Stair Calculator
Even risers and a stringer length that fits.
Stair Calculator
Risers = round(total rise / target riser); Treads = risers − 1; Stringer = √(rise² + run²)
Calculated instantly
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What This Calculator Does
The Stair Calculator works out the number of risers, the actual riser height, the number of treads, the total run, and the stringer length for a straight stair. It starts from the total floor-to-floor rise and a target riser height.
How It Works
Divide the total rise by the target riser height and round to the nearest whole number of risers. Dividing the total rise by that count gives the actual riser height, which must be identical for every step. There is always one fewer tread than riser, because the upper floor is the final step. Stringer length is the hypotenuse of the total rise and total run.
Example
A 108 inch floor-to-floor rise with a 7.5 inch target gives 14.4, which rounds to 14 risers. The actual riser is 108 / 14 = 7.71 inches. With 10 inch treads there are 13 treads, a 130 inch run, and a stringer length of about 169 inches.
Formula
Risers = round(total rise / target riser)Riser = total rise / number of risersTreads = risers − 1; Run = treads × tread depthStringer = √(total rise² + total run²)Reading the Results
Number of risers
The count of vertical steps, including the final step onto the upper floor. It is rounded to the nearest whole riser.
Actual riser height
The exact height each riser must be. Build to this figure, not the target, so the top and bottom steps match.
Stringer length
The straight-line length of the angled board that carries the treads, before allowing for plumb and level cuts.
Sources and References
International Residential Code (IRC) R311.7 — stairways
Maximum riser 7.75 in; minimum tread 10 in; minimum width 36 in
IRC R311.7 sets the prescriptive limits for residential stairs. Local amendments can be stricter.
OSHA 29 CFR 1926.1052 — stairways
Workplace stair rise and tread limits
Construction-site stairs have separate temporary-stair rules from permanent residential stairs.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How do I calculate the number of stairs?
Measure the total floor-to-floor rise and divide by your target riser height, then round to the nearest whole number. A 108 inch rise with a 7.5 inch target gives 14.4, so 14 risers.
Why are there fewer treads than risers?
The upper floor is the last step, so it does not need a tread. A stair with 14 risers has 13 treads and lands on the floor at the top.
What is the maximum riser height for a residential stair?
The IRC caps risers at 7.75 inches and requires treads at least 10 inches deep for most residential stairs. Local codes may differ, and outdoor or utility stairs can have separate rules.
How long should a stair stringer be?
It is the hypotenuse of the total rise and total run: √(rise² + run²). For a 108 inch rise and 130 inch run that is about 169 inches, before cut allowances.