Calculators

Stair Calculator

Number of steps, riser height, total run and stringer length for a straight staircase, checked against code.

How it works

Divide the floor-to-floor height by the tallest riser you will accept and round up: that is the number of steps. The actual riser height is the total rise divided by that number, so every step is equal. There is one tread fewer than risers because the upper floor is the last step. The stringer runs along the hypotenuse of the rise and the run.

risers = ⌈total rise ÷ max riser⌉; treads = risers − 1; run = treads × tread
Total rise
Finished floor to finished floor, in inches
Riser
Height of one step; at most 7¾ in under US residential code
Tread
Depth of one step; at least 10 in under US residential code
2R + T
Twice the riser plus the tread; 24–25½ in feels natural

Worked example

A stair between floors 108 in apart, with a 7¾ in maximum riser and 10 in treads.

  1. 1Risers: 108 ÷ 7.75 = 13.9, so 14
  2. 2Riser height: 108 ÷ 14 = 7.71 in (7 11/16 in)
  3. 3Treads: 14 − 1 = 13; total run: 13 × 10 = 130 in
  4. 4Stringer: √(108² + 130²) = 169 in, about 14 ft 1 in

14 steps of 7 11/16 in, taking 10 ft 10 in of floor, at an angle of 39.7°. Buy 16 ft stringer stock.

Frequently asked questions

What is the maximum riser height for stairs?

Under the International Residential Code, used in most US states, risers can be at most 7¾ in and treads at least 10 in deep, with no more than ⅜ in variation between steps. Commercial buildings are stricter: 7 in risers and 11 in treads. Local codes can differ.

How do I measure the total rise?

Measure from the top of the finished lower floor to the top of the finished upper floor. If the floors are not finished yet, add the thickness of the flooring to come. For deck stairs, measure from the deck surface to the ground or pad where the stairs will land, and check that the ground is level.

How long should stair stringers be?

At least the length along the slope, √(rise² + run²), plus a little for the top and bottom cuts. Buy the next stock length up: a 14 ft 1 in stringer needs 16 ft lumber. Most stringers are cut from 2 × 12s.

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