Stair Calculator: Rise, Run and Stringer Length

Measure the total rise from finished floor to finished floor, and enter the tread depth you want. The calculator finds the fewest risers that keep each one under your maximum height, and gives the total run, the stringer length and how many stringers to cut. It flags anything outside the IRC limits for house stairs.

Stairs & stringers calculator

How it works

Risers = total rise ÷ maximum riser, rounded up
Riser height = total rise ÷ risers (all risers equal)
Treads = risers − 1 (the top “step” is the upper floor)
Total run = treads × tread depth
Stringer length ≈ √(total rise² + total run²)

IRC R311.7 limits for stairways in houses: risers no more than 7-3/4 in, treads at least 10 in, a nosing of 3/4 to 1-1/4 in where treads are under 11 in, at least 36 in wide, 6 ft 8 in headroom, and no more than 151 in of rise between landings. For deck stairs, DCA 6 calls for at least three cut stringers and no more than 18 in between stringers.

Worked example

A 108 in floor-to-floor rise with 10-1/2 in treads, 36 in wide:

Total rise (finished floor to finished floor): 108 in
Maximum riser height: 7.75 in
Tread run (depth): 10.5 in
Stair width: 36 in

The calculator gives:

Risers: 14 at 7-11/16" (7.714 in)
Treads: 13 at 10-1/2"
Total run: 11′ 4-1/2"
Stringer length (approx.): 14′ 6-1/16", so a 16 ft board
Stair angle: 36.3°
Stringers for 36 in width: 3 (at most 18 in apart, at least 3)

108 ÷ 7.75 = 13.9, so 14 risers of 7.714 in (about 7-11/16 in) and 13 treads. The run is 13 × 10.5 = 136.5 in (11 ft 4-1/2 in). The stringer is about √(108² + 136.5²) = 174 in, so cut it from a 16 ft 2×12.

Notes and limits

Measure total rise to the finished floor surfaces. If flooring is still to go on, add its thickness at the top and subtract it at the bottom.
Stringer length here is approximate. Lay out the actual stringer with a framing square, and check that enough solid wood (the “throat”) remains below the notches for your code or deck guide.
Handrails, guards and headroom are not checked by the calculator.
Local codes can differ from the IRC, and commercial stairs follow the IBC, which is stricter.

Frequently asked questions

How many steps for a 9 ft ceiling?

A 9 ft ceiling usually means about 9 ft 10 in to 10 ft floor to floor once joists and flooring are counted. At 118 in of rise, 16 risers of 7-3/8 in. Measure your actual rise.

What is the maximum stair riser height?

7-3/4 in under the IRC for houses (R311.7.5.1). Treads must be at least 10 in deep.

How long should a stair stringer be?

About √(total rise² + total run²), plus a little for the cuts at the top and bottom. The calculator suggests the next stock length.

How many stringers do I need?

For deck stairs DCA 6 calls for at least three, no more than 18 in apart, so 36 in stairs need 3 and 48 in stairs need 4.

Sources

Last reviewed: 25 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.