Stair Calculator
Designing stairs correctly is critical both for safety and code compliance. The International Residential Code (IRC) specifies maximum riser heights (7.75 inches) and minimum tread depth (10 inches) for residential stairs. This stair calculator takes the total vertical rise in inches (the floor-to-floor height), the desired riser height, and the tread run in inches to compute the exact number of steps, the actual equal riser height that results from dividing the total rise by a whole number of steps, and the stringer length. The stringer length is the diagonal length of the structural member supporting the treads, calculated from the Pythagorean theorem using the total rise and total run. The calculator also shows a code compliance check for the IRC riser and tread requirements. All results assume a straight-run stair. For L-shaped or U-shaped stairs with a landing, calculate each flight separately. For winding stairs and curved stairs, the tread depth is measured at the walk-line 12 inches from the narrow edge. Always consult a structural engineer or licensed contractor for stairs that are part of a permitted project, and verify that your local jurisdiction has adopted the IRC or a comparable code. Stair layout should be confirmed with actual measurements before cutting stringers.
Stair calculation formulas
Steps = round(Total rise / Desired riser height)
Actual riser = Total rise / Steps
Total run = Steps x Tread run (note: treads = steps - 1 for open stairs, but run = steps x tread for stringer length)
Stringer length = sqrt(Total rise^2 + Total run^2)
IRC: Riser between 4 in and 7.75 in; Tread min 10 in
Worked example: 9 ft floor-to-floor (108 in), 7 in desired riser, 10 in tread
- Steps: round(108 / 7) = round(15.43) = 15 steps
- Actual riser: 108 / 15 = 7.20 in
- Total run: 15 x 10 = 150 in
- Stringer: sqrt(108^2 + 150^2) = sqrt(11664 + 22500) = sqrt(34164) = 184.8 in (15.4 ft)
- IRC check: 7.20 in riser (pass); 10 in tread (pass)
Stair calculator: frequently asked questions
What is the IRC code for stair riser height and tread depth?
The International Residential Code (IRC) Section R311.7 requires that stair risers be no more than 7.75 inches high and no less than 4 inches high. Treads must have a minimum run of 10 inches (measured from nosing to nosing). The maximum variation between the largest and smallest riser in a flight is 3/8 inch. This calculator checks your inputs against these requirements and shows a code compliance note.
How do I calculate the number of steps?
Divide the total vertical rise (in inches) by the desired riser height. Round to the nearest whole number of steps. Then divide the total rise by the number of steps to get the actual riser height, which will be slightly different from your desired riser height. For example, a 108-inch total rise with a desired 7-inch riser gives 108 / 7 = 15.4, rounded to 15 steps. Actual riser height = 108 / 15 = 7.20 inches.
What is the stringer length?
The stringer is the diagonal structural member that supports the treads and risers. Its length is calculated using the Pythagorean theorem: stringer length = sqrt((total rise)^2 + (total run)^2). Total run = number of treads x tread depth. For a standard flight with 15 steps at a 10-inch tread and 7.2-inch riser, stringer length = sqrt((108)^2 + (140)^2) = approximately 176 inches (about 14.7 feet).
Why is the actual riser height different from what I enter?
Stairs must have a whole number of steps to reach the floor above. The total rise must be divided evenly by the number of steps. This means the actual riser height is rarely exactly equal to your target. The calculator finds the nearest whole number of steps and divides to get the exact equal riser height that will result. All risers in a flight must be equal to within 3/8 inch per IRC requirements.
What is a comfortable stair rise-to-run ratio?
A commonly used rule of thumb is that 2 x Rise + Run should equal approximately 24 to 25 inches. For a 7-inch riser and 11-inch tread: 2 x 7 + 11 = 25, which is comfortable. For a 7.75-inch riser and 10-inch tread: 2 x 7.75 + 10 = 25.5, which is steeper but still within code. Shallower stairs with a 6.5-inch riser and 12-inch tread (2x6.5+12=25) are easier to climb for all ages.
Sources
- International Residential Code Section R311.7 Stairways: codes.iccsafe.org.
- ICC International Residential Code adoption status by state: iccsafe.org.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. Always verify with a licensed contractor and local building code.