Stair Calculator
By Taro Schenker | Updated February 11, 2026
Quick Answer: To calculate stairs, divide Total Rise by desired rise per step (7-7.5 in ideal) and round to get the number of risers. Treads = risers minus 1. Stringer length = square root of (Total Rise squared + Total Run squared). IRC code: max rise 7.75 in, min run 10 in, min width 36 in. Comfort checks: Rise + Run = 17-18 in, 2R + T = 24-25 in, angle 30-37 degrees. For a 9 ft (108 in) rise at 7.5 in desired, you get 14 risers at 7.71 in actual rise.
Calculate how many steps you need, the rise and run per step, stringer length, and stair angle. Enter your total rise (floor-to-floor height) and desired dimensions to get precise measurements with automatic IRC building code compliance checks.
How to Calculate Stairs
The key formulas for stair calculations are:
- Measure the total rise — the vertical distance from the lower finished floor to the upper finished floor.
- Divide the total rise by the desired rise per step (7-7.5 inches is ideal) and round to the nearest whole number. This gives you the number of risers.
- Divide the total rise by the number of risers to get the actual rise per step.
- The number of treads is always one less than the number of risers (the top floor acts as the final tread).
- Multiply the number of treads by the run per step to get the total run (horizontal footprint).
- Use the Pythagorean theorem to calculate stringer length from total rise and total run.
IRC Building Code Requirements
The International Residential Code (IRC) sets minimum standards for residential stairs. These are the key requirements:
| Dimension | IRC Requirement | Notes |
|---|---|---|
| Maximum Rise | 7.75 inches | Per step, 7-7.5 in is ideal |
| Minimum Run | 10 inches | Per step, measured nose to nose |
| Minimum Width | 36 inches | Clear width above handrail |
| Minimum Headroom | 6 ft 8 in | Measured vertically from nosing line |
| Handrail Height | 34-38 inches | Measured from stair nosing |
| Nosing | 0.75-1.25 inches | Overhang beyond riser face |
Stair Dimension Guide
Quick reference for common total rises and the resulting stair dimensions using a 7.5-inch desired rise:
| Total Rise | Risers | Actual Rise | Total Run (10 in run) |
|---|---|---|---|
| 8 ft (96 in) | 13 | 7.38 in | 10 ft 0 in |
| 9 ft (108 in) | 14 | 7.71 in | 10 ft 10 in |
| 10 ft (120 in) | 16 | 7.50 in | 12 ft 6 in |
| 12 ft (144 in) | 19 | 7.58 in | 15 ft 0 in |
Tips for Building Stairs
- Measure twice: Always measure the total rise from finished floor to finished floor. Account for any flooring that has not been installed yet.
- Consistency matters: All risers must be within 3/8 inch of each other per the IRC. Inconsistent rises are a leading cause of stair falls.
- Stringer material: Use construction-grade 2x12 lumber for stringers. Avoid boards with large knots near the cut lines.
- Minimum throat: After cutting, the thinnest part of the stringer (the throat) should be at least 3.5 inches thick for structural integrity.
- Subtract tread thickness: When laying out your stringers, subtract the tread thickness from the bottom riser so all visible rises are equal.
- Check for level: Use a long level to verify that both the top and bottom landing surfaces are level before cutting your stringers.
Formula Summary
Number of Risers: Divide the total rise by the desired rise per step and round to the nearest whole number.
$$N_{\text{risers}} = \text{round}\left(\frac{\text{Total Rise}}{\text{Desired Rise}}\right)$$
Actual Rise: $$R_{\text{actual}} = \frac{\text{Total Rise}}{N_{\text{risers}}}$$
Number of Treads: $$N_{\text{treads}} = N_{\text{risers}} - 1$$
Total Run: $$\text{Total Run} = N_{\text{treads}} \times \text{Run per step}$$
Stringer Length (Pythagorean): $$S = \sqrt{\text{Total Rise}^2 + \text{Total Run}^2}$$
Stair Angle: $$\theta = \arctan\left(\frac{\text{Total Rise}}{\text{Total Run}}\right)$$
| IRC Code Check | Requirement |
|---|---|
| Max Rise per Step | 7.75 in (min 4 in) |
| Min Run per Step | 10 in |
| Min Stair Width | 36 in |
| Rise + Run | 17-18 in (comfort) |
| 2R + T (Blondel) | 24-25 in (comfort) |
| Stair Angle | 30-37 degrees (comfort) |
Standard Assumptions: Stringers spaced every 16 inches of stair width with a minimum of 2. Ideal rise per step is 7-7.5 inches. Number of treads is always one less than number of risers (top floor acts as final tread). All risers must be within 3/8 inch of each other per IRC.
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Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.