Stair Calculator
Find how many steps a stair needs, how tall each riser is, the tread depth, the total run, and the length of the stringers. Enter the height from floor to floor, and the calculator lays out risers of equal height in inches and fractions or in millimeters.
Measure from the finished floor below to the finished floor above.
Default: 7 3/4 in (US IRC maximum)
Default: 10 in (US IRC minimum)
- Choose US or Metric. You can still change the unit of any single field.
- Enter the Total rise, the height from the finished floor below to the finished floor above. Measure to the floor surface, not the top of a carpet. If the flooring is not down yet, add the thickness of the flooring above and subtract the thickness of the flooring below.
- Check the Max riser height. It starts at 7 3/4 in, the US code maximum, or 190 mm in Metric. The calculator uses the fewest risers that keep every riser at or below it. Lower it to get more, shorter steps.
- Choose Set tread depth to use a tread depth you pick, which starts at 10 in or 260 mm, or Fit to total run to divide the space you have between the treads.
- Read the layout. Riser check shows what the risers add up to when you mark each one at the rounded height.
Fields accept whole numbers, decimals, fractions, and mixed numbers, such
as 95, 112.5, 3/4, or
7 3/4.
How to calculate stairs
Every riser in a flight should be the same height, so the calculator divides the total rise into equal risers, then adds the treads:
Risers = total rise ÷ max riser height, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1, total run = tread depth × treads
Stringer length = √(total rise² + total run²)
There is one fewer tread than risers because the upper floor is the last step. The stringer length is the diagonal from the bottom of the first riser to the top of the last one. Swanson Tool suggests adding 12 to 18 inches for the cuts when you buy the board. The stair angle is the slope of a line along the tread nosings, atan(riser height ÷ tread depth).
Riser height and tread depth
The riser height is measured from the top of one tread to the top of the next, and the tread depth from the front edge of one tread to the front edge of the next. A tread board with a nosing that overhangs the riser below is deeper than the tread depth by the overhang.
Swanson Tool calls a riser of 7 to 7 3/4 inches the sweet spot for a house, and gives three rules of thumb for a comfortable stair:
- Riser + tread = 17 to 18 in
- Riser × tread = 72 to 76 in²
- 2 × riser + tread = 24 to 25 in
US code limits for stairs
The 2024 International Residential Code (IRC) sets these limits for stairs in houses:
| Measurement | IRC limit |
|---|---|
| Riser height | 7 3/4 in (196 mm) or less |
| Tread depth | 10 in (254 mm) or more |
| Difference within one flight | 3/8 in (9.5 mm) or less between the tallest and shortest riser, and between the deepest and shallowest tread |
The IRC is a model code: US states and cities adopt an edition of it, sometimes an earlier one, often with their own changes. The calculator adds a note when the layout falls outside these limits. Staying inside them does not mean a stair meets the code, which also covers headroom, handrails, nosings, and more. The rules also differ outside the US, such as in the UK, Canada, and Australia. Check the code where you build.
The riser height is rounded to 1/16 in, so marking every riser at the rounded height can leave the last one a little off. Riser check shows the difference. Swanson Tool takes a small difference off the bottom riser. If the difference is more than 3/8 in, spread it out by making some risers 1/16 in taller or shorter. Flooring added after the stair is built also changes the first and last risers, which is why the total rise is measured to the finished floors.
Examples
A basement stair
A total rise of 95 in with a max riser height of 7 3/4 in needs 95 ÷ 7.75 = 12.26, rounded up to 13 risers of 7 5/16 in (95 ÷ 13 = 7.31 in). With 10 in treads, the 12 treads make a total run of 10 ft, and the stringer is 12 ft 9 1/16 in. Thirteen risers of 7 5/16 in add up to 95 1/16 in, so take 1/16 in off the bottom riser.
Fitting a set space
For the same stair in a total run of 122 in, each of the 12 treads is 122 ÷ 12 = 10.17 in, or 10 3/16 in to the nearest 1/16 in. Twelve treads of 10 3/16 in add up to 122 1/4 in. The stringer is √(95² + 122²) = 12 ft 10 5/8 in.
Metric dimensions
A total rise of 2,700 mm with a max riser height of 190 mm needs 15 risers of 180 mm. With 260 mm treads, the 14 treads make a total run of 3,640 mm, and the stringer is 4,532.1 mm.
FAQ
A total rise of 8 ft (96 in) with a max riser height of 7 3/4 in needs 13 risers of about 7 3/8 in, and 12 treads.
In the US, the IRC allows risers up to 7 3/4 in and treads of at least 10 in. Swanson Tool calls a riser of 7 to 7 3/4 inches the sweet spot for a house.
The stringer is the diagonal of the total rise and total run: √(total rise² + total run²). Buy a board 12 to 18 inches longer to leave room for the cuts at the top and bottom.
At the IRC limits of a 7 3/4 in riser and a 10 in tread, a stair is about 37.8°. Lower risers or deeper treads make it less steep.
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