Stair calculator
This lays out the geometry of a stair — spacing, count and lengths. It does not size the stringer or say whether a board will carry the load, and it does not replace the building code's riser and tread limits. Check both before you cut.
Enter the total rise between the two finished floors and, if you like, your preferred riser and tread. The tool splits the rise into equal risers, gives you the exact riser height, then works out the total run and the stringer length from the right triangle the stair forms.
Worked example
A total rise of 9 ft 1 in is 109 inches. Aiming for a 7-inch riser, 109 ÷ 7 = 15.6, which rounds to 16 risers. The exact riser is then 109 ÷ 16 = 6.81 in. Sixteen risers means 15 treads; at an 10.5-inch run that is a total run of 157.5 in. The stringer is √(109² + 157.5²) ≈ 191.5 in, at about 34.7° from horizontal.
The formula
risers = round(total rise ÷ target riser) · stringer = √(rise² + run²)
Rounding the riser count and dividing back is what keeps every riser identical — the thing your feet notice on a stair. The stringer length is pure Pythagoras: the rise and the run are the two legs, the stringer is the hypotenuse.
Questions
- How do I work out the number of steps?
- Divide the total rise — floor surface to floor surface — by your target riser height, then round to a whole number. That whole number is your riser count. Dividing the total rise back by it gives the exact riser height, which is almost never a round figure, and that is fine: every riser being equal matters far more than the height being tidy.
- What is a comfortable riser and tread?
- A riser around 7 inches (about 178 mm) with a tread run around 10 to 11 inches (255 to 280 mm) walks comfortably for most people. Those are the defaults here, and you can change them. Local building rules cap the maximum riser and set a minimum tread, so check your authority — this tool lays the stair out, it does not certify it.
- What does this tool tell me — and what does it not?
- It gives you the riser count, the exact riser height, the tread count, the total run and the stringer length, all from your rise and your target dimensions. It does not size the stringer stock or tell you whether a board will carry the load. That is a structural question for your plans or an engineer, and it is deliberately out of scope.
- How long does the stringer need to be?
- The stringer is the hypotenuse of a right triangle whose legs are the total rise and the total run, so its length is the square root of rise squared plus run squared. That is the diagonal length of the cut line — add material at the top and bottom for the connections and the plumb cuts.
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