Stair Stringer Calculator

Stair stringer calculator: length, count, and lumber

A stair stringer calculator builds the geometry from your total rise. It divides the rise into equal steps that stay under your maximum riser, finds the total run from the tread depth, and gets the stringer length as the diagonal: √(rise² + run²). For a 44-inch rise with 10-inch treads, that is 6 steps and a 5.55 ft stringer.

Stair framing is code-governed. Check riser and tread limits and the minimum remaining wood at the notch with your local code before you cut.

The formula in plain words

Think of stringers as the sloped spine that every step hangs from. The stair rises and runs like a right triangle, and the stringer is the long side of that triangle.

• Steps = total rise ÷ max riser, rounded up
• Riser height = total rise ÷ steps
• Total run = (steps − 1) × tread depth
• Stringer length = √(rise² + run²) ÷ 12, in feet
• Number of stringers = ⌊width in inches ÷ 16⌋ + 1
• Total lumber = stringer length × number of stringers

Worked examples

1. Deck stairs: 44 in rise, 7.75 in max riser, 10 in tread, 3 ft wide

• Steps: 44 ÷ 7.75 = 5.68, rounded up to 6
• Riser: 44 ÷ 6 = 7.33 in
• Run: (6 − 1) × 10 = 50 in
• Stringer: √(44² + 50²) = 66.6 in = 5.55 ft
• Stringers: ⌊36 ÷ 16⌋ + 1 = 3
• Lumber: 5.55 × 3 = 16.65 ft of 2×12, so three 8 ft boards

2. Taller deck: 60 in rise, 7.5 in max riser, 10 in tread, 4 ft wide

• Steps: 60 ÷ 7.5 = 8, riser exactly 7.5 in
• Run: 7 × 10 = 70 in
• Stringer: √(60² + 70²) = 92.2 in = 7.68 ft
• Stringers: ⌊48 ÷ 16⌋ + 1 = 4
• Lumber: 7.68 × 4 = 30.7 ft, so four 8 ft boards just fit, but ten-footers leave margin

In example 2 the stringer is 7.68 ft, so an 8 ft board leaves only a few inches for the cuts. Most builders step up to a 10 ft board.

Stringers by stair width

Stair width	Stringers (16 in max)
30 in	2
36 in	3
48 in	4
60 in	4
72 in	5

How to measure

• Total rise: Measure from the finished landing at the bottom to the top of the deck surface, not to the framing.
• Finished surfaces: If the bottom landing is concrete or pavers, measure to that surface, not to the dirt.
• Tread depth: Pick a depth that suits the tread boards you will use. Many US model codes call for at least 10 in; check local code.
• Max riser: Many US model codes allow up to 7¾ in, but local rules vary.
• Width: Measure the finished width of the stair, outside edge to outside edge.

Common mistakes

• Unequal risers. The first and last risers must match the rest, so cut the bottom of the stringer shorter by the tread thickness.
• Counting treads as steps. A stair with 6 risers has 5 treads of run.
• Buying the exact length. Cuts and the connection to the deck rim consume extra board.
• Choosing too small a board. Notches remove a large share of the width, so use deep stock like 2×12.
• Forgetting the footing. The bottom of the stringers needs a solid landing, not soft dirt.

Checking the layout before you cut

Two quick checks catch most errors. First, steps times riser height must equal the total rise: 6 × 7.33 = 44 in. Second, treads times tread depth must equal the run: 5 × 10 = 50 in.

Then check the footprint. A 50 in run is about 4.2 ft of horizontal space, and the bottom of the stringers needs a solid pad to rest on. Mark the first stringer on the board, cut it, and test it in place before you cut the others.

Related calculators

The stair calculator finds the exact riser and tread pair to mark on your square. For the deck the stairs attach to, use the deck joist calculator and the deck calculator. Poured stairs have their own tool, the concrete stairs calculator.