Stair Stringer Calculator
One measurement — total rise — and the layout falls out: riser count and exact height, treads, run, stringer length, plus the comfort check that makes stairs feel right.
Verified against independent hand calculation · sources cited below
4 risers of 7.50″
- Treads to cut
- 3 × 10.0″
- Total run
- 30.0″
- Stringer length (min)
- 42.4″
- Comfort (2R + T)
- 25.0″
Defaults follow common US residential limits (riser ≤ 7¾″, tread ≥ 10″) — your local code wins, and inspectors measure. Buy stringer stock longer than the minimum for end cuts and mounting. Riser count × height must land exactly on the total rise — never round it.
Why riser height is a division, not a choice
Stairs must land exactly on the upper floor, so riser height is always total rise ÷ number of risers — you pick the count, the height follows. Divide by the code maximum, round up, and every riser comes out identical and legal. A flight with one odd step at the top is the trip hazard building codes exist to prevent, which is why the 3/8″ variation rule gets measured.
From numbers to layout
- Set a framing square to riser × tread with stair gauges and walk it down the stringer — the calculator’s two numbers are exactly the square’s two legs.
- Drop the bottom riser by one tread-board thickness so the first step matches the rest after treads go on.
- Check the throat after marking: the wood left behind the notches carries the load.
- Dry-fit one stringer and check the top and bottom step heights in place before cutting its siblings.
Frequently asked questions
How do I measure total rise correctly?
Finished floor to finished floor, measured vertically — not along the slope, and including any flooring still to be installed on either level. A 3/4″ flooring change on one end alters every riser; this is the most common stair-layout mistake, and inspectors catch it because the top and bottom steps come out odd.
What are the code limits for risers and treads?
Common US residential (IRC) limits: risers max 7¾″, treads min 10″, and no more than 3/8″ variation between the tallest and shortest riser in a flight. Commercial and local codes differ — the calculator defaults to IRC numbers, but your local code and inspector always win.
What is the 2R + T comfort rule?
Twice the riser plus the tread should land around 24–25″ — a formula matching the human stride that’s been in use for centuries. 7½″ risers with 10″ treads give exactly 25. Outside the window stairs feel cramped (low value) or leggy (high value) even when they pass code.
What lumber do I use for stringers, and how many?
2×12s are the standard — after notching, the remaining throat must typically keep at least 5″ of solid wood. Use three stringers for spans 36″ and wider (two only for very narrow stairs), pick straight stock, and crown all stringers the same way. Buy longer than the calculated minimum for end cuts and mounting.
Sources & further reading
- Common US residential (IRC) stair limits: riser ≤ 7¾″, tread ≥ 10″, ≤ 3/8″ riser variation — verify against your local code
- Classic stair comfort formula: 2 × riser + tread ≈ 24–25″