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.
Result
Your answer appears here as you type.
Stair calculator
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.
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.
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