Stair Calculator
Precision geometry for safe and comfortable construction.
Stair Calculator
Building a staircase that's comfortable to walk up and meets code requires getting the rise and run just right. This stair calculator does the geometry for you — so you can cut lumber confidently instead of guessing.
What Does This Calculator Do?
Enter the total rise (floor-to-floor height) and your desired step dimensions, and the stair calculator returns the number of steps, individual rise and run measurements, total stringer length, and stair angle.
Key Terms to Know
- Rise: The vertical height of each step.
- Run: The horizontal depth of each step (tread depth).
- Total Rise: The total vertical distance from bottom to top landing.
- Stringer: The diagonal board that supports the steps.
How to Use This Calculator
- Measure the total rise in inches (floor to floor).
- Enter your desired individual rise (typically 7–7.75 inches).
- Enter your desired run (typically 10–11 inches).
- Click Calculate to see the step count and stringer length.
Stair Formula
Number of Steps = Total Rise ÷ Individual Rise (round to nearest whole number)
Individual Rise = Total Rise ÷ Number of Steps
Stringer Length = √(Total Rise² + Total Run²)
Example Calculation
Total rise = 108 inches (9 feet). Desired individual rise ≈ 7.5 inches.
- Steps = 108 ÷ 7.5 = 14.4 → round to 14 steps
- Adjusted rise = 108 ÷ 14 = 7.71 inches per step
- With a 10.5-inch run, total run = 14 × 10.5 = 147 inches
- Stringer length = √(108² + 147²) ≈ 182 inches (15.2 ft)
Building Code Guidelines
- Maximum rise per step: 7¾ inches
- Minimum run per step: 10 inches
- Minimum headroom: 6 ft 8 in
- Maximum variation between steps: 3/8 inch
Common Mistakes to Avoid
- Ignoring the finished floor thickness — if you're adding flooring at top or bottom, account for that thickness in your total rise.
- Making steps inconsistent — each step must be within 3/8 inch of the others or it's a tripping hazard.
- Forgetting the top tread — the landing itself serves as the top tread; don't add an extra step at the top.
- Not checking local building codes — rise/run requirements vary by region and building type.
Frequently Asked Questions
What is the ideal rise and run for stairs?
The most comfortable stairs have a 7-inch rise and 11-inch run. The rule of thumb: rise + run should equal 17–18 inches.
How do I calculate stairs for a deck?
Measure from the deck surface to the ground. Enter that as your total rise and use the same formula. Deck stairs typically use a slightly larger run for comfort.
How long should my stair stringers be?
Use the Pythagorean theorem: √(total rise² + total run²). The calculator does this for you automatically.
How many steps do I need for an 8-foot ceiling?
96 inches ÷ 7.5 = ~13 steps with a 7.38-inch rise each.
Do stairs need a handrail?
Most building codes require a handrail for any staircase with 4 or more steps. Check your local code for specifics.
Conclusion
Stairs are one of the most precise carpentry projects — a fraction of an inch off across multiple steps adds up to a real problem. Use this stair calculator to nail your measurements before you make a single cut.
Related: Roofing Calculator | Concrete Calculator | Square Footage Calculator
Carpenter's Pro-Tip
Don't forget to "drop the stringer." Cut the very bottom of the stringer shorter by the thickness of your tread material so the first step height matches all the others.
