Guide · Framing

How to Calculate Stair Stringers (Rise and Run)

A deck 36 inches off the ground needs 5 risers at 7.2 inches each, 4 treads, and a 44-inch total run — carried on 3 stringers cut from 2×12s. Each stringer is about 57 inches long, so a 6-foot board covers it.

Below is the full layout method — the 7-11 rule, riser math, tread counts, stringer spacing, and how to transfer the layout to the board with a framing square. To skip the arithmetic, use our free stair layout calculator — enter your total rise and it returns risers, treads, run, and stringer length.

The math, step by step

1. Measure the total rise. From the finished surface below to the finished surface of the deck or floor above: 36 inches in our example. Measure finished-to-finished — flooring thickness counts.
2. Divide by ~7 and round. 36 ÷ 7 = 5.14 → round to 5 risers. Seven inches is the comfort target from the 7-11 rule.
3. Get the exact riser height. 36 ÷ 5 = 7.2 inches per riser — every riser identical, which matters more than the exact number. Check it against the code maximum of 7.75″: 7.2 ≤ 7.75, so we’re good. (If the math ever lands above 7.75″, add one more riser and recompute.)
4. Count the treads: risers minus one. The top step lands on the deck itself, which acts as the final tread: 5 − 1 = 4 cut treads.
5. Total the run. 4 treads × 11″ (the 7-11 rule’s run) = 44 inches of horizontal run — 3 ft 8 in out from the deck.
6. Count the stringers. One every ~16 inches of stair width: a standard 36″-wide stair gets 3 stringers (two outside, one centered).
7. Size the board. Stringer length is the hypotenuse: √(44² + 36²) = √3232 ≈ 56.9″ → ~57 inches. A 6-foot 2×12 covers it with room to spare.

Rise and run chart

Riser counts target ~7″ each and stay at or under the 7.75″ code maximum; treads use the 11″ run from the 7-11 rule.

Note the 24″ row: 24 ÷ 7 rounds to 3 risers at 8″ — over the 7.75″ maximum — so it bumps to 4 risers at 6″ instead. Always recheck the max.

How many stringers do I need?

Space stringers about every 16 inches so the treads don’t flex or bounce between supports:

Cut stringers from 2×12 lumber — the notches remove a lot of wood, and a 2×10 can leave too little meat below the tread cuts on steep layouts.

Laying out the stringer with a framing square

The math gives you the numbers; the framing square transfers them to the board. Set stair gauges (those little brass buttons) on the square at the riser height on the tongue (7.2″) and the tread run on the blade (11″). Starting at the top end of the 2×12, align the gauges to the board’s edge and trace the L — that’s one riser plus one tread. Slide the square down so the riser gauge meets the end of the tread line you just drew, trace again, and repeat for all risers and treads. Cut the top of the stringer to drop by one riser height minus the tread thickness (so the finished top step matches the others once treads are on), and cut the bottom by the same logic against the landing. Cut the first stringer carefully, test-fit it, then use it as the template for the rest.

Code notes before you cut

Residential stair code (IRC, adopted with local amendments in most places) sets hard limits worth knowing while planning: maximum 7.75″ riser height, minimum 10″ tread depth, and riser heights within a flight must be nearly identical — the biggest and smallest can differ by no more than ⅜″. Handrails are required on stairs with four or more risers. Treat these as planning awareness, not code advice: confirm the exact requirements with your local building department before building, since amendments vary.

Pair the stair math with the deck material calculator when the stairs serve a deck — it sizes the framing, decking, and footings the stringers will tie into.