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This stair calculator works out how many steps you need between two floor levels. Enter the total rise, the maximum riser height allowed and your tread depth to get the number of risers and treads, the exact riser height, the total run, the stringer length and the stair angle, with a check against common building code limits.
How to Use the Stair Calculator
- Measure the total rise: the vertical distance from the finished lower floor to the finished upper floor or deck surface.
- Enter the maximum riser height your code allows. The US International Residential Code (IRC) allows 7¾ inches; UK private stairs allow up to 220 mm.
- Enter the tread depth (the going). The IRC requires at least 10 inches; UK private stairs require at least 220 mm.
- Read the number of risers, treads, total run, stringer length and angle.
Stair Formulas
Number of risers = Total rise ÷ Maximum riser height (rounded up)
Riser height = Total rise ÷ Number of risers
Number of treads = Number of risers − 1
Total run = Number of treads × Tread depth
Stringer length = √(Total rise² + Total run²)
Worked Example
- Total rise: 108 in (9 ft floor to floor)
- 108 ÷ 7.75 = 13.9, so 14 risers
- Riser height: 108 ÷ 14 = 7.71 in
- Treads: 13 at 10 in = 130 in of total run
- Stringer: √(108² + 130²) = 169 in, about 14.1 ft
- Angle: about 39.7°
Common Code Limits
| Rule | US IRC (residential) | UK Approved Document K (private stair) |
|---|---|---|
| Maximum riser | 7¾ in (197 mm) | 220 mm |
| Minimum tread / going | 10 in (254 mm) | 220 mm |
| Maximum pitch | Not stated directly | 42° |
| Other | Risers and treads may vary by no more than ⅜ in within a flight | 2 × rise + going between 550 and 700 mm |
These are common limits only. Local codes, handrail and headroom rules and requirements for commercial stairs differ, so confirm with your building department before you cut stringers.
Tips for Building Stairs
- Measure the rise to the finished floor surfaces, including flooring thickness.
- Keep every riser the same height; uneven steps are a trip hazard.
- Buy stringer stock at least 2 ft longer than the calculated stringer length.
- Building deck stairs? Pair this with the deck calculator.
Frequently Asked Questions
How many steps do I need for a 9 foot rise?
With a 7¾ inch maximum riser, a 108 inch rise needs 14 risers of about 7.7 inches and 13 treads.
What is the maximum stair riser height?
The US IRC allows 7¾ inches for homes. UK rules allow up to 220 mm for private stairs. Always check your local code.
How do I calculate stringer length?
Use the Pythagorean theorem: the square root of total rise squared plus total run squared.
What is a comfortable stair angle?
Most residential stairs fall between about 30° and 37°. Steeper stairs feel more like ladders and may not meet code.
Read the guide: How to Calculate Stairs: Rise, Run and Stringer Length
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