How to Lay Out Stair Stringers: Rise, Run and Code Limits

Start with the total rise

Everything on a stringer comes from one measurement: the total rise, finished floor to finished floor. Not subfloor to subfloor. If the upper floor is getting 3/4 in. hardwood and the lower level is bare slab, those are different numbers, and a stringer cut to the wrong one gives you a top or bottom step that is off by the thickness of the flooring. Measure with a level and a straightedge if the two floors are not plumb over each other, because a sloped slab adds up over a 12 ft run. The stair stringer calculator turns that one number into the riser count, exact riser height, total run and stringer length, and checks them against the code.

Riser height, riser count and tread depth

The 2021 IRC (R311.7) sets the limits for one- and two-family dwellings. Your local code may amend them, and commercial stairs under the IBC are stricter, so confirm with the authority having jurisdiction.

Pick a target riser, then divide the total rise by it and round up. Rounding down makes each riser taller than your target and can push it past 7-3/4 in. The actual riser height is the total rise divided by that whole number of risers, and every riser in the flight gets that same number. A flight has one fewer tread than risers, because the upper floor is the last tread.

Tread depth is measured horizontally from nosing to nosing, which is the same as the riser-face-to-riser-face run you cut on the stringer. So a 10-1/2 in. run cut gives a 10-1/2 in. code tread, and the tread board itself is wider by the nosing: 11-1/2 in. with a 1 in. overhang.

Run, stringer length and the comfort rule

Total run = treads × tread depth. Check the floor space: a 105 in. rise with 10-1/2 in. treads uses 136-1/2 in. of floor before the landing, and the 2021 IRC (R311.7.6) wants a landing at least 36 in. deep in the direction of travel. Then check comfort with the two rules carpenters have used for a century: riser + tread = 17 to 18 in., or 2 × riser + tread = 24 to 25 in. A basement stair at 7-1/2 and 10 comes in at 17-1/2 and 25; a main stair at 7 and 11 comes in at 18 and 25. A 7-3/4 riser with a 10 in. tread, the code maximum and minimum together, is legal and feels like a ladder.

The stringer length along the cut edge is Pythagoras: √(total rise² + total run²). That tells you whether a 12, 14 or 16 ft 2×12 covers it; you lose a little at each end to the plumb and level cuts.

Marking, cutting and dropping the stringer

Use a straight 2×12 with no big knots along the cut edge. Clamp stair gauges on the framing square: the riser height on the tongue, the tread run on the body. Set the square on the edge of the board with both gauges touching, and trace the two legs. Slide the square along until the riser line meets the end of the tread you just drew, and trace again. Repeat for every riser, and count again before you cut. The gauges are what keep every step identical; marking freehand is how you end up with the 3/8 in. variation an inspector finds with a tape.

Cut to the line with a circular saw and finish the inside corners with a handsaw or jigsaw. Overcutting weakens the throat, the narrowest solid wood behind the notches. On a 2×12 with a 7-1/2 in. rise and 10-1/2 in. run the throat is about 5 in. The American Wood Council's deck guide (DCA 6) calls for at least 5 in. of throat on a cut stringer, no more than 6 ft of unsupported horizontal span, and cut stringers no more than 18 in. apart. Those are sound minimums for interior stairs too.

Lay out the stringer with equal rises and then drop it: cut the tread thickness off the bottom. Once a tread is nailed onto the first step, that step grows by the tread thickness, while the top step, whose tread is the upper floor itself, does not. Skip the drop and the bottom step is one tread thickness too tall and the top step the same amount too short, the most common way a flight fails the 3/8 in. rule. If the lower floor will get finish flooring after the stairs are set, subtract that thickness from the drop. Square the top cut to whatever carries it, a hanger board or stringer hangers, and keep the bottom off a slab with a pressure-treated kicker.

Worked example

Total rise measures 105 in. Target riser 7-1/2 in.: 105 ÷ 7.5 = 14, so 14 risers at exactly 7.5 in., under the 7-3/4 in. limit. That makes 13 treads. At 10-1/2 in. each the total run is 13 × 10.5 = 136-1/2 in., and riser plus tread is 18 in., at the top of the comfort band. Stringer length = √(105² + 136.5²) = √(11,025 + 18,632) = 172.2 in., a hair over 14 ft 4 in., so you buy 16 ft 2×12s. With 1 in. treads, drop the bottom of each stringer by 1 in.

Check it before you nail it

Set the first stringer in place, tack it, and measure every rise with a scrap of tread on the bottom step. Stringers cut from the same layout should stack and match edge for edge. BuildSmith carries the stair, rafter and concrete calculators on the job site without a signal, and the building inspector practice exam covers the IRC stair and guard questions if you are studying for a certification.