Stair Stringer Calculator
Enter the total rise to get the number of risers, the riser height, tread count, total run, and stringer length, with an IRC R311.7 code check on riser and tread. Free, no sign-up.
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Explanation
A flight of stairs is set by one measurement: the total rise, the vertical height from one finished floor to the next. From there the geometry follows. You divide the rise into equal risers (step heights), the number of treads is one less than the risers, and the stringer — the notched board that carries the steps — is the diagonal found with the Pythagorean theorem. This calculator does that and checks the result against the IRC R311.7 limits so the stair will pass inspection.
The IRC R311.7 dimension rules
Residential stairs in the IRC are governed by a handful of dimensional limits. The riser cannot be too tall, the tread cannot be too shallow, and — the rule most often missed at inspection — every riser and every tread in a flight must be within 3⁄8 inch of each other, which is why the risers are sized equally.
| Dimension | IRC R311.7 limit |
|---|---|
| Maximum riser height | 7¾ in |
| Minimum tread depth | 10 in |
| Max variation, risers or treads | 3⁄8 in |
| Minimum headroom | 6 ft 8 in |
| Minimum width | 36 in |
Source: IRC R311.7. A common comfort guide is 2 × riser + tread ≈ 24–25 in, which a 7 in riser with a 10–11 in tread satisfies.
Laying out the stringer
With the riser height and tread depth fixed, mark a framing square set to those two numbers and step it down the stringer board once per tread; the diagonal length this calculator returns tells you how long a board to start with. Stairs off a deck are the most common case — size the deck framing with the deck beam span calculator first — and the bottom of the stringer should land on a solid base, so pour a landing pad or footing sized with the concrete calculator.
Notes and limitations
The tread depth here is the net horizontal run between nosings. Where treads are less than 11 inches and the risers are solid, the IRC requires a nosing of 3⁄4 to 1¼ inch, which overhangs but does not add to the run. A stair more than 12 feet of vertical rise needs an intermediate landing. Cutting the stringer notches reduces its remaining depth, so check that at least 3½ inches of solid wood remains below the cut.
This calculator sizes the risers, treads, and stringer from your total rise and checks the riser and tread against IRC R311.7. It does not design the stringer for load, the handrails, guards, landings, or headroom, or replace a stamped plan. Confirm the governing code edition and any local amendments with the authority having jurisdiction.