Stair Calculator

Plan a staircase from the floor-to-floor height. Get the number of risers and treads, exact riser height, total run, stringer length and stair angle.

Number of risers (steps)
–
Riser height–
Treads–
Total run–
Stringer length–
Stair angle–
2R + T–

How the stair calculation works

Risers = total rise ÷ max riser height, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1
Total run = treads × tread depth
Stringer ≈ √(total rise² + total run²)

Worked example

A total rise of 108 inches (9 feet) with a 7¾ in maximum riser needs 108 ÷ 7.75 = 13.9, rounded up to 14 risers. Each riser is 108 ÷ 14 = 7.71 in. With 13 treads at 10 in, the total run is 130 inches (10 ft 10 in), and the stringer is about √(108² + 130²) ≈ 169 inches.

Common stair rules (IRC residential)

ItemTypical requirement
Maximum riser height7¾ in
Minimum tread depth10 in (plus ¾–1¼ in nosing on closed risers)
Riser and tread variationNo more than ⅜ in between the largest and smallest
Minimum headroom6 ft 8 in
Minimum width36 in
Handrail height34 to 38 in above the nosing

Codes vary by location and change over time, so confirm with your local building department. Need the angle or length of any right triangle? Use the Pythagorean theorem calculator. For converting measurements, try the feet and inches calculator.

Last reviewed October 2026.

Frequently asked questions

How do I calculate the number of stairs?

Measure the total rise, the vertical distance from the finished floor below to the finished floor above. Divide it by your maximum riser height (7¾ inches under the U.S. residential code) and round up. That is the number of risers. There is usually one fewer tread than risers.

What are the code limits for residential stairs?

The International Residential Code (IRC), used in most U.S. states, sets a maximum riser height of 7¾ inches and a minimum tread depth of 10 inches, with at least 6 ft 8 in of headroom. Local codes can differ, so check with your building department before you build.

What is the 2R + T rule?

A traditional comfort rule says two risers plus one tread should add up to about 24 to 25 inches. For example, 7.5 in risers with 10 in treads gives 25 inches. Stairs that follow it tend to feel natural to climb.

How long should the stringer be?

The stringer is the diagonal board that carries the treads. Its length is roughly the square root of (total rise² + total run²). Buy lumber a little longer to allow for cuts at the top and bottom.

What is the difference between rise and run?

Rise is vertical height; run is horizontal distance. Each step has a unit rise (riser height) and a unit run (tread depth), and the whole staircase has a total rise and total run.