About the Stair Calculator
This stair calculator turns one measurement — the total rise from the finished lower floor to the finished upper floor — into a complete stair layout. It finds the fewest risers that stay under your maximum riser height, divides the rise evenly between them, and then works out the number of treads, the total horizontal run, the stair angle and the length of the diagonal stringer you need to cut.
It is useful for DIY builders laying out deck steps, basement stairs or a loft stair, and for anyone checking a carpenter’s plan against common code limits. The defaults follow the US International Residential Code (IRC, 2021 edition, Section R311.7.5) limits of a 7¾-inch maximum riser and a 10-inch minimum tread depth, with no more than 3/8 inch variation between risers or treads in a flight, but you can change both to suit local rules or a gentler, more comfortable stair.
Measure the rise between finished surfaces (include flooring or decking that is still to be installed). The stringer length is the straight-line hypotenuse of rise and run; buy lumber at least a foot longer to allow for the top and bottom cuts.
With the default inputs, the number of steps (risers) is 14. Change any value above to recalculate instantly.
How to use the stair calculator
- 1Measure the total rise from the finished lower surface to the finished upper surface.
- 2Enter your maximum riser height (7¾ in for most US homes, lower for a gentler stair).
- 3Enter the tread depth you plan to use (at least 10 in under the IRC).
- 4Read the number of risers, the exact riser height and the total run.
- 5Use the stringer length and step table to mark out and cut your stringers.
Formula and method
The calculator divides the total rise H by your maximum riser height and rounds up, which gives the fewest risers N that keep every step at or below the limit. The rise is then split evenly, so every riser is exactly H ÷ N — building codes require risers within a flight to vary by no more than 3/8 inch.
A straight flight has one fewer tread than risers because the top step is the upper floor itself. Multiplying the tread count by the tread depth gives the total run, and the stringer length is the hypotenuse of the rise and run. The stair angle is the arctangent of riser ÷ tread; most comfortable residential stairs fall between 30° and 38°. The comfort check uses the classic rule that two risers plus one tread should total roughly 24–25 inches; up to about 25½ inches is still comfortable, which is where a code-maximum 7¾-inch riser with a 10-inch tread lands.
- H
- Total rise between finished floors (in)
- Rmax
- Maximum allowed riser height (in)
- N
- Number of risers (steps)
- R
- Actual riser height (in)
- D
- Tread depth (in)
- L
- Stringer length (in)
Worked examples
Basement stair with a 9-foot rise
A 108-inch rise divided by the 7¾-inch limit is 13.94, so you need 14 risers of 108 ÷ 14 = 7.714 in. Thirteen 10-inch treads give a 130-inch run, the stringer is √(108² + 130²) ≈ 169 in, and the stair angle is about 37.7°.
Deck steps with a 30-inch drop
Thirty inches at a 7½-inch maximum divides exactly into 4 risers of 7.5 in. Three 11-inch treads make a 33-inch run, so each stringer is √(30² + 33²) ≈ 44.6 in long at an angle of about 34.3°.
Comfortable 10-foot stair
Keeping risers at 7 in or less for a 120-inch rise needs 18 risers of 6.667 in. With 17 treads of 11 in the run is 187 in, the stringer is about 222 in (18½ ft) and the stair is a gentle 31°.
Frequently asked questions
How many steps do I need for a 9-foot ceiling?+
With a 9-foot (108 in) ceiling plus a typical 10–12 inch floor structure, the finished floor-to-floor rise is usually about 118–120 inches. At a 7¾-inch maximum riser that means 16 risers of roughly 7.4–7.5 in and 15 treads. For an 8-foot ceiling (about 105–108 in of rise) you need 14 risers.
What is the code for stair rise and run?+
The US International Residential Code limits risers to 7¾ inches and requires treads of at least 10 inches, with no more than 3/8 inch variation between the largest and smallest riser or tread in a flight. Commercial codes are stricter (7 in max riser, 11 in min tread).
How do I calculate stair stringer length?+
Multiply the number of treads by the tread depth to get the total run, then use Pythagoras: stringer = √(rise² + run²). Buy a board at least a foot longer to allow for the plumb and level cuts at each end.
Why is there one less tread than risers?+
In a straight flight the top riser lands directly on the upper floor or deck, which acts as the final tread. So a stair with 14 risers has only 13 treads that you actually build.
What is the most comfortable stair angle?+
Most people find stairs between about 30° and 37° comfortable. A common check is the 2R + T rule: two riser heights plus one tread depth should be about 24–25 inches (up to roughly 25½ is fine).