Stair Calculator: Rise, Run and Risers
Enter the height from finished floor to finished floor and a tread depth. The calculator picks the number of risers, works out the riser height, the run and the angle, and checks them against the IRC limits for residential stairs: a riser of 7¾ inches or less and a tread of at least 10 inches.
Last checked 11/10/2026. Sources: Knox County (Tennessee) Code Administration: 2018 IRC residential stairs handout (7¾ in riser, 10 in tread, ⅜ in uniformity, 6 ft 8 in headroom).
Guide
How the calculator works
Take a 9 ft (108 in) floor-to-floor height and a target riser of 7 in. 108 ÷ 7 = 15.43, which rounds to 15 risers. Each riser is 108 ÷ 15 = 7.2 in, so every step is the same height. A flight with 15 risers has 14 treads. At 10 in each, the run is 14 × 10 = 140 in, or 11.67 ft. The stringer, the long diagonal board, is √(9² + 11.67²) = 14.73 ft, and the angle is 35.8°.
You can type your own number of risers instead. With 14 risers each is 7.71 in, still under 7¾ in. With 13 risers each is 8.31 in, which is too tall.
The rules it checks, and where they come from
For houses built to the International Residential Code the riser is at most 7¾ in, measured between the leading edges of adjacent treads, and the tread is at least 10 in deep. The biggest riser in a flight may not exceed the smallest by more than ⅜ in, and the same applies to tread depth. Headroom must be at least 6 ft 8 in. Treads need a nosing of ¾ to 1¼ in when the risers are solid, unless the tread is at least 11 in deep. Source: Knox County Code Administration handout on the 2018 IRC (linked below).
Many states and cities adopt the IRC with changes, so check yours. The calculator checks riser height and tread depth.
A tight space
If the stair has to fit a short space, the going is what suffers. With 9 in treads the run falls to 10.5 ft, but 9 in is below the 10 in minimum and the calculator flags it. The fixes are a longer run, a turn or landing, or a steeper riser within the limit. Put your space in the optional box and it tells you whether the run fits and by how much.
What it does not check
- Headroom above the stairs, handrail and guard heights, landings, winders and tapered treads, which all have their own rules.
- Structure: stringer size and spacing, fixings and the floor above. Ask a builder or engineer.
- Local amendments. Councils and states can be stricter than the national or model code, and a permit or inspection may be needed.
- Existing stairs. Old houses are sometimes allowed to differ; ask your building authority.
Common questions
What riser height should I aim for?
The code gives a maximum of 7¾ in, not a target. Many people aim for 7 to 7½ in because lower risers are easier to climb. Use the target box to try values.
Does the top step count as a tread?
No. A flight has one fewer tread than risers because the floor above is the last step. The calculator uses that.
Do I need a permit for stairs?
Often yes for new stairs and structural changes. Ask your local building department.
Why must all the risers be the same?
People trip on uneven steps. The IRC lets the biggest riser in a flight exceed the smallest by no more than ⅜ in, and the calculator divides the height into equal risers.
Planning a related job?
- Stairs for a deck? Work out the boards, joists and screws. Deck Board Calculator
- A landing or footings? Work out the concrete. Concrete, Gravel and Sand Calculator
More calculators
Paying for the project?
- Work out the repayment on a loan with the calcfig personal loan calculator.