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14 risers of 7 11/16 in and 13 treads of 10 in, total run 10 ft 10 in
| Risers | Riser | Treads | Tread | Total run | Angle | 2R + T |
|---|---|---|---|---|---|---|
| 14 | 7 11/16 in | 13 | 10 in | 10 ft 10 in | 37.6° | 25.4 in |
| 15 | 7 3/16 in | 14 | 10 in | 11 ft 8 in | 35.8° | 24.4 in |
| 16 | 6 3/4 in | 15 | 10 in | 12 ft 6 in | 34° | 23.5 in |
| 17 | 6 3/8 in | 16 | 10 in | 13 ft 4 in | 32.4° | 22.7 in |
This layout meets the 2021 IRC limits checked here for risers, treads, flight rise and width.
With treads under 11 in (279 mm) and solid risers, the IRC requires a nosing that projects ¾ to 1¼ in (19–32 mm).
Four or more risers need a handrail on at least one side, 34 to 38 in (864–965 mm) above the nosings.
Headroom must be at least 6 ft 8 in (2032 mm), measured straight up from the line of the nosings. Local codes and amendments may differ; check with your building department.
How to use the stair calculator
- Measure the total rise from the finished floor at the bottom to the finished floor at the top. Allow for flooring that isn't down yet.
- Keep the maximum riser at the code limit or enter a lower one for an easier stair.
- Enter the tread depth you want, or switch to Fit a total run if the stair has to fit a fixed space.
- Optionally fix the number of risers, enter the stair width and the floor thickness at the opening above to check headroom.
Stair formulas
Treads = risers − 1 · Total run = treads × tread depth
Stringer length = √(total rise² + total run²) · Angle = arctan(riser ÷ tread)
Rule of thumb: 2 × riser + tread ≈ 24 to 25 in (610–635 mm)
Floor opening for headroom = (6 ft 8 in + floor thickness) × tread ÷ riser
Dividing the rise into equal risers matters more than any other number: the IRC allows no more than ⅜ in (9.5 mm) between the tallest and shortest riser in a flight, and the same for tread depths. There is one tread fewer than risers because the upper floor is the last step. The stringer length is measured along the nosings; the board you cut it from must be longer to allow for the plumb cut at the top and the level cut at the bottom. The comfort rule of thumb, 2R + T, is not a code requirement, but stairs far outside 24 to 25 inches tend to feel awkward.
Worked example: a 108 in floor-to-floor rise
108 ÷ 7.75 = 13.94, so the stair needs 14 risers of 108 ÷ 14 = 7.714 in, about 7 11/16 in. With 13 treads of 10 in, the total run is 130 in (10 ft 10 in) and the stringer is 169 in (14 ft 1 in) long, at 37.6°. 2R + T is 25.4 in, a little over the comfort range. Adding one riser gives 15 risers of 7.2 in and 2R + T of 24.4 in, at the cost of 10 in more run. With 10½ in of floor structure at the top, the opening above must be at least 117.3 in (9 ft 9 5/16 in) long for 6 ft 8 in of headroom.
IRC stair limits (2021 edition, R311.7)
| Item | Limit | Section |
|---|---|---|
| Riser height | 7¾ in (196 mm) maximum | R311.7.5.1 |
| Tread depth | 10 in (254 mm) minimum, nosing to nosing | R311.7.5.2 |
| Variation within a flight | ⅜ in (9.5 mm) for risers and for treads | R311.7.5.1, .2 |
| Nosing projection | ¾–1¼ in (19–32 mm), not required with treads of 11 in (279 mm) or more | R311.7.5.3 |
| Headroom | 6 ft 8 in (2032 mm) minimum | R311.7.2 |
| Width | 36 in (914 mm) minimum clear above handrail height | R311.7.1 |
| Rise of one flight | 12 ft 7 in (151 in, 3835 mm) maximum between floors or landings | R311.7.3 |
| Handrail | Required with 4 or more risers; 34–38 in (864–965 mm) high | R311.7.8 |
These are model-code figures. States and cities adopt the code with amendments: some allow risers up to 8¼ in, and other countries use different rules entirely. Your building department has the final word.
