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Stair Calculator

The number of stair risers is calculated by dividing the total vertical rise by the target riser height and rounding to the nearest whole number. The actual riser height is then the total rise divided by that count. Total run equals the number of treads (risers minus one) times the tread depth, and stringer length is the hypotenuse of the total rise and total run. Enter your measurements below to calculate stair dimensions and check compliance with the International Residential Code (IRC).

Quick Answer

A total rise of 108 inches (9 feet) with a 7.5-inch target riser height produces 14 risers at 7.71 inches each, a total run of 130 inches, and a stringer length of 169.02 inches.

Total vertical height from finished floor to finished floor

Common Examples

Input Result
108" total rise, 7.5" target riser, 10" tread 14 risers, 7.71" actual riser, 130" run, 169.02" stringer
96" total rise, 7.5" target riser, 10" tread 13 risers, 7.38" actual riser, 120" run, 153.60" stringer
36" total rise, 7" target riser, 11" tread 5 risers, 7.20" actual riser, 44" run, 56.86" stringer
120" total rise, 7.5" target riser, 10" tread 16 risers, 7.50" actual riser, 150" run, 192.09" stringer

How It Works

This calculator uses standard stair construction formulas along with IRC (International Residential Code) compliance checks.

Number of Risers = round(Total Rise / Target Riser Height)

Actual Riser Height = Total Rise / Number of Risers

Number of Treads = Number of Risers - 1 (the top floor serves as the final landing)

Total Run = Number of Treads x Tread Depth

Stringer Length = sqrt(Total Rise^2 + Total Run^2)

Where:

  • Total Rise = the vertical distance from the surface of the lower finished floor to the surface of the upper finished floor, measured in inches
  • Target Riser Height = the desired height of each individual riser step
  • Tread Depth = the horizontal depth of each tread (the part stepped on), measured from nose to nose
  • Stringer Length = the diagonal length of the stair stringer, calculated as the hypotenuse

IRC Compliance Requirements

The International Residential Code (IRC Section R311.7) sets the following limits for residential stairs:

  • Maximum riser height: 7.75 inches (196 mm). Individual risers must not exceed this height.
  • Minimum tread depth: 10 inches (254 mm). Measured from the leading edge (nosing) of the tread to the leading edge of the next tread.

The IRC also requires uniform riser heights throughout the staircase. The maximum variation between the tallest and shortest riser is 3/8 inch. Since this calculator computes equal riser heights, uniformity is guaranteed.

Worked Example

For a total rise of 108 inches (a standard 9-foot ceiling height), a target riser height of 7.5 inches, and a tread depth of 10 inches: Number of risers = round(108 / 7.5) = round(14.4) = 14. Actual riser height = 108 / 14 = 7.714 inches. Number of treads = 14 - 1 = 13. Total run = 13 x 10 = 130 inches (10 feet 10 inches). Stringer length = sqrt(108^2 + 130^2) = sqrt(11,664 + 16,900) = sqrt(28,564) = 169.01 inches (14 feet 1 inch). The riser height of 7.71 inches is under the 7.75-inch IRC maximum, and the 10-inch tread meets the minimum, so this staircase is IRC compliant.

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Frequently Asked Questions

How do I measure total rise for stairs?
Total rise is the vertical distance from the surface of the lower finished floor to the surface of the upper finished floor. Measure straight up from the bottom landing to the top landing. Include the thickness of any flooring material that will be installed, since risers are measured from finished surface to finished surface.
What is the IRC maximum riser height?
The International Residential Code (IRC) sets a maximum riser height of 7.75 inches (196 mm) for residential stairs. This limit applies to each individual riser. Local building codes may have stricter requirements, so checking with the local building authority is advisable.
Why is there one fewer tread than risers?
The top floor (or landing) serves as the final stepping surface, so no separate tread is needed at the top. A staircase with 14 risers has 13 treads because the 14th step up places the person on the upper floor itself.
What is a stair stringer?
A stringer is the structural board that runs diagonally along each side (and sometimes the center) of a staircase. Treads and risers are attached to the stringer. The stringer length is calculated as the hypotenuse of the total rise and total run, and it determines the length of lumber needed to cut the stringer.
What if my calculation shows the stairs are not IRC compliant?
If the actual riser height exceeds 7.75 inches, try increasing the number of risers by lowering the target riser height. If the tread depth is below 10 inches, increase it to at least 10 inches. Adjusting these inputs until both checks pass ensures the design meets the IRC residential code minimums. Local codes may impose additional requirements.