An 8 ft deck (96 in total rise) needs about 13 risers at ~7.38 in each — that is 12 treads — not twelve neat 8-inch steps that feel fine on paper and wrong on the last cut.
The short count shows up when you notch the first 2×12. You divided rise by 8, skipped the max-riser check, and the top landing sat proud. Someone drove back for another board while the deck crew waited.
The stair step formula is:
risers = ceil( total rise ÷ max riser ) · tread count = risers − 1
Measure finished-floor to finished-floor. Pick a max riser you will not exceed (often about 7¾ in in US residential practice). Divide, round up, then set every riser equal: total rise ÷ riser count. Treads = risers minus one on a straight flight that lands on a finished surface. Estimates only — not engineering or building-code advice.
| Total rise | Risers | Riser ht | Treads | Stringer |
|---|---|---|---|---|
| 36 in (3 ft) | 5 | 7.20 in | 4 | ~4 ft 6 in |
| 48 in (4 ft) | 7 | 6.86 in | 6 | ~6 ft 5 in |
| 60 in (5 ft) | 8 | 7.50 in | 7 | ~7 ft 8 in |
| 72 in (6 ft) | 10 | 7.20 in | 9 | ~9 ft 7 in |
| 84 in (7 ft) | 11 | 7.64 in | 10 | ~10 ft 11 in |
| 96 in (8 ft) | 13 | 7.38 in | 12 | ~12 ft 10 in |
How many steps for an 8 ft deck?
Total rise is 96 in. At a 7¾ in max, ceil(96 ÷ 7.75) = 13 risers. Each riser is 96 ÷ 13 ≈ 7.38 in. Treads on that flight = 12. With a 10 in unit run, total run = 120 in and the stringer is about √(96² + 120²) ≈ 153.7 in (~12 ft 10 in) before you pick stock length at the yard.
Divide by 8 and you get 12 risers of exactly 8 in — over a common max and a trip hazard on the last step if one riser drifts. Equal risers beat a round number that fails the max.
The formula, on a tape
Work in inches (or millimetres — same math). US decks often stay in inches; AU/UK/NZ crews can toggle mm in the app.
- Total rise = finished upper floor − finished lower floor (include finishes)
- Riser count = ceil(total rise ÷ max riser)
- Riser height = total rise ÷ riser count (all equal)
- Tread count = riser count − 1 (straight flight to a landing/floor)
- Total run = tread depth × tread count
- Stringer length ≈ √(total rise² + total run²)
Example: 36 in porch rise. ceil(36 ÷ 7.75) = 5 risers → 7.2 in each → 4 treads. At 10 in going, stringer ≈ 53.8 in.
In Stair Stringer Master you enter total rise once — optional total run or going per tread — and get step count, riser height, tread depth, stringer length, angle, and a left/right cut list on your phone. Free for one straight flight; Pro unlocks extras at $9.99. Inches default; mm toggle for metric regions.
Three mistakes that ruin the stringer
Dividing by a nice 8 in. Round numbers feel clean. Max-riser rules and equal risers do not care. Ceil against your max, then divide so every step matches.
Mixing risers and treads. People say “12 steps” and buy twelve treads for twelve risers. On a straight flight to a floor, tread count is usually one less than riser count. Order lumber from the stringer math, not the nickname.
Forgetting finishes — or unit run. Carpet, decking, or a tile thickness changes total rise. A shallow tread shortens total run and steepens the angle. Measure finished surfaces; set going before you mark the 2×12.
Risers vs treads — the naming trap
Risers are the vertical faces (how many times you lift your foot). Treads are the horizontal boards you stand on. Search “how many steps” usually means riser count in conversation and tread count at the lumber desk — say both numbers out loud before you buy.
Worked example for a 96 in deck stair
Rise 96 in → 13 equal risers at ~7.38 in → 12 treads. At 10 in unit run, total run 120 in → stringer ~153.7 in. Buy stock long enough for that hypotenuse plus waste for the crown and cuts. For the count plus printable layout on your phone, open Stair Stringer Master. Related: Stair Stringer Master review, how many deck boards, how many studs.
Estimates only — not engineering or building-code advice. Measure finished rise. Keep risers equal. Round the count up against your max.
Open Stair Stringer MasterEstimate only — not engineering or building-code advice.