Stairs are where small math errors become trip hazards. Every riser must be the same height — codes typically allow only about 3/8" variation between the tallest and shortest riser. This calculator divides your total rise into equal risers, then works out treads, total run and stringer length.
How the numbers work
- Number of risers = total rise ÷ target riser, rounded up.
- Actual riser height = total rise ÷ number of risers.
- Number of treads = risers − 1 (the top floor is the last "tread").
- Total run = treads × tread depth.
- Stringer length = √(total rise² + total run²) — the Pythagorean theorem again.
Worked example
Total rise is 108" with a 7-1/4" target: 108 ÷ 7.25 = 14.9, so 15 risers at exactly 7.2" (7-3/16"). That gives 14 treads; at 10-1/2" each, the total run is 147". The stringer is √(108² + 147²) ≈ 182.4", so cut from 2×12 stock at least 16 ft long.
Comfort rule
Two risers plus one tread should land between about 24" and 25" (610–635 mm). Steps in that range match a natural stride. The calculator shows this number so you can trade a little riser height for tread depth.
Watch: how it is done
Video by Whitehead Woodworks on YouTube. More in our video library.
Frequently asked questions
How do I measure total rise?
Measure from the finished surface of the lower floor to the finished surface of the upper floor. If flooring is not installed yet, add its thickness at both ends.
Why is there one fewer tread than risers?
The upper floor acts as the final step, so a stair with 15 risers has 14 treads.
What size lumber do stringers need?
Most residential stringers are cut from 2×12. After notching, at least 3-1/2" to 5" of solid wood should remain below the notches, depending on code and span.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.