How to use the stair stringer calculator
- Measure the total rise — from the top of the finished lower floor (or ground) to the top of the finished upper floor or deck.
- Choose your tread depth (10–11 in / 250–280 mm is typical).
- Pick the code limits that apply: IRC for homes and decks in most of the US, IBC for commercial buildings, or enter your own.
- Use the cut layout to mark each step on the stringer with a framing square.
Stair formulas
Risers = Total rise ÷ Maximum riser → round up Riser height = Total rise ÷ Risers Treads = Risers − 1 Total run = Treads × Tread depth Stringer length = √(Total rise² + Total run²) Angle = arctan(Riser ÷ Tread)
Worked example
A basement stair with a total rise of 105 in and 10.5 in treads:
- 105 ÷ 7.75 = 13.55 → 14 risers
- Riser height = 105 ÷ 14 = 7.5 in
- Treads = 13; total run = 13 × 10.5 = 136.5 in (11 ft 4.5 in)
- Stringer = √(105² + 136.5²) = 172.2 in (14.35 ft) → use 16 ft boards
- Angle = 35.5°; comfort check 2 × 7.5 + 10.5 = 25.5 in
Measuring the total rise correctly
Measure to finished surfaces. If the floor at the bottom will get tile, carpet or a concrete pad later, add that thickness to your plan. For deck stairs, measure straight down from the top of the deck boards to the landing spot, and check level ground at the bottom — sloping ground changes the rise. If the ground slopes, pour or set a level landing pad first and measure to that. A mistake of even half an inch shows up as one odd-height step, which is exactly the kind of step people trip on.
Key IRC stair rules (US homes)
| Item | Requirement |
|---|---|
| Maximum riser height | 7¾ in (197 mm) |
| Minimum tread depth | 10 in (254 mm) |
| Variation between risers or treads | No more than ⅜ in (9.5 mm) |
| Nosing (if treads under 11 in) | ¾ to 1¼ in |
| Minimum width | 36 in |
| Minimum headroom | 6 ft 8 in |
| Handrail | Required with 4 or more risers |
Always confirm with your local building department — some areas amend these rules.
Tips for cutting stringers
- Use 2 × 12 lumber for stringers so at least 5 in (127 mm) of wood remains below the cuts.
- Use stair gauges on a framing square set to the riser and tread to mark each step accurately.
- Drop the stringer: cut the bottom of the stringer down by one tread thickness so the first step isn’t taller than the rest.
- Space stringers no more than about 16 in apart for 5/4 decking treads, closer for composite.
- Cut one, test it, then copy it for the other stringers.