Stair Stringer Calculator
Calculate stair stringer length, how many stringers you need, and total lumber from total rise, riser height, tread depth, and stair width.
How this calculator works
The stringer is the sloped board the treads sit on. Its length is the hypotenuse of the total rise and run; the number of stringers depends on the stair width. The calculator finds equal risers under your limit, then derives the stringer geometry.
Formula: Stringer length = √(rise² + run²). Number of stringers = ⌊(width in ÷ 16)⌋ + 1. Total lumber = stringer length × number of stringers.
Worked example
A 44-inch total rise, 7.75-inch max riser, 10-inch treads, 3-foot-wide stair:
- Steps: 6, riser height ≈ 7.33 in, total run = 50 in
- Stringer length: √(44² + 50²) ÷ 12 ≈ 5.55 ft
- Number of stringers (36 in wide): ⌊36 ÷ 16⌋ + 1 = 3
- Total stringer lumber: 5.55 × 3 ≈ 16.65 ft (buy three 2×12 at 8 ft)
Reference (stringers by width)
| Stair width | Stringers (16 in max) |
|---|---|
| 36 in | 3 |
| 48 in | 4 |
| 60 in | 4 |
How to measure
Measure total rise floor-to-floor and choose your riser and tread from the Stair Calculator. Measure the finished stair width to get the stringer count, and buy 2×12 stock a foot longer than the calculated length.
Stair framing is code-governed. Verify riser, tread, and stringer sizing with local code before cutting.
Frequently asked questions
How long does a stair stringer need to be?
The stringer is the diagonal hypotenuse of the total rise and total run, so it's longer than either. For a 44-inch rise and 50-inch run it's about 5.55 feet — but always buy stringer stock a foot or so longer to allow for the top and bottom cuts.
How many stringers do I need?
Space stringers no more than 16 inches apart for most decking, plus one to close the far side. A 36-inch-wide stair needs three stringers; a 48-inch stair needs four. Heavier loads or thinner treads need tighter spacing.
What size lumber are stringers cut from?
Stringers are typically cut from 2×12 stock, because notching for the treads and risers removes a lot of material and a 2×12 leaves enough solid wood behind the notches. Never cut stringers from a 2×8 or 2×10 for a full flight.
Should I use notched or solid stringers?
Notched (cut) stringers carry the treads on top of the cuts and are standard for decks and porches. Solid stringers with cleats or routed housings are used where you don't want to weaken the board. This calculator sizes the board length either way.
How do I lay out the notches?
Use a framing square set to your riser height on one leg and tread depth on the other, and step it down the board once per step. The Stair Calculator gives the exact riser and tread to set on the square.
Estimates only. Verify quantities with your supplier before purchasing.