Stair Length Calculator

Total run is the horizontal distance from the first riser to the last, measured along the floor. It's what you check against available space in a room or hallway.

Stringer length (also called stringer run or the diagonal) is the actual length of lumber a stringer needs, measured along the slope from the bottom cut to the top cut. It's always longer than the total run because it accounts for rise as well.

Overall footprint adds anything else in the path — a landing, a turn, or clearance at top and bottom — to the total run. This is the number you need for a floor plan or permit drawing.

Total run = number of treads × tread depth. If you have 8 treads at 10½ in each, run is 84 in, or 7 ft.

Stringer length = √(rise² + run²), the same Pythagorean formula you'd use for any right triangle. For a 90 in rise and 84 in run, that's √(90² + 84²) = √(8100 + 7056) = √15156 ≈ 123.1 in.

Footprint = total run + landing depth (typically 36 in minimum) + any extra clearance you're planning for at the bottom, such as a walkway offset.

More risers means a shorter, gentler stair with a longer run, because you're spreading the same rise over more steps. Fewer risers means a steeper stair with a shorter run but often a code problem if the riser height gets too tall.

This is why you can't pick total length in isolation — it's locked to riser height and tread depth, which in turn are constrained by commonly cited IRC reference values (max 7¾ in riser, min 10 in tread for residential stairs) and by comfort rules like rise + run ≈ 17–18 in.

This table uses a 7¼ in riser height and a 10½ in tread depth, scaling the number of risers from 8 to 16, so you can see how quickly run and stringer length grow.

Measure total rise with a level and tape from finished floor to finished floor, not from the subfloor — flooring thickness at top and bottom changes the number by half an inch or more.

If space is tight, work backward: measure the available run first, divide by your target tread depth to see how many treads fit, then check whether the resulting riser height still falls in a reasonable range.

1. Measure total rise (finished floor to finished floor).
2. Pick a target riser height, usually 7 to 7¾ in for residential stairs.
3. Divide rise by target riser and round to the nearest whole number of risers.
4. Recalculate actual riser height = rise ÷ risers, then treads = risers − 1.
5. Multiply treads by tread depth to get total run, then find stringer length with the Pythagorean formula.

Confusing stringer length with total run is the most common one — a lumber order based on total run alone will come up short, sometimes by a foot or more on a steep stair.

Forgetting nosing overhang is another. Nosing adds a bit to each tread's visual depth without changing the stringer layout, so it doesn't belong in the run or stringer formulas, but it does matter for headroom clearance.

If you're cutting an actual 2x12 stringer with a circular saw, the stair stringer calculator gives you layout marks, not just a length. If you only care about the slope angle, the stair angle calculator isolates that one number.