Stair Calculator
Calculate steps, rise, run, and stringer length for decks, basements, and interior staircases.
Your Stair Measurements
- Measure total rise — the vertical height from the lower floor or ground to the upper floor or deck surface.
- Choose tread depth — 11 inches is standard for residential stairs. IRC code requires a minimum of 10 inches.
- Check rise per step — IRC maximum is 7¾ inches per riser. If your result exceeds this, increase the number of steps.
- Use stringer length to buy lumber — purchase 2×12 boards at least this long, plus 10% for waste and end cuts.
- Count your stringers — two stringers for stairs up to 30 inches wide; add a center stringer beyond that.
How to Calculate Stair Dimensions
Every stair calculation starts with two measurements: total rise (the vertical height the stairs must climb) and tread depth (how deep each step is). Divide the total rise by your desired rise-per-step to get the number of steps. From there, total run, stringer length, and stair angle all follow from basic geometry.
The IRC building code limits riser height to a maximum of 7¾ inches and requires treads to be at least 10 inches deep. All risers in a single flight must be within ⅜ inch of each other — so always divide total rise evenly across the number of steps rather than rounding individual risers.
Stair Dimensions by Application
Typical rise and run values by stair type:
| Application | Rise per Step | Tread Depth | Angle |
|---|---|---|---|
| Interior residential | 7–7¾ in (178–196mm) | 10–11 in (254–279mm) | 33–38° |
| Deck / exterior | 6–7 in (152–178mm) | 11–12 in (279–305mm) | 27–33° |
| Basement | 7–7¾ in (178–196mm) | 10–11 in (254–279mm) | 33–38° |
| Gentle / accessible | 5–6 in (127–152mm) | 12–14 in (305–356mm) | 20–27° |
| Commercial IBC | 4–7 in (102–178mm) | 11 in min (279mm min) | — |
Shallower stairs are more comfortable but need more horizontal space. Steeper stairs save floor space but are harder on the knees — especially for elderly users.
Stair Quick Reference — 96 in (8 ft) Total Rise
| Tread Depth | Steps | Rise/Step | Total Run | Stringer Length |
|---|---|---|---|---|
| 10 in (254mm) | 13 | 7.38 in | 130 in (10'10") | 163 in (13'7") |
| 11 in (279mm) | 13 | 7.38 in | 143 in (11'11") | 174 in (14'6") |
| 12 in (305mm) | 13 | 7.38 in | 156 in (13'0") | 185 in (15'5") |
| 10 in (254mm) | 14 | 6.86 in | 140 in (11'8") | 171 in (14'3") |
| 11 in (279mm) | 14 | 6.86 in | 154 in (12'10") | 183 in (15'3") |
Stringer length = √(Total Rise² + Total Run²). Buy lumber 10% longer than calculated to allow for end cuts.
Stair Formulas
Standard stair geometry:
Rise per step = Total Rise ÷ Number of Steps
Total run = Number of Steps × Tread Depth
Stringer length = √(Total Rise² + Total Run²)
Stair angle = arctan(Total Rise ÷ Total Run)
Comfort rule: 2 × Rise + Run = 24–25 inches. For a 7-inch rise, the ideal tread is 10–11 inches. This ratio produces a natural walking stride.
Related Calculators
Stairs are just one part of the build. See our full construction calculator collection for concrete, roofing, and framing tools.
Frequently Asked Questions
What is the maximum riser height allowed by building code?
The International Residential Code (IRC) sets the maximum riser height at 7¾ inches (196mm) and the minimum at 4 inches. All risers in a single flight must be within ⅜ inch of each other. Local codes may be stricter — always verify with your local building department before construction.
How many stringers do I need?
Use two stringers for stairs up to 30 inches wide. Add a third center stringer for widths between 30 and 36 inches. For stairs wider than 36 inches, space intermediate stringers no more than 16 inches apart. Stringers are typically cut from 2×12 lumber.
What lumber do I need for stair stringers?
Stringers are cut from 2×12 dimensional lumber (actual size 1½" × 11¼"). Buy boards 2–3 feet longer than your calculated stringer length to allow for end cuts and to ensure enough solid wood remains at each notch after cutting.
How do I calculate stringer length?
Stringer length uses the Pythagorean theorem: √(Total Rise² + Total Run²). For example: 84 inches of rise and 110 inches of run gives √(7,056 + 12,100) = √19,156 ≈ 138 inches. Buy lumber slightly longer and trim to the exact cut angle.
How do I measure total rise and total run?
Total rise is the vertical height from the finished floor at the bottom to the finished floor at the top. Total run is the horizontal distance available for the stair. Once you have those, the calculator can estimate riser and tread layout.