This stair calculator works out the number of risers and treads, the riser height, the total run, an approximate stringer length and the stair angle from the total rise you measure. It also checks the layout against a common comfort rule and common residential code limits, so you can plan basement, deck or attic stairs before cutting a single stringer.
How to Use the Stair Calculator
- Measure the total rise in inches. This is the vertical distance from the finished floor at the bottom to the finished floor at the top, not to the subfloor.
- Enter the maximum riser height you want to allow. The default of 7.75 inches matches a common residential code limit. A lower number gives more, gentler steps.
- Enter the tread depth in inches. This is the horizontal run of each step, not counting any nosing. The default of 10 inches is a common minimum.
- Read the number of risers and treads, the exact riser height, total run, stringer length and angle.
- Check the note under the results. It tells you whether the layout falls inside the common code limits.
How the Calculation Works
The total rise is divided by the maximum riser height and rounded up to get the number of risers. The total rise is then divided by that number to get the actual riser height, so every step is exactly the same. Treads are one fewer than risers, because the top step is the upper floor itself.
Total run is the number of treads times the tread depth. The stringer length is the diagonal of a right triangle with the total rise and total run as its two sides: the square root of rise squared plus run squared. The stair angle comes from the same triangle. The comfort check adds two riser heights to one tread depth. A result between 24 and 25.5 inches is marked as comfortable. The code note checks that the riser is no more than 7.75 inches and the tread is at least 10 inches.
Worked example with the default values: a 108 inch total rise (9 feet), 7.75 inch maximum riser and 10 inch treads.
- Risers: 108 / 7.75 = 13.94, rounded up to 14 risers
- Riser height: 108 / 14 = 7.71 inches, about 7 11/16 inches
- Treads: 14 – 1 = 13 treads
- Total run: 13 x 10 = 130 inches, or 10 feet 10 inches
- Stringer length: square root of (108 x 108 + 130 x 130) = square root of 28,564 = 169 inches, about 14 feet 1 inch
- Stair angle: about 39.7 degrees
- Comfort check: 2 x 7.71 + 10 = 25.4 inches, inside the 24 to 25.5 inch range
- Code check: 7.71 inches is under 7.75 and the tread is 10 inches, so it passes the common limits
Stair Layouts for Common Total Rises
These results use the default 7.75 inch maximum riser and 10 inch tread. They are useful for a quick check of how much floor space a stair will take.
| Total rise | Risers | Riser height | Treads | Total run | Stringer (approx.) |
|---|---|---|---|---|---|
| 24 in (2 ft) | 4 | 6.00 in | 3 | 30 in | 3.2 ft |
| 36 in (3 ft) | 5 | 7.20 in | 4 | 40 in | 4.5 ft |
| 48 in (4 ft) | 7 | 6.86 in | 6 | 60 in | 6.4 ft |
| 60 in (5 ft) | 8 | 7.50 in | 7 | 70 in | 7.7 ft |
| 72 in (6 ft) | 10 | 7.20 in | 9 | 90 in | 9.6 ft |
| 96 in (8 ft) | 13 | 7.38 in | 12 | 120 in | 12.8 ft |
| 108 in (9 ft) | 14 | 7.71 in | 13 | 130 in | 14.1 ft |
| 120 in (10 ft) | 16 | 7.50 in | 15 | 150 in | 16.0 ft |
Tips and Common Mistakes
- Always confirm local code. The 7.75 inch riser and 10 inch tread limits are common in US residential codes, but your local building department sets the actual rules. Headroom, handrails, guards, nosing and landings all have their own requirements, and many stair projects need a permit.
- Measure to finished floors. Flooring added later changes the top or bottom step. A thick tile floor at the bottom makes the first riser shorter than the rest, which is a common trip hazard.
- Measure the rise in more than one place. Floors are not always level. Measure the rise where the stair will actually land, using a level and a straight board if the lower floor slopes.
- Buy longer stringer stock. The stringer length is the plain diagonal. You need extra for the plumb cut at the top and the level cut at the bottom, so buy lumber at least one size longer than the result.
- Allow for tread thickness. When laying out a cut stringer, the bottom riser is usually shortened by the thickness of one tread so all steps finish at the same height.
- Check the total run fits. Make sure the space at the bottom of the stair has room for the full run plus a clear landing before you build.
Frequently Asked Questions
How many steps do I need for a 9 foot rise?
A 108 inch rise with a 7.75 inch maximum riser needs 14 risers of about 7 11/16 inches and 13 treads. With 10 inch treads the stair runs about 10 feet 10 inches.
What is the 2R + T rule?
It is a comfort rule of thumb: two riser heights plus one tread depth should land around 24 to 25.5 inches. Stairs in that range tend to match a natural walking stride.
Why are there fewer treads than risers?
The last step up lands on the upper floor, which acts as the final tread. So a stair with 14 risers has 13 treads you build.
How long should a stair stringer be?
Roughly the square root of rise squared plus run squared, which the calculator gives as the approximate stringer length. Add extra for the top and bottom cuts and buy the next standard lumber length.
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