2x12 Stair Stringer Calculator

Like all dressed lumber, a 2x12 is smaller than its name: actual dimensions are 1½ in by 11¼ in. That 11¼ in is the entire budget you have to work with once a triangular notch is cut out of it for each step.

A 2x10 is actually 9¼ in wide — two full inches less material to spare below the notch, which matters a lot once the notch itself gets deep.

Throat depth (sometimes called the remaining depth) is the strip of uncut wood left below the deepest point of the notch — the inside corner where the riser cut meets the tread cut. That strip is the only thing carrying the load down to the next joist or ledger below it.

Cut the notch too deep relative to the board width and that remaining strip gets thin enough to crack or flex under foot traffic, especially with a rim of only a couple inches left at the corner.

A commonly cited minimum for the remaining throat on a notched sawn stringer is around 5 in, though this varies by code and jurisdiction — always confirm with your local authority having jurisdiction rather than treating it as a hard universal number.

A rough way to estimate remaining throat: throat ≈ board width − (unit rise × unit run ÷ unit diagonal length). For a 7.2 in rise, 10.5 in run, the unit diagonal is √(7.2² + 10.5²) ≈ 12.7 in, so the notch depth term is (7.2 × 10.5) ÷ 12.7 ≈ 5.95 in.

On a 2x12 (11.25 in wide): 11.25 − 5.95 ≈ 5.3 in of throat remaining — right around that commonly cited 5 in minimum, and fine for most residential geometry.

On a 2x10 (9.25 in wide) with the same rise and run: 9.25 − 5.95 ≈ 3.3 in remaining — under the commonly cited minimum, which is exactly why 2x10 stringers only work on shallow-rise, shallow-run stairs and fail once the stair gets steeper.

As risers get taller and treads get deeper, the notch grows and eats further into the board. Past a certain point — commonly somewhere around a 7½–8 in riser with a 10 in-plus tread — a 2x10's remaining throat drops below what most jurisdictions will accept for a notched sawn stringer.

There's no universal exact cutoff because it depends on the specific rise, run, and local code minimum, which is exactly why running your actual numbers through a throat-depth check matters more than following a rule of thumb from a different stair.

For most residential stairs, three stringers spaced roughly 16 in on center handle stairs up to about 36 in wide with standard wood treads. Composite decking and thinner tread stock often call for tighter spacing — commonly cited around 12–16 in on center, but this depends on the specific product's span rating, so check the manufacturer's data.

Wider stairs, heavier tread material, or longer unsupported spans between stringers may need a fourth stringer down the middle, especially on open (cut) stringers where the outer stringers alone carry more visible load.

If throat depth on a notched stringer doesn't work for your rise and run, a closed or housed stringer sidesteps the problem entirely. Instead of cutting triangular notches out of the board's edge, a closed stringer has dadoes routed into its inner face, and the full board width stays intact along the outer edge.

This is more work — each dado has to be routed to the exact riser and tread thickness, often with a wedge-and-glue joint — but it removes the throat-depth question completely since no material is cut away from the outer profile.

For the full notch layout on whichever board size passes your throat check, go back to the main stair stringer calculator.

Building a metal stair instead of wood? The steel stair stringer calculator covers channel and plate stringer geometry, which doesn't have this throat-depth limitation.