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Stringer Length
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Number of Steps (Risers)
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Actual Riser Height
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Total Run (Length)
Building stairs requires precise measurements and careful calculation to ensure safety and comfort. To begin calculating stair dimensions, you first need to determine the Total Rise. The total rise is the exact vertical distance from the finished floor of the lower level to the finished floor of the upper level. Do not measure from subfloor to subfloor unless both finished floors will be the exact same thickness.
Once you know the total rise, you must divide it by your Target Riser Height. A comfortable standard is generally around 7 to 7.5 inches. Dividing the total rise by the target riser height gives you the number of steps (risers) required. Since you cannot have a fraction of a step, round this number to the nearest whole number. Finally, divide the total rise by this whole number to find your true, exact riser height for every step.
The International Residential Code (IRC) provides guidelines for stair construction to prevent tripping hazards and ensure safe egress. While local codes can vary and should always be checked, the general IRC guidelines for residential stairs are:
Our calculator automatically checks your calculated riser height and chosen tread depth against the maximum 7.75" rise and minimum 10" run rules. Note that many carpenters prefer the "7-11 rule" (7" rise and 11" run) for the most comfortable stride.
The stringer is the structural board (usually a 2x12) cut with zig-zag notches that the treads and risers attach to. To determine how long a board you need to buy for your stringers, you must calculate the hypotenuse of the triangle formed by the Total Rise and Total Run.
The total run is calculated by multiplying the number of treads by the target tread depth. Remember that the number of treads is always one less than the number of risers, because the top floor or landing acts as the final "tread." Use the Pythagorean theorem (A² + B² = C²) with the total rise and total run to find the minimum stringer length. Always buy lumber that is 1-2 feet longer than your exact calculated stringer length to account for the top and bottom cuts.
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