
Accuracy and limitations
The stair geometry calculations are mathematically exact given the inputs. The critical real-world variable is total rise: it must reflect finished floor surface to finished floor surface, because flooring thickness at both levels directly affects every riser in the flight. An error of half an inch in total rise changes the actual riser height and can push a borderline design over the IRC uniformity limit or maximum riser height.
This calculator covers straight single-flight stairs only. Winding stairs, quarter-turn landings, curved stairs, and alternating-tread devices involve additional code requirements and geometric calculations beyond the scope of this tool. Always verify your final stair design against your local jurisdiction's adopted building code — some states and municipalities amend or exceed the IRC — and obtain the required building permit before construction begins.
Stair terms defined
About this stair calculator
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