Stair Calculator
Use this stair calculator to work out how many risers and treads a staircase needs from the total rise. Enter the floor-to-floor rise, a target riser height and the tread depth to get the actual riser height, total run, stringer length and stair angle, plus a typical IRC check.
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Number of risers
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Actual riser height
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Number of treads
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Total run
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Stringer length
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Stair angle
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Tread area
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Typical IRC check (confirm with your local code)
Riser height (max 7.75 in)
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Tread depth (min 10 in)
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Comfort check: 2 × riser + tread
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Risers = total rise ÷ target riser height, rounded to the nearest whole number. Actual riser = total rise ÷ risers. Treads = risers − 1, because the upper floor acts as the last tread. Run = treads × tread depth. Stringer length = √(rise² + run²), the straight-line distance before cutting, so order longer stock. Angle = arctan(riser ÷ tread). The limits shown (IRC R311.7.5.1 and R311.7.5.2) are typical residential values: riser at most 7¾ in, tread at least 10 in. Your local code and inspector decide what applies.
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How it works
- How do you calculate the number of stair risers?
- Divide the total rise by the riser height you want and round to the nearest whole number. For a 108 in rise and a 7.5 in target, 108 ÷ 7.5 = 14.4, so use 14 risers of 108 ÷ 14 = 7.714 in each.
- How many treads does a staircase have?
- One fewer than the risers, because the upper floor serves as the last step. 14 risers need 13 treads, and at 10 in per tread the total run is 130 in (10 ft 10 in).
- What are the typical stair riser and tread limits?
- The model International Residential Code (R311.7.5) sets a maximum riser height of 7¾ in (7.75 in) and a minimum tread depth of 10 in, with no more than ⅜ in variation within a flight. These are typical values; local codes differ, so check yours.
- How do you find the stringer length?
- Use the Pythagorean theorem on the total rise and run: √(108² + 130²) = 169.0 in (about 14 ft 1 in). Add length for the cuts at the top and bottom, and order stock longer than this figure.
- What is the 2R + T stair comfort rule?
- Twice the riser height plus the tread depth should be about 24–25 in for a comfortable stride. A 7 in riser with an 11 in tread gives 25 in. It is a design guide, not a code requirement.