Stair Calculator
Enter your floor-to-floor rise and the calculator divides it into equal risers, then returns treads, total run, stringer length and whether the result passes residential code.
Unit
Total rise is finished floor to finished floor, not floor to ceiling — it includes the upper floor structure.
Every riser in a flight must be within 3/8 in (1 cm) of every other one, which is why the calculator divides the rise into equal risers rather than using your target exactly. Figures follow IRC R311.7 for residential stairs; your local code, and commercial or public stairs, may differ — check before cutting a stringer.
Frequently asked questions
How many stairs do I need for a given height?
Divide the total floor-to-floor rise by a comfortable riser height of about 7 inches, then round to the nearest whole number — that is your riser count. Divide the rise by that count again to get the actual riser height, because every riser in a flight has to be identical. A 9 ft rise gives 15 risers at 7.2 inches each.
What is the standard riser height and tread depth?
US residential code (IRC R311.7) caps risers at 7¾ inches and requires treads of at least 10 inches. Most comfortable stairs land near a 7 inch riser with an 11 inch tread. Commercial and public stairs are governed by different limits, usually shallower risers and deeper treads.
Why is there one fewer tread than riser?
The top riser lands on the upper floor itself, so the floor acts as the final tread. A flight with 15 risers has 14 treads. Counting a tread for the top riser is the single most common error in a first stair layout.
What is the 2R + T rule?
Two riser heights plus one tread depth should total between 24 and 25 inches. It is a comfort rule derived from an average stride: stairs outside that range feel either cramped or stretched even when they pass code. The calculator checks it for you.
How long does the stringer need to be?
The stringer is the hypotenuse of the total rise and total run, so it is the square root of rise squared plus run squared. Buy a board longer than that figure — the cuts at the top and bottom eat into the length, and stock lumber comes in fixed sizes.
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