Stair calculator
Find the number of steps, riser height, tread depth, total run and stringer length for a straight stair, checked against the IRC.
Finished floor to finished floor.
The calculator uses the fewest risers that stay at or under this height.
Measured level, nosing to nosing.
Riser count and stair width
Leave empty to use the maximum riser height.
Clear width. Used only for the rule check.
Stair layout
- Risers
- 1413 treads: the upper floor is the last step
- Riser height
- 7.5 inNearest 1/16 in: 7 1/2 in
- Tread depth
- 10 in
- Total run
- 10 ft 10 in130 in
- Stringer length
- 13 ft 11 1/8 in167.11 in on the diagonal, before the end cuts
- Stair angle
- 36.87°
Checked against the 2024 International Residential Code, R318.7
- Riser height 7 3/4 in or lessThis stair: 7.5 inPass
- Tread depth 10 in or moreThis stair: 10 inPass
- Rise of one flight 12 ft 7 in or lessThis stair: 8 ft 9 inPass
- Width 36 in or moreThis stair: 36 inPass
Straight flight only. Headroom, nosings, handrails and guards are not checked, and local amendments can differ.
Estimate only. Check product labels and local building rules before you buy or build.
Quick answer
Divide the total rise by the tallest riser you accept and round up to get the number of risers, then divide the total rise by that number for the riser height. A 105 in floor-to-floor height at the 7 3/4 in IRC limit needs 14 risers of 7 1/2 in, and 13 treads of 10 in take 130 in of floor. The stringer diagonal is √(105² + 130²) = 167.11 in.
How to use the stair calculator
- Measure the total rise from the finished floor at the bottom to the finished floor at the top. If a floor will get new flooring, allow for its thickness first.
- Keep the maximum riser height at 7 3/4 in, the International Residential Code limit, or lower it for an easier stair. The calculator picks the fewest risers that stay at or under it.
- Set the tread: type a tread depth, or choose Available run, type the floor space you have and let the calculator share it between the treads.
- Read the risers, riser height, total run, stringer length and angle, then the pass or fail list under the results. Open "Riser count and stair width" to force a number of risers or to check the width.
How do you calculate stair risers and treads?
Divide the total rise by the largest riser you accept and round up: that is the fewest risers that keep every step at or under the limit. Then divide the total rise by that whole number to get the riser height, so every riser comes out the same.
A straight stair has one tread fewer than it has risers, because the upper floor is the last step. The total run is the number of treads times the tread depth, measured nosing to nosing. The stringer length shown is the diagonal from the foot of the first riser to the edge of the upper floor; the board you cut it from has to be longer to leave room for the cuts at each end.
risers = total rise ÷ maximum riser height, rounded upriser height = total rise ÷ riserstreads = risers − 1tread depth from available run = available run ÷ treadstotal run = treads × tread depthstringer length = √(total rise² + total run²)stair angle = arctan(riser height ÷ tread depth)What are the code rules for stair risers and treads?
The International Residential Code covers houses, townhouses and the stairs to their decks; the International Building Code covers other buildings. The 2024 IRC puts the stair rules in R318.7, with the same figures as R311.7 in the 2021 edition. Cities and counties adopt these codes with their own amendments, so the local building department has the final say, and a licensed professional should check any structural stair work.
| Requirement | House (IRC R318.7) | Other buildings (IBC 1011) |
|---|---|---|
| Riser height | 7 3/4 in (196 mm) or less | 4 to 7 in (102 to 178 mm) |
| Tread depth, nosing to nosing | 10 in (254 mm) or more | 11 in (279 mm) or more |
| Largest minus smallest riser or tread in a flight | 3/8 in (9.5 mm) or less | 3/8 in (9.5 mm) or less |
| Nosing projection | 3/4 to 1 1/4 in; none needed with treads of 11 in or more | 1 1/4 in (32 mm) or less |
| Headroom | 6 ft 8 in (2032 mm) | 80 in (2032 mm) |
| Clear width | 36 in (914 mm) above handrail height | 44 in (1118 mm), or 36 in for fewer than 50 occupants |
| Rise of one flight | 12 ft 7 in (3835 mm) or less | 12 ft (3658 mm) or less |
At and below handrail height the IRC allows a clear width of 31 1/2 in with a handrail on one side and 27 in with handrails on both sides.
How many steps do I need for my floor height?
Worked out with a 7 3/4 in maximum riser and 10 in treads. The total run changes with the tread depth you choose; the number of risers does not.
