How to calculate a staircase
Stairs start from the total rise — the finished floor-to-floor height. From there:
Steps (risers) = total rise ÷ target riser height (rounded), then riser height = total rise ÷ steps
The number of treads is always one less than the number of risers, because the top floor itself acts as the last "step." Total run is the number of treads times the tread depth.
Comfort and code
| Measurement | Typical target |
|---|---|
| Riser height | 7–7.75 in (lower is easier) |
| Tread depth | 10–11 in minimum |
| Comfort rule | 2 × riser + tread ≈ 24–25 in |
Know the stair terms
- Rise — the total floor-to-floor height; riser is the vertical face of one step.
- Run — the total horizontal distance the stair covers; tread is the part you step on.
- Nosing — the lip where the tread overhangs the riser (about ¾–1¼ in).
- Stringer — the sawtooth board that carries the treads and risers.
- Headroom — the clear height above the stairs (code minimum 6 ft 8 in).
Code essentials (typical IRC)
| Item | Limit |
|---|---|
| Max riser height | 7¾ in |
| Min tread depth | 10 in |
| Min stair width | 36 in |
| Riser/tread variation | ≤ ⅜ in across the flight |
| Handrail height | 34–38 in |
| Min headroom | 6 ft 8 in |
Figures are common IRC values — always confirm against your local code.
Stringers and layout
Most stairs use three stringers (two outside, one center) for a 36-inch width, with more for wider stairs or heavy use. Make sure the total run fits the space before you frame — a steeper stair saves floor space but is harder to climb, and a tall flight (more than about 12 ft of rise) usually needs an intermediate landing.
Good to know
- Keep risers equal. Codes require all risers within about 3/8 in of each other — uneven steps are a trip hazard.
- Mind the headroom and total run. A longer run needs more floor space; check it fits before you frame.
- Confirm local code. Riser/tread limits and handrail rules vary by jurisdiction.
Laying out and cutting stair stringers
Once you know your riser height and tread depth, transfer them to a 2×12 stringer with a framing square and a pair of stair gauges (the little brass buttons that clamp to the square). Set one gauge at your riser height on the tongue and the other at your tread depth on the body, then step the square down the board, marking each triangle. The key gotcha is the first-riser deduction: cut the bottom of the stringer down by the thickness of one tread, so that once the treads are installed, every finished riser — including the first and last — ends up equal. Cut the notches with a circular saw, finish the inside corners with a handsaw, and test-fit before cutting the matching stringers.
Straight, L-shaped, or U-shaped?
The layout you choose affects the run and whether you need a landing:
- Straight. The simplest and cheapest — one run from bottom to top, but it needs the most uninterrupted floor length.
- L-shaped. Turns 90° at a landing, which softens a fall and tucks the stair into a corner.
- U-shaped. Doubles back 180° around a landing, saving floor space in a compact footprint.
Any flight taller than about 12 feet of rise generally needs an intermediate landing, and a landing is a smart break point on long or steep stairs regardless.