Stair Stringer Calculator
Enter the total rise to get the number of steps, exact riser height, total run, stringer length and angle. It also tells you how many 2×12 stringers to buy and checks the design against US residential code limits.
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Risers = total rise ÷ 7¾ in, rounded up, and treads = risers − 1. With a 105 in rise that is 14 risers of 7½ in and 13 treads. At 10 in treads the run is 130 in and each stringer is about 13 ft 11 in long, so buy 3 × 2×12 at 16 ft for a 36 in wide stair.
How to use this stair stringer calculator
- Measure the total rise from the finished floor (or ground pad) at the bottom to the finished floor or deck surface at the top. Use finished surfaces, not subfloor.
- Set the maximum riser. 7¾ in is the US residential maximum; 7–7½ in feels more comfortable.
- Enter the tread depth (the run of each step, without nosing) and the stair width.
- Choose stringer spacing to suit the tread material.
- Read the results and code checks. Adjust the riser or tread until every line shows a tick.
Risers and run by total rise
| Total rise | Risers | Riser height | Treads | Run at 10 in treads |
|---|---|---|---|---|
| 24 in (deck) | 4 | 6 in | 3 | 30 in |
| 36 in (deck) | 5 | 7.2 in | 4 | 40 in |
| 48 in (deck) | 7 | 6⅞ in | 6 | 60 in |
| 105 in (8 ft ceiling) | 14 | 7½ in | 13 | 130 in |
| 117 in (9 ft ceiling) | 16 | 7.31 in | 15 | 150 in |
Stair code limits (US IRC, residential)
| Rule | Limit |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Variation within a flight | ⅜ in maximum between the largest and smallest riser or tread |
| Nosing | ¾–1¼ in where treads are under 11 in |
| Width | 36 in minimum |
| Headroom | 6 ft 8 in minimum |
| Handrail | Required with 4 or more risers, 34–38 in above the nosings |
Commercial buildings and some local codes are stricter, so check with your building department before you cut.
Comfortable stairs: the 2R + T rule
A stair feels natural when twice the riser plus the tread is 24–25 in. A 7½ in riser with a 10 in tread scores 25 in. A 7 in riser with an 11 in tread also scores 25 in, at a gentler angle that suits main stairs. Steep combinations above about 37° feel like a ladder.
Stringers: how many and what lumber
- 2×12 for cut stringersAfter notching, a solid band of wood must remain below the cuts. Many deck guides ask for at least 5 in.
- SpacingAbout 16–18 in for 2× lumber treads, which means 3 stringers for a 36 in stair. Composite treads usually need 12 in, which means 4.
- Bottom riserCut the bottom of the stringer short by one tread thickness so every step ends up the same height.
- OutdoorsUse pressure-treated lumber rated for ground contact, and land the stringers on concrete, not soil.
Worked example
- Total rise 105 in ÷ 7¾ in
- 13.5, so 14 risers
- Exact riser height
- 105 ÷ 14 = 7½ in
- Treads (14 − 1) at 10 in
- 13 treads, 130 in total run
- Stringer length √(105² + 130²)
- 167⅛ in (13 ft 11⅛ in)
- 36 in wide, 2× treads
- 3 × 2×12 at 16 ft
Frequently asked questions
How do you calculate stair stringers?
Risers = total rise ÷ max riser, rounded up. Treads = risers − 1. Run = treads × tread depth. Stringer length = √(rise² + run²).
How many stairs do I need for an 8 foot ceiling?
With about 105 in floor to floor, 14 risers of 7½ in and 13 treads.
What is the maximum riser height?
7¾ in under the US IRC, with a 10 in minimum tread and no more than ⅜ in variation in a flight.
How many stringers do I need for 36 inch wide stairs?
Three with 2× lumber treads, or four for composite treads at 12 in spacing.
What size lumber is used for stair stringers?
2×12, so enough solid wood remains below the cuts. Use pressure-treated lumber outdoors.
What this calculator assumes
- A straight flight where the upper floor forms the top step, so treads = risers − 1.
- Stringer length is the straight-line distance from floor to floor. Stock length adds one tread depth and rounds up to the next even length.
- Code checks use US IRC residential limits. Local codes can differ.