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Staircase Calculator: Stair Stringers, Risers and Treads With IRC Checks
Enter the floor-to-floor height and your tread depth. Get the number of steps, exact riser height, total run, stringer length and angle, a drawing of your stringer, and a check against the residential building code.
Stair and Stringer Calculator
- ✓Riser 7 3/16": the IRC maximum is 7 3/4 in.
- ✓Tread 10 1/2": the IRC minimum is 10 in, measured nosing to nosing, with a 3/4 to 1 1/4 in nosing on solid risers.
- !15 risers: a handrail is required (4 or more risers), 34 to 38 in above the nosings.
- !Keep at least 6 ft 8 in of headroom above every tread and landing.
Checks use the 2018 International Residential Code (R311.7). Your city may have adopted a different edition or amendments; confirm with your building department.
How Do You Calculate Stairs?
Divide the floor-to-floor height by about 7 1/4 inches and round up to get the number of risers, then divide the height by that number for the exact riser. Treads are one fewer than risers. A 9-foot rise needs 15 risers of 7.2 inches and 14 treads, about 12 feet 3 inches of run.
Checked against the sources below · How we build and test our calculators
Stair Chart by Total Rise
Aiming for 7 1/4-in risers and 10 1/2-in treads, standard mount.
| Total rise | Risers | Riser height | Treads | Total run | Stringer |
|---|---|---|---|---|---|
| 2 ft 0 in | 4 | 6.00 in | 3 | 2 ft 7.5 in | 3 ft 3.625 in |
| 2 ft 6 in | 4 | 7.50 in | 3 | 2 ft 7.5 in | 3 ft 7.5 in |
| 3 ft 0 in | 5 | 7.20 in | 4 | 3 ft 6 in | 4 ft 7.375 in |
| 4 ft 0 in | 7 | 6.86 in | 6 | 5 ft 3 in | 6 ft 7.25 in |
| 7 ft 0 in | 12 | 7.00 in | 11 | 9 ft 7.5 in | 11 ft 10.875 in |
| 8 ft 0 in | 13 | 7.38 in | 12 | 10 ft 6 in | 13 ft 2.375 in |
| 8 ft 6 in | 14 | 7.29 in | 13 | 11 ft 4.5 in | 14 ft 2.375 in |
| 9 ft 0 in | 15 | 7.20 in | 14 | 12 ft 3 in | 15 ft 2.375 in |
| 10 ft 0 in | 17 | 7.06 in | 16 | 14 ft 0 in | 17 ft 2.5 in |
How to Lay Out a Stair Stringer
- Measure the total rise from the finished lower floor to the finished upper floor.
- Work out risers and treads with the calculator.
- Set a framing square on the 2×12 with the riser height on the tongue and the tread depth on the blade; use stair gauges to repeat it.
- Mark each step, then drop the bottom by the thickness of one tread so every step comes out equal.
- Cut one stringer, test it, then use it as the pattern for the rest.
Residential Stair Code at a Glance
- Riser: 7 3/4 in maximum.
- Tread: 10 in minimum, nosing to nosing.
- Nosing: 3/4 to 1 1/4 in on solid risers (not needed with treads of 11 in or more).
- Variation: 3/8 in maximum between the largest and smallest riser or tread.
- Headroom: 6 ft 8 in minimum.
- Handrail: 34 to 38 in high on flights with 4 or more risers.
From the 2018 IRC (R311.7) as summarized by Knox County, Tennessee. Check the edition and amendments your city uses.
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Questions
How do I calculate stairs?
Measure the total rise from finished floor to finished floor. Divide it by about 7 to 7 1/2 inches and round up to get the number of risers; divide the total rise by that number for the exact riser height. You need one fewer tread than risers, and the total run is the number of treads times the tread depth.
How many steps do I need for a 9-foot ceiling?
A 9-ft floor-to-floor rise (108 in) is usually 15 risers of 7.2 in with 14 treads. At 10 1/2-in treads, the total run is about 12 ft 3 in. Your actual rise includes the floor framing and finishes, so measure it.
What is the maximum stair riser height?
The International Residential Code limits risers to 7 3/4 inches and requires treads of at least 10 inches, and the largest riser in a flight may not exceed the smallest by more than 3/8 inch. Commercial buildings follow the IBC, which is stricter (7 in risers, 11 in treads).
How do I calculate stringer length?
Stringer length is the hypotenuse: the square root of (total rise² + total run²). Buy a board at least a foot longer to allow for the cuts at the top and bottom.
What size lumber is used for stair stringers?
Most residential stringers are cut from 2×12 lumber, which leaves enough wood below the notches. Use pressure-treated lumber outdoors.
When is a handrail required?
Under the IRC, stairs with four or more risers need a handrail on at least one side, 34 to 38 inches above the tread nosings. Keep at least 6 ft 8 in of headroom along the whole stair.
What is the 2R + T rule?
A comfort rule used by stair builders: twice the riser height plus the tread depth should be about 24 to 25 inches. A 7 1/4-in riser with a 10 1/2-in tread gives 25 in.