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 the total rise and the result updates instantly. There is no waste allowance here: this is a geometry layout, not a materials count. Buy the stringer 6 to 12 in longer than the calculated length for the plumb and seat cuts.

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, Clear stair width 36 in. Result: 15 .

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 up to the next whole number of risers. The actual riser height is then the total rise divided by that whole number. Rounding up is not a detail, it is what keeps the result legal: adding a riser always lowers each one, so the actual riser lands at or below your target and therefore under the 7.75 inch code maximum, while rounding to the nearest can push it over. A 96 inch rise with a 7.75 inch target rounds to 12 risers of exactly 8 inches, which fails inspection; rounded up it is 13 risers of 7.38 inches, which passes.

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 comparison uses three provisions from the 2021 International Residential Code: general clear width in R311.7.1, maximum riser height in R311.7.5.1 and minimum tread depth in R311.7.5.2. Passing these three checks is not full code compliance. The calculator does not check the permitted variation within a built flight, landings, headroom, nosings, handrails, guards, winders, illumination, structural capacity or local amendments. Confirm the adopted edition and complete stair design before cutting material.

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

Sources and references

These references and structured source records support the method and material data used on this page.

• Stairs (Wikipedia)
• 2021 IRC R311.7.1: Width
• 2021 IRC R311.7.5.1: Risers
• 2021 IRC R311.7.5.2: Treads
• Pythagorean theorem (Wikipedia)

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