Stair Calculator
Number of steps, riser height, total run and stringer length for a staircase.
Good to know. Two risers plus one tread should come to about 63 cm, roughly the length of a comfortable stride.
Approximate. Real-world values vary.
How it works
In plain words
Divide the total rise by the tallest riser allowed and round up for the number of risers; each riser is then the rise divided by that number. There is one tread fewer than risers, since the top step is the floor above. The run is the treads times the tread depth, and the stringer is the diagonal of the rise and the run.
The formula
risers = ⌈total rise ÷ max riser⌉ · riser = total rise ÷ risers treads = risers − 1 · run = treads × tread depth stringer = √(rise² + run²) · comfort: 2 × riser + tread ≈ 60–66 cm
Worked example
270 cm ÷ 19 cm is 14.2, so 15 risers of 18 cm each. 14 treads of 25 cm make a run of 350 cm, and 2 × 18 + 25 = 61 cm is a comfortable stride.
Sources
- International Residential Code 2021, Section R311.7.5 Stair treads and risers Maximum riser 7¾ in and minimum tread 10 in for US homes.
- Approved Document K: Protection from falling, collision and impact (England) Private stairs: rise up to 220 mm, going at least 220 mm, pitch up to 42°.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the stair calculator
A staircase is designed from one measurement: the total rise from the finished floor below to the finished floor above. Divide it into equal steps no taller than the building rules allow, and the rest follows: the number of treads, the length of the staircase and the angle.
All risers in a flight must be the same height, since people remember the step and trip over one that differs by even a centimetre. US homes allow risers up to 7¾ inches and require treads of at least 10 inches (IRC R311.7.5). In England, private stairs can have risers up to 22 cm and goings of at least 22 cm, with a maximum pitch of 42° (Approved Document K). Most comfortable stairs are 17 to 19 cm (6¾ to 7½ in) high with 25 to 28 cm (10 to 11 in) treads.
The comfort check uses the 17th-century rule of François Blondel: twice the riser plus the tread should equal a normal stride, about 60 to 66 cm (24 to 26 in). Too little and the stair feels steep and cramped; too much and each step feels like a lunge.
The stringer length is the diagonal of the rise and run, before trimming at the top and bottom. Remember headroom (at least 2 m, or 6 ft 8 in) and handrails, and check local rules, which can differ from these common figures.
Questions people ask
How many steps for a 9 ft ceiling?
With 108 in from floor to floor and a maximum riser of 7¾ in, 14 risers of about 7.7 in, and 13 treads.
What is the standard riser height?
About 7 to 7¾ in (17 to 19.5 cm) in the US and 15 to 22 cm in the UK. All risers in a flight must be equal.
How do I calculate stair stringer length?
Use Pythagoras on the total rise and total run: √(270² + 350²) ≈ 442 cm for the example here.