Stair Calculator

How to use this stair calculator

What this calculator does

This calculator estimates the geometry of one straight stair flight. It converts a measured floor-to-floor height into a whole number of equal risers, computes the resulting actual riser height, determines the number of horizontal treads for a standard or flush top mount, and then calculates total run, stringer length, stair angle, and a simple two-risers-plus-one-tread comfort check. It is useful for preliminary layout, material estimating, and comparing candidate dimensions. It does not certify structural capacity, code compliance, headroom, landing dimensions, guard design, handrail details, or the suitability of a particular stringer size.

When to use it

Use it while laying out deck stairs, checking whether a stair flight fits an available horizontal space, estimating stringer stock length, or comparing tread depths and target riser heights before making full-size marks. For workplace stairs, review the applicable OSHA stairway requirements; residential and commercial projects should also be checked against the code adopted by the local authority.

How to calculate

1. The calculator opens with a complete demonstration: 108 inches of total rise, a 7.5-inch target riser, a 10.5-inch tread run, and a standard top mount. The example results and Excel workbook are ready immediately.
2. Replace Total rise with the finished vertical distance between the lower and upper walking surfaces. Enter a positive decimal in the selected unit system.
3. Set Target riser height to the rise you want the layout to approach. The calculator rounds the quotient to a whole riser count, then divides the total rise evenly so every riser is identical.
4. Enter the horizontal Tread run. Choose Standard mount when the top floor acts as the final walking surface, or Flush mount when the stair layout includes a tread at the upper level.
5. Use Units to switch between inches/feet and centimeters/meters. The current values are converted rather than relabeled. Review the live results and the step layout table, then select Download Excel for a current-state workbook.
6. Reset clears the demonstration inputs and results. Download Excel is then disabled until all required dimensions are entered again.

Input guide

Total rise is required and must be a positive finite number, such as 108 inches or 274.32 centimeters. Measure finished surface to finished surface; measuring only framing can create a mismatched first or last riser. A greater total rise generally increases the number of risers, the stringer length, and often the required horizontal space.

Target riser height is required and must be positive, such as 7.5 inches. It guides the whole-number riser count but is not necessarily the final rise. The actual value is total rise divided by the selected integer count. A common mistake is assuming the target can be used unchanged when it does not divide the total rise evenly.

Tread run is required and must be positive, such as 10.5 inches. Increasing it lengthens the total run and stringer and lowers the stair angle. Do not confuse tread run with the physical board depth or nosing projection.

Top mount is required. Standard mount produces one fewer tread than risers because the upper floor serves as the last tread. Flush mount uses an equal count. Units is required and controls both input display and output formatting; changing it converts all three dimensions.

Output guide

Stringer length is the straight diagonal from the lower start to the upper endpoint, calculated with the Pythagorean theorem. Number of risers is the whole count selected from total rise divided by target rise. Actual riser height is the equal vertical increment after that count is fixed. Number of treads depends on mount type. Total run is tread run multiplied by tread count. Stair angle is the arctangent of total rise divided by total run; a larger angle means a steeper stair. 2R + T comfort check is twice the actual rise plus one tread run. It is a comparison aid, not a code approval. The summary pills repeat riser count, actual rise, and angle from the same model. The step table lists each riser, cumulative rise, whether a tread follows it, cumulative run, and the diagonal distance from the origin.

Worked example

With a 108-inch total rise and a 7.5-inch target, 108 ÷ 7.5 = 14.4, which rounds to 14 equal risers. The actual rise is 108 ÷ 14 = 7.7143 inches. A standard mount has 13 treads, so the total run is 13 × 10.5 = 136.5 inches. The stringer length is √(108² + 136.5²) = 174.058 inches, displayed as 14 feet 6.06 inches. The stair angle is arctan(108 ÷ 136.5) = 38.35°, and the comfort check is 2 × 7.7143 + 10.5 = 25.93 inches.