What the stair calculator does
Enter the total rise — the vertical distance from the lower floor to the upper floor — and a target riser height, and the calculator works out the number of steps, the exact riser height, a matching tread depth, the total run the staircase will occupy, the stringer length and the angle, with a drawing to scale. It is the arithmetic every deck, loft, basement and garden staircase starts with, and the numbers building codes check. Runs in your browser.
The calculation
Number of risers = round(total rise ÷ target riser)
Actual riser = total rise ÷ number of risers (every riser identical)
Number of treads = risers − 1 (the top floor is the last "tread")
Tread depth = chosen to satisfy 2 × riser + tread ≈ 24–25 in (610–635 mm)
Total run = treads × tread depth
Stringer length = √(total rise² + total run²)
Angle = arctan(total rise ÷ total run)
Total rise 108 in, target riser 7.5 in:
108 ÷ 7.5 = 14.4 → 14 risers → actual riser 7.71 in
13 treads at 10 in → run 130 in
stringer = √(108² + 130²) = 169 in; angle = 39.7°Comfort and code rules
| Rule | US (IRC) | UK (Part K, private) | Comfort guideline |
|---|---|---|---|
| Max riser | 7¾ in (197 mm) | 220 mm | 7–7½ in (175–190 mm) |
| Min tread (going) | 10 in (254 mm) | 220 mm | 10–11 in (250–280 mm) |
| Riser variation | ≤ ⅜ in between any two | Must be equal | Identical — a 5 mm difference trips people |
| Max pitch | — | 42° | 30–37° |
| Min width | 36 in | 800 mm typical | 900 mm+ for comfort |
| Headroom | 6 ft 8 in (2,032 mm) | 2,000 mm | — |
| Blondel's rule | — | 2R + G = 550–700 mm | 2R + G ≈ 24–25 in / 620–640 mm |
Blondel's rule (twice the riser plus the tread) matches a normal stride; stairs that violate it feel wrong even when each dimension is legal. The calculator picks a tread that satisfies it for the riser you get.
Measuring the total rise
- Measure from finished floor to finished floor — include the thickness of any flooring not yet laid on either level.
- Measure in several places; floors are rarely level. Use the rise at the stair's position.
- For a deck, measure from the deck surface to the ground where the bottom step will land, allowing for a landing pad.
- Do not round: a 3 mm error per riser across 14 risers puts the top step 42 mm out.
Cutting the stringer
Lay out the riser and tread on a framing square along the stringer board, marking each step; the calculator's riser and tread are the two numbers you set on the square. Drop the bottom of the stringer by one tread thickness so the first and last risers end up equal once treads are fitted. Check the number of risers against the drawing before cutting — an off-by-one error is the classic stair mistake, and the calculator's diagram is there to catch it.