Stair Stringer Calculator

Punch in a total rise — the vertical distance from the lower finished floor to the upper one — and a target riser height, and the calculator first solves for the number of risers: total rise divided by target riser, rounded to a whole number. It then divides the rise evenly across that many steps.

From there it builds the notch layout: unit rise, unit run, the number of tread cuts (always one fewer than the number of risers on a stringer that lands flush at the top), the diagonal stringer length, and the angle of the stringer against the floor.

It also handles the two cuts that trip people up: the top plumb cut, which is set back one tread depth from the top riser, and the bottom cut, which drops by one tread thickness so the first step off the floor is the same height as every other step.

Total rise is the single most important number and the one people guess wrong most often. Measure it with a level and tape from the actual finished lower floor to the actual finished upper floor — not the subfloor, not an estimate from a plan.

If flooring hasn't gone down yet, add its thickness to your measured rise on both ends. A ¾ in hardwood floor at the top and 2 in of concrete topping at the bottom both change your total rise before a single stringer is cut.

Riser height and tread depth are targets, not guarantees — the calculator rounds the riser count to a whole number and recalculates the exact riser height so every step in the flight ends up identical.

Riser count = round(total rise ÷ target riser height). Actual riser height = total rise ÷ riser count. Tread count = riser count − 1, because the top tread is the upper floor itself.

Total run = tread count × tread depth. Stringer length (the diagonal, corner to corner) = √(rise² + run²). Stair angle = arctan(rise ÷ run), measured from horizontal.

For a 108 in rise with a 7.25 in target riser: 108 ÷ 7.25 = 14.9, which rounds to 15 risers. Actual riser height becomes 108 ÷ 15 = 7.2 in. With 14 treads at 10.5 in, total run is 147 in, and the diagonal length is √(108² + 147²) ≈ 182.4 in, an angle of about 36.3°.

The top of the stringer gets a plumb cut set back one full unit run from the top riser line, so the stringer's top end sits flush against the header or ledger rather than leaving a triangular overhang.

The bottom of the stringer needs to drop by the thickness of one tread. Without that drop, the first step would be one tread thickness taller than the rest, because the bottom notch normally measures full riser height straight up from the floor, and the tread board sitting on top of every other notch adds thickness that the floor itself doesn't have.

Skip the bottom drop and you'll find out on move-in day when someone catches a toe on the first step going up.

Every notch on the stringer uses the same rise and run — that's the point of dividing the rise evenly instead of just stacking 7¼ in steps until you run out of board.

The framing square gets set to the actual riser height and tread depth for every mark, not the target numbers you originally typed in.

Most stringer failures on inspection trace back to one of these, and they're all avoidable with five minutes of arithmetic before the saw comes out.

• Using the target riser height instead of the recalculated actual riser height for every mark
• Forgetting the bottom drop and ending up with a tall first step
• Measuring rise to the subfloor instead of the finished floor
• Rounding riser count down when it should round up, producing risers that exceed code maximums
• Not accounting for tread thickness when the treads sit on top of the stringer rather than being let into a housed notch

Commonly cited IRC reference values for residential stairs are a maximum 7¾ in riser, minimum 10 in tread, maximum ⅜ in variation between the largest and smallest riser or tread in a flight, and 6 ft 8 in of headroom. Commercial work commonly follows IBC reference values instead: max 7 in riser, min 11 in tread.

Codes vary by state, city, and sometimes by the specific inspector, so treat these as a starting point and confirm with your local authority having jurisdiction before you finalize a design.

If you just need the raw geometry — rise, run, angle, and length — without the notch layout, the stair rise and run calculator gives you that in a simpler form.

Building outdoors instead of indoors? The deck stair calculator and deck stair stringer calculator use the same math but default to composite-friendly tread sizes and pressure-treated board widths.