Stair calculator
Calculate riser count, actual riser height, tread count, total run, and stringer length for IRC-compliant residential stairs.
Inputs
Stringer mount type
Deck/landing face is top tread — most common for decks
Results
Number of risers
Enter your measurements to see the result
risers
Actual riser height
—
in — IRC max 7¾"
Number of treads
—
treads
Total horizontal run
—
Stringer length
—
— buy next standard length up
Stair angle
—
Rule of thumb (R + T)
—
in — 17–18" rule of thumb
IRC code check
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riser · tread · width
⚠ Risers = ⌈Total Rise ÷ Target Riser⌉. Actual riser = Total Rise ÷ Risers (equal distribution per 2024 IRC §R318.7.5.1 ⅜" uniformity rule). Stringer = √((Rise − one riser)² + Run²) for standard mount, √(Rise² + Run²) for flush mount. 2024 IRC: max riser 7¾" and min tread 10" (R318.7.5), min width 36" (R318.7.1), max 151" rise per flight (R318.7.3). Carpenter's rule of thumb (not code): Rise + Run ≈ 17–18". A cut 2×12 stringer spans at most 6'-0" horizontally without a support post or landing (AWC DCA 6). Measure finished floor to finished floor — include flooring thickness on both ends. Always verify with your local building department before cutting stringers.
Frequently asked questions
How do I calculate stair rise and run?
Divide your total floor-to-floor height by your target riser height and round up to get the number of risers. Then back-derive the actual riser by dividing total rise by riser count — this ensures all risers are equal. Treads = risers minus 1. Total run = treads × tread depth.
What is the maximum riser height per building code?
The 2024 IRC (International Residential Code, R318.7.5) sets a maximum riser height of 7¾ inches (7.75") and a minimum tread depth of 10 inches for residential stairs. All risers in a single flight must be within ⅜ inch of each other.
How do I calculate stringer length?
Use the Pythagorean theorem. For a standard mount stair, stringer length = √((total rise − one riser)² + total run²), because the deck face acts as the top step; for a flush mount, use the full total rise. Add 6–12 inches when purchasing lumber to allow for the top plumb cut and bottom seat cut.
How steep can residential stairs be?
The IRC limits the dimensions, not the angle: a riser of no more than 7¾ inches and a tread of at least 10 inches (2024 IRC R318.7.5). At those limits the stair angle is about 37.8°; a 7-inch rise with an 11-inch run is about 32.5°. A common carpenter's rule of thumb (not a code requirement) is Rise + Run ≈ 17–18 inches.
How many steps do I need for a 9-foot ceiling?
A 9 ft floor-to-floor height is 108 inches. At a 7-inch target riser: ⌈108 ÷ 7⌉ = 16 risers, actual riser = 108 ÷ 16 = 6.75 inches. At 7.5-inch target: ⌈108 ÷ 7.5⌉ = 15 risers, actual riser = 7.2 inches. Either is code-compliant.