Stair calculator
Risers, treads, run and angle
From the finished floor below to the finished floor above.
The IRC allows risers up to 7 3/4 in (196 mm) and treads from 10 in (254 mm). A lower riser and a deeper tread make a gentler stair.
Leave empty to use the fewest risers that stay within the limit.
Checked against the 2021 International Residential Code, section R311.7. Local codes, and codes outside the US, can differ, so confirm before you build. View the source
Result
Your answer appears here as you type.
About this tool
Enter the floor-to-floor height to get the number of risers and treads, riser height, total run, stair angle and stringer length — checked against the International Residential Code and the comfort rule.
How the stair layout is worked out
The calculator divides the floor-to-floor rise by the tallest riser you allow and rounds up to get the fewest risers that fit, unless you set the number yourself. Every riser is then the same height, which matters more than the exact number: uneven steps are a common cause of falls. A straight flight has one tread fewer than it has risers.
What it checks
The checks follow the 2021 International Residential Code, section R311.7, the model code for houses in the United States. Beyond the riser and tread limits, it flags:
- a flight that rises more than 151 in (3,835 mm) without a landing;
- the clear width of at least 36 in (914 mm) and headroom of at least 80 in (2,032 mm);
- a nosing on treads shallower than 11 in (279 mm);
- a handrail, 34–38 in (864–965 mm) high, once a flight has 4 or more risers.
Commercial buildings follow different, stricter rules, and so do other countries, whose building regulations set their own riser, going and pitch limits. Outside US homes, use the layout and the comfort check, and take the limits from your local rules.
Measuring the rise
Measure from finished floor to finished floor. If the flooring is not down yet, add its thickness at the top and subtract it at the bottom, or the first or last step will come out a different height from the rest. For a stair to a sloping garden, a slope reading of the ground helps plan the run.
Frequently asked questions
How many stairs do I need?
Divide the total rise, measured from finished floor to finished floor, by the tallest riser you want, and round up. That gives the number of risers. A straight flight has one tread fewer than risers, because the upper floor is the last step.
Example: a rise of 108 in divided by 7.75 in, the tallest riser the International Residential Code allows, is 13.94, so you need 14 risers of 7.71 in and 13 treads. Enter your own rise above for the full layout in inches or millimetres.
How do you calculate stair rise and run?
The riser height is the total rise divided by the number of risers, and the total run is the number of treads multiplied by the tread depth. The International Residential Code allows risers up to 7.75 in (196 mm) and treads from 10 in (254 mm).
A stair feels comfortable when two risers plus one tread come to about 24–25 in (600–650 mm), roughly one walking stride. The stair angle is arctan(riser ÷ tread); at the code limits it is about 38°.
How do you calculate and lay out stair stringers?
The stringer follows the diagonal of the flight, so its pitch line is the square root of the total rise squared plus the total run squared. Buy longer stock than that, because the cuts at the top and bottom need extra. The calculator above gives the pitch-line length for your stair.
To lay one out, clamp stair gauges to a framing square at the riser height on one arm and the tread depth on the other. Set the square on the board with both gauges against the edge, trace the step, then slide it along to the end of that mark and repeat for each tread. Finally, cut the bottom down by the thickness of one tread so the first step is not taller than the rest.
About these measurements
Readings are estimates taken from rendered pixels and consumer device sensors. Do not rely on them for structural engineering, surveying, manufacturing inspection, navigation, or medical decisions. Check anything safety-critical with a calibrated instrument.
All processing happens in your browser. No image, camera frame or sensor reading is ever uploaded.
More calculators
Slope calculator
Rise, run, length and grade
Enter any two of rise, run, sloped length and angle to solve the rest. See the slope as degrees, percent grade, a 1:N ratio, a pitch and per mille.
Roof pitch calculator
Pitch, degrees, rafters and roof area
Convert a roof pitch between x:12, degrees and percent, and get rafter and hip factors, cut angles, rafter length and roof area — with the minimum pitch for each roof covering from the International Residential Code.
Wheelchair ramp calculator
Ramp length and landings to ADA
Enter the height to climb and get the ramp length, the number of runs and landings, and whether handrails are needed — checked against the 2010 ADA Standards.