Stair Calculator
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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.
Frequently Asked Questions
How many stairs do I need for my floor height?
Divide the total rise by 7.75″ (the residential max riser) and round up — that's your riser count; steps you actually walk on are one fewer. A 106″ rise needs 14 risers (13 treads). Measure the real finished-floor-to-finished-floor height, not just the ceiling height.
What is the 7-11 rule for stairs?
The commercial (IBC) standard: risers no taller than 7″, treads no shallower than 11″. Residential codes (IRC) allow steeper stairs — 7¾″ risers on 10″ treads — because homes get more layout latitude than public buildings.
What is the 17-inch rule?
A comfort guideline: one riser plus one tread should total about 17–18″. It's one of a family of stride-based formulas — the better-known one is 2 × riser + tread = 24–25″. This calculator reports the 2R+T score so you can tune tread depth toward comfort.
What's the maximum stair riser height?
7¾″ for residential stairs under the widely adopted IRC; 7″ for commercial under the IBC. Codes also cap the variation between risers in one flight at ⅜″ — equal risers matter as much as short ones, which is why the calculator divides the rise evenly.
How long a board do I need for the stringer?
The stringer length is the diagonal: √(rise² + run²). A 106″ rise with a 130″ run needs 167.7″ (14 ft) of carry — so a 16-ft 2×12 per stringer, since you also lose length to the top and bottom cuts. Three stringers is standard for 36″-wide stairs.
Does this include landings and headroom?
No — it computes one straight flight. Codes require a landing at least as deep as the stair is wide at top and bottom, 6′8″ of headroom over every tread nosing, and flights over 12′ of rise need an intermediate landing. Those checks are on you and your local inspector.
Methodology. This calculator uses standard construction and material-estimation formulas. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Results are estimates for general use; verify critical figures independently.