Stair Stringer Calculator
Turn a total rise into an even riser count (IRC 7.75 inch max), tread count, total run, and the 2x12 stringer length to buy.
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
How to measure total rise
Total rise is the vertical distance from the lower finished surface to the upper finished surface: landing to deck surface, slab to porch floor. Measure it where the stairs will actually land, not at the structure, because grades slope. Hold a level board out from the upper surface and measure down to the landing point.
Get this number right to the quarter inch. Every other figure in the stairs comes from it, and the classic stringer mistake (forgetting to subtract the tread thickness from the top or bottom of the stringer) starts with a sloppy rise measurement. If the stairs land on dirt, plan a landing pad first, since its thickness changes the rise; the concrete calculator sizes a small pad.
Tread depth is your second input. 10 inches is the code minimum, 10.5 inches matches two 5/4x6 deck boards with a small overhang, and 11 inches is the comfortable choice if you have the run to spare.
The formula and what it assumes
Riser count equals total rise divided by 7.75 and rounded up. The actual riser height is then the total rise divided evenly by that count, which is the step everyone skips and shouldn’t: dividing evenly is what keeps every riser identical. Treads number one fewer than risers, because the top surface itself acts as the final step. Total run is treads times tread depth, and the stringer length is the hypotenuse: the square root of rise squared plus run squared.
A 36 inch rise with 10.5 inch treads: 36 ÷ 7.75 = 4.6, round up to 5 risers, each 36 ÷ 5 = 7.2 inches. That gives 4 treads, a 42 inch (3.5 ft) total run, and a stringer of √(36² + 42²) = 55.3 inches, about 4.6 ft. The calculator adds a foot of cutting allowance and rounds up, with an 8 ft minimum since that’s the shortest 2x12 worth buying: an 8 footer here.
A taller example: a 55 inch deck rise needs 8 risers at 6.88 inches, 7 treads, a 6.1 ft run, and a 7.7 ft stringer, so you’d buy 9 ft 2x12s (in practice, 10 footers, since lumber comes in even lengths).
The 7.75 inch ceiling comes from the International Residential Code. IRC R311.7.5.1 sets the maximum riser at 7-3/4 inches and limits the spread between the tallest and shortest riser in a flight to 3/8 inch, with R311.7.5.2 setting the 10 inch minimum tread. Local amendments can be stricter, so check before cutting.
Deck stairs versus interior stairs
The math is identical; the details differ. Deck stairs use cut 2x12 stringers in pressure-treated lumber, hung from the rim with a hanger or ledger and landing on a pad or footings, never bare dirt. Plan three stringers for a 36 inch width. Interior stairs more often use housed (routed) stringers with the treads let into them, and they have a fixed floor-to-floor rise you can’t fudge, which is why interior riser heights end up as numbers like 7-9/16.
Deck stairs also need a graspable handrail once there are four or more risers, and guards above a 30 inch drop, under the same IRC chapter. If the stairs are part of a bigger deck build, the decking calculator covers the boards and the stairs’ treads come out of the same order.
Cutting tips
Lay out the stringer with a framing square and stair gauges clamped at the riser and tread dimensions, and cut the notches with a circular saw, finishing each corner with a handsaw so you don’t overcut and weaken the stringer. Check the first stringer against the actual deck and landing before tracing the other two from it. Then remember the bottom adjustment: drop the bottom of the stringer by one tread’s thickness, or the first step ends up taller than the rest and blows the 3/8 inch rule on day one.
Frequently asked questions
How do I calculate stair stringers?
Measure the total rise, divide by 7.75 and round up to get the riser count, then divide the rise by that count for the actual riser height. Treads are one fewer than risers, total run is treads times tread depth, and the stringer length is the square root of rise squared plus run squared. A 36 inch deck rise works out to 5 risers at 7.2 inches, 4 treads, and a 4.6 ft stringer cut from an 8 ft 2x12.
What is the maximum stair riser height?
7-3/4 inches under the IRC (section R311.7.5.1), measured vertically between the leading edges of adjacent treads. Some local codes are stricter; a few amend it to 7 inches. The companion rule is the minimum 10 inch tread depth. Comfortable stairs usually land below the maximums, around 7 inch risers with 10.5 to 11 inch treads.
Do all the risers have to be the same height?
Within 3/8 inch, yes. The IRC says the tallest riser in a flight can't exceed the shortest by more than 3/8 inch, because people unconsciously calibrate to the first step and an odd riser is a trip hazard. This is why you divide the total rise evenly instead of building 7.75 inch steps and letting the last one be whatever's left.
How many stringers do deck stairs need?
Three for a standard 36 inch wide stair with 2x lumber treads, spaced 16 to 18 inches apart, and more for wider stairs. Two stringers only works for very narrow stairs with thick treads. Cut stringers from 2x12s, since notching for 7+ inch risers and 10+ inch treads leaves too little wood in anything smaller.