Stair Calculator
Work out the number of steps, riser height, total run, stringer length and angle for a straight stair, with US code checks.
Results
| Item | Value |
|---|
Quick Answer
Divide the total rise by the maximum riser height and round up to get the number of steps. For a 108 in rise with 7¾ in risers, you need 14 risers of 7.71 in and 13 treads. With 10 in treads, the total run is 130 in (10 ft 10.0 in).
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Table of Contents
- Stair Calculator
- Quick Answer
- Related Calculators
- How This Stair Calculator Works
- How to Use the Stair Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Rise vs Run
- Steps for Common Floor Heights
- US Code Basics (IRC R311.7)
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Stair Calculator Works
Measure the total rise from the finished floor below to the finished floor above. The calculator splits it into equal risers no taller than your maximum, then works out the run, stringer length and angle.
Quick explanation: risers = total rise ÷ max riser height, rounded up.
- Riser: the height of each step.
- Tread: the depth you step on.
- Treads: one fewer than risers, as the top floor is the last step.
How to Use the Stair Calculator
- Measure the total rise from the finished floor below to the finished floor above, in inches.
- Enter the tallest riser you want. The IRC limit for homes is 7¾ in.
- Enter the tread depth you plan to use. The IRC minimum is 10 in.
- Press Calculate.
- Read the number of steps, riser height, total run, stringer length, angle and code checks.
Core Formulas
- Risers: n = round up (total rise ÷ max riser)
- Riser height: total rise ÷ n
- Treads: n − 1
- Total run: treads × tread depth
- Stringer length: √(rise² + run²)
- Angle: arctan(rise ÷ run)
Key Terms
- Riser: the vertical part of a step, or its height.
- Tread: the flat part you step on.
- Nosing: the front edge of a tread that sticks out past the riser.
- Stringer: the sloped board that supports the treads and risers.
- Total rise and total run: the full height and full horizontal length of the stair.
Worked Example
A deck is 42 in above the ground. You want risers up to 7¾ in and 11 in treads.
- Risers = 42 ÷ 7.75 = 5.42, rounded up to 6
- Riser height = 42 ÷ 6 = 7.00 in
- Treads = 5, so total run = 5 × 11 = 55 in
- Stringer length = √(42² + 55²) = 69.20 in
Real-Life Examples
- Basement stairs: a 96 in rise with 7¾ in risers needs 13 risers of 7.38 in. With 10 in treads, the run is 120 in (10 ft 0.0 in).
- Garden steps: a 30 in slope with gentle 6 in risers needs 5 risers. With deep 14 in treads, the run is 56 in.
- Short on space: if the run must stay under 100 in for a 108 in rise, 10 in treads give 130 in, which is too long. You may need a landing or a turn.
Rise vs Run
- Rise: the vertical height of each step and of the whole stair.
- Run: the horizontal depth of each tread and of the whole stair.
- The balance: taller risers make a shorter but steeper stair. Deeper treads make a longer but easier one.
- Comfort rule: 2 × riser + tread between 24 and 25 in suits most people’s stride.
Steps for Common Floor Heights
Using a maximum riser of 7¾ in.
| Total rise | Risers | Riser height | Run (10 in treads) |
|---|---|---|---|
| 18 in | 3 | 6.00 in | 20 in |
| 30 in | 4 | 7.50 in | 30 in |
| 42 in | 6 | 7.00 in | 50 in |
| 60 in | 8 | 7.50 in | 70 in |
| 96 in | 13 | 7.38 in | 120 in |
| 108 in | 14 | 7.71 in | 130 in |
| 120 in | 16 | 7.50 in | 150 in |
US Code Basics (IRC R311.7)
- Riser height: 7¾ in maximum.
- Tread depth: 10 in minimum, measured nose to nose.
- Even steps: risers and treads should not vary by more than ⅜ in.
- Headroom: at least 6 ft 8 in.
- Width: at least 36 in.
Your local code may differ. Always check before you build.
Common Mistakes to Avoid
- Measuring to the subfloor. Measure between finished floor surfaces, including flooring.
- Uneven first or last step. All risers should match within ⅜ in.
- Forgetting headroom. Leave at least 6 ft 8 in above the stairs.
- Counting treads as steps. There is one fewer tread than risers.
- Skipping the permit. Many areas need a permit and inspection for new stairs.
Frequently Asked Questions
Divide the total rise by the maximum riser height and round up. That gives the number of risers.
A 108 in rise with risers up to 7¾ in needs 14 risers of about 7.71 in.
The IRC allows up to 7¾ in for homes. Commercial codes are usually stricter.
The top floor acts as the last step, so it does not need a tread.
A common rule is 2 × riser + tread = 24 to 25 in.
It depends on the stair width and tread material. Many builders use stringers about 16 in apart, so a 36 in stair often uses three. Check your local code.
Run a level board or string from the top landing out over the steps, then measure straight down to where the bottom step will land.
Use the stringer length from the calculator, then add extra for cutting and fixing. It is √(rise² + run²).
References & Sources
| Source | Used For | Link |
|---|---|---|
| International Code Council, 2021 IRC Section R311.7 Stairways | US residential stair rules. | https://codes.iccsafe.org/content/IRC2021P2/chapter-3-building-planning |
| Wikipedia, Stairs | Stair parts and design rules. | https://en.wikipedia.org/wiki/Stairs |