Stair tread calculator

Tread depth is the structural, nose-to-nose measurement used in every stair calculation: total run, angle, and riser-tread ratio all use this number. It's the horizontal distance your foot actually travels from one step to the next.

Tread board width is bigger, because most treads project a nosing past the face of the riser below. A tread board sized for a 10.5 in tread depth with a 1 in nosing needs to be about 11.5 in wide, not 10.5 in.

Nosing is the lip of the tread that hangs past the riser face. It gives a little extra foot room without adding to the stair's total run, and it's part of why open stringers with visible tread ends look slightly deeper than the math suggests.

Typical nosing projection runs about ¾ in to 1¼ in on residential stairs, though exact limits are set locally. Commonly cited IRC reference values call for a nosing between about ¾ in and 1¼ in on closed-riser stairs with treads under 11 in deep, but codes vary by jurisdiction and your local authority has final say.

Sometimes the constraint isn't a target riser, it's a fixed opening in the floor. If you know your available run and your riser count, divide: tread depth = available run ÷ (risers − 1).

A 130 in stairwell opening with 14 risers gives 13 treads, so 130 ÷ 13 = 10 in per tread, right at the commonly cited IRC minimum. Any less floor space and you'd need to add a landing, steepen the risers, or choose a different stair shape.

1. Confirm your total rise and pick a riser count.
2. Subtract one from the riser count to get the tread count.
3. Divide your available floor run by the tread count for tread depth.
4. Add your chosen nosing projection to get the actual tread board width to buy.

This trips people up constantly: a 15-riser stair has only 14 treads, because the top riser lands you on the upper floor, which acts as the final "tread" without needing a separate board.

Order lumber for the tread count, not the riser count, or you'll end up with one extra board and a stringer layout that doesn't match your material list.

Cut every tread board to the same width, even if your stringers have very slightly different notch depths from saw variation. A stair reads as one consistent plane; a wandering tread edge is far more visible than a wandering riser.

If you're using dimensional lumber like 2x12s for outdoor treads, remember the actual width is 11.25 in, not 12 in, and check that against your target tread depth plus nosing before you buy.

Multiply tread count by stair width to get total lineal footage of tread stock, then divide by your board length to get a board count with reasonable waste allowance.

Example: 14 treads at a 36 in stair width need 14 × 36 in = 504 in, or 42 lineal feet of tread stock. At 8 ft boards that's about six boards before waste.

Commonly cited IRC reference values set a minimum tread depth of 10 in for residential stairs; IBC reference values for most commercial stairs raise that minimum to 11 in. Both are minimums, not targets, and both vary by jurisdiction.

If you're solving primarily for riser height rather than tread layout, use the stair riser calculator. For the full stringer cut list once tread depth is locked in, move to the stair stringer calculator.