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Stair Stringer Calculator

Source: International Residential Code R318.7.5.1 (Risers) in the 2024 edition, numbered R311.7.5.1 in 2021 and earlier, reproduced with ICC commentary by the Routt County Regional Building Department · Source verified August 21, 2026

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A stair stringer calculator is a carpentry planning tool used to determine the dimensions of the structural boards (stringers) that support a staircase. It divides the total vertical rise, measured between walking surfaces at both ends, by a target step rise to compute the total number of risers and treads, then calculates the actual rise, actual run, incline angle, and the exact stringer board length needed. This helps carpenters lay out and cut the sawtooth patterns accurately while meeting local building codes.

Find the lumber length and exact cut measurements for your stair stringers. Input your total vertical rise and horizontal run to determine the exact number of steps, individual riser heights, tread depths, and incline angle.

Quick Answer

Calculate the required lumber length, step counts, and rise/run cutting dimensions for a stair stringer from your walking-surface total rise and total run.

Staircase Boundaries

in

Height from bottom floor to top landing surface.

in

Total horizontal distance the stairs will cover.

in

Standard comfortable rise is around 7 to 7.5 inches.

Stringer Layout Cuts

Minimum lumber board length

136 in

Recommend buying at least a 12 ft 2x12 board

Number of steps (risers)11
Number of treads10
Actual riser height (rise per step)7.273 in
Actual tread depth (run per step)11 in
Stringer incline angle36°
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Examples

80 in total rise, 110 in total run, 7.5 in target rise

11 risers at 7.27 in rise, 10 treads at 11 in run, 136 in stringer length

36 in total rise, 48 in total run, 7 in target rise

5 risers at 7.20 in rise, 4 treads at 12 in run, 60 in stringer length

100 in total rise, 130 in total run, 7.5 in target rise

13 risers at 7.69 in rise, 12 treads at 10.83 in run, 164 in stringer length

How it works

Stair stringer geometry uses the Pythagorean theorem to calculate the length of lumber required, alongside proportional division of the total rise and run to ensure uniform step heights:

Step Count (Risers):

Number of Risers (N) = Round(Total Rise ÷ Target Riser Height)

Step Dimensions:

Actual Step Rise = Total Rise ÷ N

Actual Step Run = Total Run ÷ (N − 1)

Stringer Length & Angle:

Stringer Length = √((Total Rise)² + (Total Run)²)

Incline Angle = Arctan(Total Rise ÷ Total Run) × (180 ÷ π)

Verify dimensions against your local building codes (e.g., maximum riser height of 7.75 inches and minimum tread depth of 10 inches under the IRC) before purchasing lumber or making cuts.

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Stair CalculatorPlan stair risers and treads from total rise and preferred riser height. Returns the riser count, actual riser height, total run, and an estimated stair angle.

Frequently asked questions

A stair stringer is the structural support board that runs along the sides of a staircase. Treads (the steps you walk on) and risers (the vertical boards between steps) are fastened to it. The stringer is cut in a sawtooth pattern to support the stair layout.

Divide the total vertical rise of the stairs by a target step rise (usually 7 to 7.5 inches) and round to the nearest whole number to find the number of risers. Then, divide the total rise by this number of risers to get the exact actual rise per step.

Under the International Residential Code (IRC), the maximum riser height is 7.75 inches and the minimum tread depth (run) is 10 inches. A common rule of thumb for comfortable stairs is that the sum of one rise and one run should be between 17 and 18 inches.

To the walking surface at both ends, counting any flooring that is not down yet. The calculator divides the total rise into equal risers, so it cannot tell which surface you measured from. Measure to a bare subfloor, cut the stringer, then lay three-quarter-inch flooring at the bottom only, and every riser keeps its cut height except the bottom one, which loses three quarters of an inch. That single short riser is what people trip on, because a flight is only comfortable when every step is the same height.

Because walking a flight is a rhythm rather than a series of decisions. After the first two steps a person stops looking and lets the legs repeat the same motion, so a step that differs from its neighbours arrives unannounced. The International Residential Code puts a number on it: section R318.7.5.1 in the 2024 edition, numbered R311.7.5.1 in the 2021 and earlier editions, requires that the greatest riser height within a flight not exceed the smallest by more than 3/8 inch, measured vertically between the leading edges of adjacent treads. Three quarters of an inch of unaccounted flooring is double that allowance, from a mistake made before any wood was cut. Note that the code measures between tread edges and excludes carpets, rugs and runners; it does not use the phrase finished floor, which is trade shorthand rather than code text.

In a standard staircase layout, the top riser meets the edge of the upper landing or floor level directly. Because the upper landing acts as the top tread, you need one fewer physical wooden tread board than the total number of vertical rises.