Stair calculator (IRC) | Construction Calc HQ

Work out the number of risers, riser height, total run, and stringer length for a staircase — and see whether it meets IRC code limits.

Formula

risers = round(total_rise ÷ target); stringer = √(rise² + run²)

Method & sources

Formula basisIRC R311.7

How we calculateRisers = round(total rise ÷ 7); tread = run ÷ (risers − 1); stringer = √(rise² + run²); checked to IRC limits. Full method →

Formula verified against the published standard above. Method last reviewed June 2026. Estimates are for planning — confirm against supplier quotes.

How stairs are sized

Divide the total rise by a comfortable target riser (about 7.5″) and round to a whole number of risers; treads are one fewer than risers.

IRC code limits

The IRC sets a maximum riser of 7.75″ and a minimum tread depth of 10″. This calculator flags whether your design passes and explains why if it doesn't.

Worked example

A staircase for a 9 ft floor-to-floor height

For a total rise of 9 ft (108 inches) with a comfortable target riser of 7.5 inches: 108 ÷ 7.5 = 14.4, which rounds to 14 risers. That gives an actual riser height of 108 ÷ 14 = 7.71 inches — just under the 7.75-inch IRC maximum, so it passes.

With 14 risers you have 13 treads. At a 10.5-inch tread depth, the total run is 13 x 10.5 = 136.5 inches, about 11.4 ft of horizontal space. The stringer length — the diagonal board the steps attach to — is the square root of (108² + 136.5²) = 174 inches, or about 14.5 ft. Order stringer stock longer than this to allow for the cuts.

What affects your result

Total rise — measured floor surface to floor surface, including the upper floor thickness.
Riser height — the IRC caps this at 7.75 inches; comfort favors 7-7.5 inches.
Tread depth — the IRC requires at least 10 inches; deeper is more comfortable.
Headroom — a separate IRC requirement (6 ft 8 in minimum) this calculator doesn't check.

Tips from the trade

Keep risers uniform

The IRC allows no more than 3/8 inch variation between the tallest and shortest riser in a flight. Uneven risers are a trip hazard and a common inspection failure — divide the total rise evenly and don't fudge the top or bottom step.

Common mistakes to avoid

Measuring total rise to the subfloor instead of the finished floor surface.
Letting the riser exceed 7.75 inches to save a step — it fails inspection.
Forgetting that treads are always one fewer than risers.
Ignoring headroom, which this tool doesn't calculate but code requires.

Where the numbers come from

Limits are from IRC R311.7. The pass/fail check shows the exact reason for a failure (e.g. “riser exceeds 7.75″ max”).

Frequently asked questions

What is the maximum stair riser height?

7.75 inches under the IRC, with a minimum tread depth of 10 inches.