Code-compliant stair layouts by total rise
Fewest risers at the 7¾ in limit, with 10 in and 11 in treads.
| Total rise | Risers | Riser height | Run, 10 in treads | Run, 11 in treads | Stringer, 10 in treads |
|---|---|---|---|---|---|
| 18 in | 3 | 6 in | 1 ft 8 in | 1 ft 10 in | 2 ft 2 15/16 in |
| 24 in | 4 | 6 in | 2 ft 6 in | 2 ft 9 in | 3 ft 2 7/16 in |
| 30 in | 4 | 7 1/2 in | 2 ft 6 in | 2 ft 9 in | 3 ft 6 7/16 in |
| 36 in | 5 | 7 3/16 in | 3 ft 4 in | 3 ft 8 in | 4 ft 5 13/16 in |
| 42 in | 6 | 7 in | 4 ft 2 in | 4 ft 7 in | 5 ft 5 5/16 in |
| 48 in | 7 | 6 7/8 in | 5 ft 0 in | 5 ft 6 in | 6 ft 4 13/16 in |
| 60 in | 8 | 7 1/2 in | 5 ft 10 in | 6 ft 5 in | 7 ft 8 3/16 in |
| 72 in | 10 | 7 3/16 in | 7 ft 6 in | 8 ft 3 in | 9 ft 7 1/4 in |
| 84 in | 11 | 7 5/8 in | 8 ft 4 in | 9 ft 2 in | 10 ft 10 5/8 in |
| 96 in | 13 | 7 3/8 in | 10 ft 0 in | 11 ft 0 in | 12 ft 9 11/16 in |
| 102 in | 14 | 7 5/16 in | 10 ft 10 in | 11 ft 11 in | 13 ft 9 1/4 in |
| 108 in | 14 | 7 11/16 in | 10 ft 10 in | 11 ft 11 in | 14 ft 1 in |
| 114 in | 15 | 7 5/8 in | 11 ft 8 in | 12 ft 10 in | 15 ft 9/16 in |
| 120 in | 16 | 7 1/2 in | 12 ft 6 in | 13 ft 9 in | 16 ft 1/16 in |
| 132 in | 18 | 7 5/16 in | 14 ft 2 in | 15 ft 7 in | 17 ft 11 1/4 in |
| 144 in | 19 | 7 9/16 in | 15 ft 0 in | 16 ft 6 in | 19 ft 2 1/2 in |
Stair building tips
- Measure the finished rise. If 1 in of flooring is going on the upper floor later, the first or last riser will be off by that much.
- Cut one stringer and test it. Set it in place, check every tread with a level, then use it as the pattern for the rest.
- Mind the bottom riser. When you cut a stringer, take the tread thickness off the bottom so the first step isn't taller than the others.
- Plan the headroom early. The floor opening above a stair is usually framed long before the stair is built; check it before the joists go in.
Building a deck stair? Size the deck itself with the deck calculator, footings with the post hole concrete calculator, or check a ramp with the slope calculator.
Frequently asked questions
How many steps do I need for an 8 ft rise?
A 96 in rise divided by the 7¾ in maximum riser is 12.4, so you need 13 risers of 96 ÷ 13 ≈ 7⅜ in. That gives 12 treads; at 10 in each the stair runs 120 in (10 ft) horizontally.
What are the IRC stair rules for risers and treads?
The 2021 International Residential Code allows risers up to 7¾ in (196 mm) and requires treads of at least 10 in (254 mm), measured nosing to nosing. Risers and treads in a flight may vary by no more than ⅜ in, headroom must be at least 6 ft 8 in and the clear width at least 36 in. Local codes may amend these figures.
How do I calculate stringer length?
Use the Pythagorean theorem on the total rise and total run. A 108 in rise with 13 treads of 10 in has a 130 in run, so the stringer is √(108² + 130²) ≈ 169 in, about 14 ft 1 in, along the nosings. Buy a longer board to allow for the end cuts.
What is the 2R + T rule for stairs?
It is a comfort rule of thumb, not a code requirement: twice the riser height plus the tread depth should be about 24 to 25 inches, close to a normal stride. Risers of 7.2 in with 10.5 in treads give 2 × 7.2 + 10.5 = 24.9 in.
How long does the stair opening need to be for headroom?
Add the 6 ft 8 in (80 in) headroom to the thickness of the floor above, multiply by the tread depth and divide by the riser height. With 10½ in of floor, 10 in treads and 7.71 in risers that is (80 + 10.5) × 10 ÷ 7.71 ≈ 117 in, about 9 ft 9 in, measured from the top riser.