About the Stair Calculator
Stairs are the most regulated framing in a house for good reason — they're where geometry errors become injuries. The layout problem is always the same: a fixed floor-to-floor rise must divide into equal risers under the code maximum, and the treads that result must fit the space available. Get the riser count wrong by one and every step is off.
Enter the total rise (finished floor to finished floor — include the flooring thickness on both levels), pick residential or commercial limits, and set your tread depth. You get the riser count and exact riser height, tread count, total run, stringer length for lumber ordering, and a comfort-formula check.
Working out the landing or the floor space the stairs consume? Measure it with the Square Footage Calculator.
The Stair Math, Step by Step
Four calculations, in order — riser count first, everything else follows:
Risers = ceil( total rise ÷ max riser ) (7.75″ IRC / 7″ IBC) Riser ht = total rise ÷ risers (all equal) Treads = risers − 1 Run = treads × tread depth Stringer = √(rise² + run²)
Worked example: a 106″ total rise (typical 9′ ceiling with framing) needs ceil(106 ÷ 7.75) = 14 risers of 7.57″ each, giving 13 treads; at 10″ that's a 130″ (10′10″) run and a 167.7″ (14′) stringer. One fewer riser would mean 8.15″ steps — over code.
Riser Counts for Common Rises
IRC residential limits with 10″ treads — every row computed by this calculator:
| Total rise | Risers | Riser height | Total run (10″ treads) |
|---|---|---|---|
| 96″ (8′ floor) | 13 | 7.38″ | 120″ (10′) |
| 100″ | 13 | 7.69″ | 120″ (10′) |
| 106″ (9′ floor) | 14 | 7.57″ | 130″ (10′10″) |
| 110″ | 15 | 7.33″ | 140″ (11′8″) |
| 118″ (10′ floor) | 16 | 7.38″ | 150″ (12′6″) |
Notice 100″ and 96″ both need 13 risers — the count only changes when the rise crosses a multiple of 7.75″. That's why measuring the real rise on site beats assuming from ceiling height.
Code Limits & Comfort Rules
The commonly adopted 2021 IRC residential limits (section R311.7): risers max 7¾″, treads min 10″, no more than ⅜″ variation between the tallest and shortest riser, minimum 6′8″ headroom, and stairs at least 36″ wide. Commercial stairs under the IBC are gentler: 7″ max riser, 11″ min tread — the “7-11 rule.” Local amendments vary, so confirm with your building department before cutting stringers.
Within code, comfort formulas pick the sweet spot: 2 × riser + tread should land between 24″ and 25″ (the classic rule based on natural stride), riser + tread around 17–18″, and riser × tread near 70–75. A 7.5″ riser with a 10″ tread scores 25″ on the first rule — right at the comfortable edge; 7″/11″ scores 25″ too, which is why commercial stairs feel easy.