Stair Calculator
This stair calculator lays out a straight flight from the total rise. Enter the floor-to-floor height, the tallest riser you want and the tread depth, and you get the number of risers and treads, the riser height to the nearest 1/16 in, the total run, the stringer length and the angle. It also checks the layout against the main limits of the International Residential Code.
Updated
Results
- Riser height, nearest 1/16 inFor your tape measure
- Treads (count)One fewer than the risers
- Total run (in)Treads × tread depth
- Stringer diagonal (in)Buy a longer board for the cuts
- Stair angle (°)
- 2R + T (in)Comfort rule of thumb: 24–25 in
- Comfort (2R + T)Rule of thumb, not a code requirement
- Riser height (7¾ in max)
- Tread depth (10 in min)
- Rise of the flight (151 in max)
- Headroom (6 ft 8 in min)Shown when you enter the headroom
- Width (36 in min)Shown when you enter the width
- NosingRequired when the tread is under 11 in
| Risers | Riser (in) | Treads | Total run (in) | 2R + T (in) | Angle (°) | Riser and tread vs IRC |
|---|
Results are estimates. See the methodology below.
How to use the stair calculator
- Measure the total rise from the finished lower floor to the finished upper floor, in inches. If flooring goes in later, adjust for it now: new flooring upstairs adds to the rise, new flooring downstairs reduces it.
- Enter the maximum riser you want. The default 7.5 in sits under the IRC's 7¾ in limit; lower it for a gentler stair.
- Enter the tread depth, measured nose to nose (the run of each step).
- In Advanced options, add the headroom and the clear width if you want those checked too.
- Compare the alternatives in the table: one, two or three extra risers give lower steps and a longer run.
How the calculator works
- Risers = total rise ÷ maximum riser, rounded up
- Riser height = total rise ÷ risers, so every riser is equal
- Treads = risers − 1, because the upper floor is the last step
- Total run = treads × tread depth
- Stringer length = √(rise² + run²)
- Angle = arctan(riser ÷ tread)
- 2R + T = 2 × riser + tread
IRC limits checked
In the 2024 IRC the stair rules are in section R318.7 (R311.7 in the 2018 and 2021 editions, with the same values):
| Item | IRC limit |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum |
| Nosing | ¾ to 1¼ in, not required when the tread is 11 in or more |
| Rise of one flight | 12 ft 7 in (151 in) before a landing |
| Headroom | 6 ft 8 in |
| Width | 36 in above the handrail height |
| Uniformity | 3/8 in between the largest and smallest riser or tread |
Because the calculator makes all risers equal, the layout meets the 3/8 in rule; on site, the bottom and top risers change if finished flooring is added after the stringers are cut. The 2R + T comfort range of 24 to 25 in is a long-standing rule of thumb, not a code requirement.
Worked examples
Basement stair, 108 in rise
With a 7.5 in maximum riser and 10.5 in treads: 15 risers of 7.200 in (7 3/16 in), 14 treads, a total run of 147 in and a stringer diagonal of 182.41 in at 34.4°. 2R + T is 24.90, inside the comfort range, and a nosing is required because the tread is under 11 in. The table shows the gentler options: 16 risers of 6.750 in need 157.5 in of run.
Deck stair, 94 in rise
With the maximum riser at 7.75 in and 10 in treads, as in the City of Eden Prairie, Minnesota, deck stair handout: 13 risers of 7.231 in (7 1/4 in) and 12 treads for a run of 120 in. With 14 risers the step drops to 6.714 in and the run grows to 130 in.
A tall rise
A 160 in rise gives 22 risers of 7.273 in. Each step is within the limits, but the flight is taller than 151 in, so the result reads landing required: split it into two flights.
Reading the result
"Within the IRC limits checked" means only that the riser, tread, flight rise and, if entered, headroom and width are inside those numbers. The IRC is a model code: what applies to you is the edition your state or city adopted, with its local amendments, and permits and inspections are handled by your local building department. Handrails (the IRC requires one on stairs with four or more risers), guards, landings, winders, spiral stairs and commercial stairs are not calculated. The stringer length is the diagonal only; the board you buy must be longer for the cuts and the bearing at the top. For the floor finish on the treads, our flooring calculator helps; for the stairwell walls, our drywall calculator.
Frequently asked questions
What is the maximum riser height for stairs?
The IRC sets a maximum of 7¾ in for residential stairs. Your local code may differ, so confirm the adopted edition with your building department.
What is the minimum tread depth?
10 in, measured from nosing to nosing. Treads under 11 in also need a nosing of ¾ to 1¼ in.
How many steps do I need for a 9 ft rise?
A 108 in floor-to-floor rise with a 7.5 in maximum riser needs 15 risers of 7.2 in and 14 treads. Measure the real rise between finished floors: it includes the floor framing, so it is more than the ceiling height.
How do I calculate the stringer length?
Use the Pythagorean theorem with the total rise and the total run. For the 108 in example, √(108² + 147²) gives 182.41 in. Buy a longer board to allow for the cuts at both ends.
What is the 2R + T rule?
Twice the riser plus the tread, which many builders like to keep between 24 and 25 in for a comfortable stride. It is a rule of thumb, not part of the IRC.
When does a stair need a landing?
Under the IRC, one flight may not rise more than 12 ft 7 in (151 in) between floors or landings. The calculator flags any rise above that.
Sources
- Fine Homebuilding — Stairway Basics (2024 IRC riser, tread, nosing, uniformity, flight rise, headroom, width, handrail)
- Jefferson County, Colorado — Stairways (2024 IRC R318.7.1 width, R318.7.2 headroom)
- Humboldt County, California — Stairways, CRC § R318.7 (riser, tread, variation, nosing)
- Town of Timnath, Colorado — 2018 Residential Stair Guide (R311.7.5.1, R311.7.5.2)
- City of Eden Prairie, Minnesota — deck stair handout (94 in rise example)
This is a layout aid, not a code approval. Building codes, permits and inspections are set locally; check with your building department before you cut stringers.