| Total rise | Risers | Riser height | Treads | Total run with 10 in treads |
|---|---|---|---|---|
| 24 in (2 ft) | 4 | 6 in | 3 | 30 in (2 ft 6 in) |
| 30 in (2 ft 6 in) | 4 | 7.5 in (7 1/2 in) | 3 | 30 in (2 ft 6 in) |
| 36 in (3 ft) | 5 | 7.2 in (7 3/16 in) | 4 | 40 in (3 ft 4 in) |
| 48 in (4 ft) | 7 | 6.86 in (6 7/8 in) | 6 | 60 in (5 ft) |
| 60 in (5 ft) | 8 | 7.5 in (7 1/2 in) | 7 | 70 in (5 ft 10 in) |
| 72 in (6 ft) | 10 | 7.2 in (7 3/16 in) | 9 | 90 in (7 ft 6 in) |
| 96 in (8 ft) | 13 | 7.38 in (7 3/8 in) | 12 | 120 in (10 ft) |
| 102 in (8 ft 6 in) | 14 | 7.29 in (7 5/16 in) | 13 | 130 in (10 ft 10 in) |
| 105 in (8 ft 9 in) | 14 | 7.5 in (7 1/2 in) | 13 | 130 in (10 ft 10 in) |
| 108 in (9 ft) | 14 | 7.71 in (7 11/16 in) | 13 | 130 in (10 ft 10 in) |
| 112 in (9 ft 4 in) | 15 | 7.47 in (7 7/16 in) | 14 | 140 in (11 ft 8 in) |
| 116 in (9 ft 8 in) | 15 | 7.73 in (7 3/4 in) | 14 | 140 in (11 ft 8 in) |
| 120 in (10 ft) | 16 | 7.5 in (7 1/2 in) | 15 | 150 in (12 ft 6 in) |
| 132 in (11 ft) | 18 | 7.33 in (7 5/16 in) | 17 | 170 in (14 ft 2 in) |
| 144 in (12 ft) | 19 | 7.58 in (7 9/16 in) | 18 | 180 in (15 ft) |
Fractions are rounded to the nearest 1/16 in. Cut to the decimal figure where you can, so the risers add up to the total rise.
Worked examples
Basement stair with a 105 in total rise
The finished basement floor is 105 in below the finished floor above. At the 7 3/4 in maximum, 105 ÷ 7.75 = 13.55, which rounds up to 14 risers. Each riser is 105 ÷ 14 = 7 1/2 in.
With 10 in treads, the 13 treads take 13 × 10 = 130 in, or 10 ft 10 in, of floor. The stringer diagonal is √(105² + 130²) = 167.11 in, about 13 ft 11 1/8 in, and the stair angle is arctan(7.5 ÷ 10) = 36.87°. All four IRC checks in the calculator pass with a 36 in wide stair.
Deck stair that has to fit in 36 in
The top of the decking is 30 in above the patio, and the stair can reach no more than 36 in out from the deck. 30 ÷ 7.75 = 3.87 rounds up to 4 risers of 7 1/2 in, with 3 treads.
Type 30 in as the total rise, choose Available run and type 36 in. Each tread is 36 ÷ 3 = 12 in, the stringer diagonal is √(30² + 36²) = 46.86 in, about 3 ft 10 7/8 in, and the angle is 32.01°. With treads of 11 in or more the IRC does not require a nosing.
Why measure the total rise between finished floors?
Because flooring changes only the top or bottom riser, and the IRC lets the tallest and shortest riser in a flight differ by no more than 3/8 in. A stair laid out between bare subfloors fails that rule if 3/4 in hardwood later goes on the upper floor and nothing goes on the lower one: the top riser ends up 3/4 in taller than the rest.
Measure from finished floor to finished floor, or add the planned flooring thickness before you type the total rise. For a deck stair, measure from the top of the decking down to the surface the stair will land on.
Questions
What is the maximum stair riser height?
The maximum riser height in a house is 7 3/4 inches (196 mm) under section R318.7.5.1 of the 2024 International Residential Code. Stairs in other buildings fall under the International Building Code, which requires risers from 4 to 7 inches and treads of at least 11 inches. In both codes the tallest and shortest riser in one flight may differ by no more than 3/8 inch.
How many steps do I need for a 9 ft rise?
A 9 ft (108 in) rise needs 14 risers of about 7 11/16 in each: 108 ÷ 7.75 = 13.94 rounds up to 14, and 108 ÷ 14 = 7.71 in. That stair has 13 treads, so with 10 in treads it takes 130 in, or 10 ft 10 in, of floor. You climb 14 steps to reach the upper floor.
Why does a stair have one fewer tread than risers?
A straight stair has one tread fewer than risers because the upper floor acts as the last tread. A stair with 14 risers has 13 treads, so its total run is 13 times the tread depth. Counting 14 treads is a common layout mistake that makes the stair one tread longer than it needs to be.
How long is a stair stringer?
The stringer diagonal is the square root of the total rise squared plus the total run squared. For a 105 in rise and a 130 in run, √(105² + 130²) = 167.11 in, about 13 ft 11 1/8 in. Buy a board longer than the diagonal, because the plumb cut at the top and the level cut at the bottom extend past it.
How much headroom does a stair need?
A stair in a house needs at least 6 ft 8 in (2032 mm) of headroom under the IRC, measured straight up from the sloped line along the tread nosings and from the floor of each landing. The International Building Code asks for 80 in, the same height. The calculator does not check headroom, so measure the opening above the stair separately.
How wide do stairs have to be?
A stair in a house must be at least 36 in (914 mm) wide above handrail height under the IRC. At and below the handrail, the clear width may drop to 31 1/2 in with a handrail on one side and 27 in with handrails on both sides. Stairs in other buildings need 44 in, or 36 in when fewer than 50 people use them.
Sources
- International Code Council, 2024 International Residential Code, Chapter 3 Building Planning, R318.7 Stairways (checked October 19, 2026)Riser, tread, uniformity, nosing, width, headroom and flight rise limits for houses.
- International Code Council, 2021 International Residential Code, Chapter 3 Building Planning, R311.7 Stairways (checked October 19, 2026)The same stair figures under the earlier section number.
- International Code Council, 2024 International Building Code, Chapter 10 Means of Egress, Section 1011 Stairways (checked October 19, 2026)Riser, tread, uniformity, nosing, width, headroom and flight rise limits for other buildings.
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