Stair stringer layout: rise, run, and cutting with a framing square (IRC limits + worked example)
The riser-count formula (total rise / 7.75, round up), a full 58-inch deck example (8 risers at 7-1/4, 7 treads at 10-1/2), the bottom-riser drop most first-timers miss, and framing-square gauge settings.
For a deck 58" above grade you need 8 risers at 7-1/4" and 7 treads at 10½", cut from a 10-ft 2×12 (the stringer measures about 93.6" tip to tip). The math is one division: total rise ÷ 7.75, round up — that's your riser count — then divide back to get the exact riser height. Treads are always one fewer than risers, because the deck or floor itself is the top "tread." Lay it out with a framing square and stair gauges, and drop the bottom of the stringer by one tread thickness before you cut.
The math
Total rise is measured finished floor to finished floor (or finished deck to landing) — not to the framing, not to the dirt where a concrete pad will go. Then:
Riser count = total rise / 7.75, ROUNDED UP Actual riser = total rise / riser count Tread count = riser count - 1 Total run = tread count x tread depth Stringer = sqrt(total rise^2 + total run^2) Rounding UP is what keeps every riser under the 7-3/4" code max. Never round down.
Why divide back after rounding? Because code doesn't just cap the riser at 7-3/4" — it caps the variation at 3/8". You can't take a 58" rise as seven 7-3/4" steps plus one 3-3/4" runt at the bottom. Every riser must be identical, so the total rise gets divided evenly across the count.
Worked example: 58-inch deck
Total rise: 58" (deck surface to grade/landing pad)
Riser count: 58 / 7.75 = 7.48 -> round UP = 8 risers
Actual riser: 58 / 8 = 7.25 -> 7-1/4" each (under 7-3/4" ✓)
Treads: 8 - 1 = 7 treads at 10-1/2"
(two 5/4x6 deck boards + a small nose overhang;
10-1/2" clears the 10" code minimum ✓)
Total run: 7 x 10.5 = 73.5" horizontal
Stringer: sqrt(58^2 + 73.5^2) = sqrt(3364 + 5402) = 93.6"
-> an 8-ft 2x12 (96") technically fits.
Buy a 10-footer: you need square ends, room to
dodge knots, and margin for the layout.Eight risers also means this staircase needs a graspable handrail (4+ risers triggers it) and the landing at the bottom should be a solid, level pad — a poured concrete pad is the standard answer.
The bottom-riser drop (don't skip this)
This is the #1 rookie mistake in stair building, and it's why so many first attempts fail the 3/8" variation rule. When you cut the stringer with every notch identical, then install treads on top, every step gains one tread thickness — except the bottom rise, which starts from the ground. Result: the first step is one tread thickness taller than all the others. With a 1" (5/4) tread, that's a 1" variation — nearly triple the legal limit, and enough to trip people in real life.
The fix: cut one tread thickness off the bottom of the stringer (rip it off the level bottom cut) so the first rise comes out equal once treads are on. At the top, run the same check in reverse — measure from the finished deck surface, and account for the finished tread on the top notch, so the last rise onto the deck matches the rest. Measure the finished assembly, not the bare stringer.
Laying it out with a framing square
The framing square is the whole tool kit here. Buy a pair of stair gauges (the little brass buttons, a few dollars at any hardware store) — they clamp to the square and turn a repetitive layout into a fence you just slide down the board.
- Clamp a stair gauge at 7-1/4" (the riser) on the tongue — the short, narrow leg.
- Clamp the second gauge at 10½" (the run) on the blade — the long, wide leg.
- Lay the 2×12 across sawhorses, crown up. Set the square on the edge so both gauges ride the same edge of the board, and trace the outside of the square: one riser line, one tread line.
- Slide the square down, register the gauges so the new tread line starts exactly where the last riser line ended, and trace again. Step it down the board 8 times for 8 risers.
- Mark the plumb cut at the top (bears against the deck rim) and the level cut at the bottom (sits on the pad) — then mark the tread-thickness drop on the bottom cut before anything touches a saw.
- Cut to the lines with a circular saw, then stop at the inside corners and finish each notch with a handsaw or jigsaw. Overcutting the corners with the circ saw slices into the throat and weakens the stringer.
Picking the board matters too: choose a straight 2×12 (SYP or SPF) with few knots in the throat area — a knot in the uncut strip below the notches is a hinge waiting to happen. Knots up in the notch zone get cut away and don't matter much.
How many stringers
- 3 stringers minimum for 36"-wide stairs with 2× lumber treads — one each side, one center.
- 16" on center for composite treads (Trex, TimberTech, etc.) — composite spans far less than wood, so a 36" stair gets 4 stringers. Check the tread manufacturer's span chart.
- Cut all stringers from the first one as a template — trace it, don't re-layout. Re-measuring each board is how you get three subtly different staircases in one stair.
Comfort check
Code sets the limits; these rules of thumb set the feel. A stair that passes both walks well:
Rule 1: riser + tread = 17" to 18"
Rule 2: (2 x riser) + tread = 24" to 25"
Our example: 7.25 + 10.5 = 17.75" ✓
(2 x 7.25) + 10.5 = 25.0" ✓If your numbers land outside those windows, adjust the tread depth (the run is yours to choose; the rise is dictated by the height) — a deeper tread with a shallow riser, or vice versa, until the sums land in range.
Let the app do the division
The BuilderCalc stair calculator takes your total rise in feet-inches and returns the riser count, exact riser height, tread count, total run, and stringer length — with the IRC checks (7-3/4" max riser, 10" min tread) applied automatically, so the layout numbers you clamp into the stair gauges are right before you touch the 2×12. Free, offline, on the App Store and Google Play.
Related
- Deck joist span and spacing (the deck these stairs land against)
- How many bags of concrete (for the landing pad)
- All free calculators
- BuilderCalc — step-by-step DIY project system + calculators
Note: Stairs are one of the most code-inspected assemblies in residential construction, and IRC R311.7 is a model code — local amendments change riser limits (some allow 8-1/4"), guard requirements, and throat-reinforcement details. Verify with your local building department before cutting. Reviewed 2026-07.