Stair Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

Worked example

A 9 ft floor height (108 in total rise) with a 7.5 in target riser and 11 in tread. Inputs: Total rise (floor to floor) 108 in, Target riser height 7.5 in, Tread depth (run) 11 in, Stair width 36 in. Result: 14 .

Stairs for common floor heights

With a 7.5 in target riser and 11 in tread. Tap a row to load it in the calculator above.

Method & assumptions

A staircase is laid out from one controlling measurement: the total rise, the vertical distance from the lower finished floor to the upper finished floor. You cannot pick the riser height freely, because the risers must divide the total rise into equal parts. So you start from a target riser, divide the total rise by it, and round to the nearest whole number of risers. The actual riser height is then the total rise divided by that whole number of risers, and you confirm it stays under the 7.75 inch code maximum. If it comes out over, add one more riser, which lowers each one.

There is always one fewer tread than riser, because the upper floor itself serves as the final step, so a flight with 14 risers has 13 treads. The total run is the number of treads times the tread depth (the run). The stringer, the diagonal board the steps are cut from, is the hypotenuse of the right triangle formed by the total rise and total run, found with the Pythagorean theorem: stringer length is the square root of rise squared plus run squared. Add 6 to 12 inches of extra stringer stock for the plumb cut at the top and the seat cut at the bottom.

The result is checked against the International Residential Code, IRC section R311.7. For residential stairs the riser height must not exceed 7.75 inches, the tread depth must be at least 10 inches, and the stair width at least 36 inches. The code also requires consistency: the tallest and shortest risers in a flight cannot differ by more than 3/8 inch, which is why the math must be exact, an inspection fails on a quarter-inch error. For comfort, aim for a riser plus tread sum near 17 to 18 inches, or the classic rule that two risers plus one tread falls between 24 and 25 inches. Commercial stairs follow the stricter IBC (7 inch riser, 11 inch tread), so always confirm the code that applies with your local building department.

Pro tips and common mistakes

• Measure total rise to the finished floors. Include the thickness of the flooring at the top and bottom, not just the subfloor. A half-inch error here throws off every riser and can fail inspection.
• Round risers up, not to nearest. Dividing the total rise and rounding up keeps the actual riser at or below your target, so it stays under the 7.75 inch code maximum. Rounding down can push the riser over the limit.
• Keep every riser equal. Code allows no more than 3/8 inch difference between the tallest and shortest riser in a flight. Mark your stringer from the calculated height, do not eyeball the last step.
• Add stock for the stringer cuts. Buy the stringer 6 to 12 inches longer than the calculated length to allow for the plumb cut at the top and the seat cut at the bottom where it meets the floor.
• Aim for comfort, not just code. The minimums are legal, not necessarily pleasant. A 7 inch riser with an 11 inch tread (rise plus run of 18) walks far better than the tightest 7.75 by 10 allowed.

Frequently asked questions

References

• Stairs (Wikipedia)
• IRC R311.7: Stairways (UpCodes)
• Pythagorean theorem (Wikipedia)

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