🪜 Stair Calculator
Plan stairs from the total rise: number of risers and treads, riser height, total run, stringer length and angle, checked against residential code limits.
Total rise is measured from the finished floor below to the finished floor above. Tread depth here is the horizontal run of each step, not counting the nosing.
108" ÷ 15 risers = 7.200" each. 14 treads × 10.5" = 147.00" of run. Each tread board is 11 1/2" deep including the nosing. First riser cut on the stringer: 6 3/16" (riser minus tread thickness).
Checks
| ✓ Riser height | 7 3/16" (limit 7.75" max) |
|---|---|
| ✓ Tread depth | 10 1/2" (limit 10" min, measured nosing to nosing) |
| ✓ Comfort rule 2R + T | 24.90" (a rule of thumb: about 24–26" feels natural) |
| ✓ Stair angle | 36.3° (most comfortable around 30–37°) |
Limits shown are from the International Residential Code (R311.7) used as the model for most U.S. homes; your local code may differ, and commercial stairs follow stricter rules. Headroom of at least 6′8″ and a stair width of at least 36″ are also required. Check with your building department before building.
What Stair Calculator Does
This stair calculator lays out a straight run of stairs from one measurement — the total rise from finished floor to finished floor. It works out how many risers you need, the exact height of each, the number of treads, the total run, the stringer length and the stair angle, and draws the staircase to scale so you can see the profile.
Results are given in inches and sixteenths, ready for a tape measure, and checked against the U.S. model residential code limits (risers no taller than 7¾ inches, treads at least 10 inches deep) and a common comfort rule of thumb. If your target riser would exceed the limit, the calculator adds a riser automatically.
How to Use Stair Calculator
- Measure the total rise from finished floor to finished floor
- Enter a target riser height and the tread depth
- Add the nosing overhang and tread thickness
- Read the risers, treads, total run, stringer length and angle
- Check each value against the code and comfort checks
Formula Used by Stair Calculator
Risers, treads and run
Risers = round(total rise ÷ target riser) · riser height = total rise ÷ risers · treads = risers − 1 · run = treads × tread depth
Worked example
A 108-inch rise, 7¼-inch target riser, 10½-inch treads.
- 108 ÷ 7.25 = 14.9 → 15 risers
- 108 ÷ 15 = 7.2 in each
- 14 treads × 10.5 = 147 in of run
Result: 15 risers of 7 3/16", 14 treads, 12′ 3″ of run.
Stringer length
Stringer = √(rise² + run²)
Worked example
108-inch rise and 147-inch run.
- 108² + 147² = 33,273
- √33,273 = 182.4 in
Result: About 15′ 2 3/8″ — buy a longer board to allow for the end cuts.
U.S. Residential Stair Requirements (IRC Model Code)
Summary of the International Residential Code as commonly adopted; local amendments apply.
| Dimension | Requirement |
|---|---|
| Riser height | 7¾ inches maximum |
| Tread depth | 10 inches minimum (nosing to nosing) |
| Nosing projection | ¾ to 1¼ inches where required |
| Headroom | 6 feet 8 inches minimum |
| Stair width | 36 inches minimum |
| Handrail height | 34 to 38 inches above the nosings |
| Riser variation | ⅜ inch maximum between the largest and smallest |
How to Read Your Result
Every riser the same
People set their stride on the first few steps, so a riser that is even a fraction of an inch different is a trip hazard — which is why codes limit the variation to ⅜ inch. Measure the total rise carefully after the finished flooring is known, and remember the first-riser adjustment for tread thickness.
Steep vs shallow
Shorter risers with deeper treads make an easier, safer stair but take more floor space. If the run does not fit, a landing that turns the stair 90° or 180° can help — calculate each straight flight separately.
Limitations & Accuracy Notes
- Straight stairs only; winders, spirals and landings need separate calculations.
- Code limits shown are the U.S. residential model code; commercial buildings and some jurisdictions are stricter.
- Structural sizing of stringers and connections is not included — check with your building department.