Stair Calculator
Find how many steps your deck stairs need, how tall and deep each step is, how far they reach into the yard, and what length of 2×12 to buy for stringers.
Building code caps each step's rise at 7¾ inches, so the deck's height decides how many steps you need, and each step you build adds about 11 inches to how far the stairs reach into the yard.
How high is the deck?
From the ground where the stairs will land to the top of the deck boards.
How wide are the stairs?
Code minimum is 36 in; 48 in lets two people pass.
Steps to build
4 steps
Reaching about 3 ft 8 in out from the deck's edge
From the ground to the deck is 5 equal rises; the last one is onto the deck. Code allows rises up to 7¾ inches and steps at least 10 inches deep; two deck boards make an 11-inch step. Stringers go no more than 18 inches apart. Stairs this tall need a handrail on at least one side.
💡About this calculator▼
Deck stairs are mostly math the building code has already done for you. Each step can rise at most 7¾ inches, so your deck's height sets how many steps you need, and each step you build adds about 11 inches to how far the stairs reach into the yard.
Enter how high the deck is, measured from the ground where the stairs will land to the top of the deck boards. Enter how wide the stairs will be: 36 inches is the code minimum, and 48 inches lets two people pass. You get the number of steps to build, how far out they reach from the deck's edge, how tall and deep each step is, and the stringers to cut.
Rises. The residential building code allows each rise, the height from one step to the next, to be at most 7¾ inches, and all the rises in a flight must be nearly the same. So the calculator divides your deck's height by 7¾, rounds up to a whole number of rises, and splits the height evenly between them. A 36-inch deck needs 5 rises of 7.2 inches each, about 7¼.
Steps and reach. The last rise lands you on the deck itself, so you build one fewer step than there are rises: 4 steps for a 36-inch deck. The code requires steps at least 10 inches deep; two standard deck boards side by side make an 11-inch step, which also needs no overhanging nosing. Four steps at 11 inches reach 44 inches, 3 feet 8 inches out from the deck's edge.
Stringers. Stringers are the zig-zag boards the steps sit on: a 2×12 cut with a sawtooth of notches, one notch for each step. The deck building guide most code offices follow calls for 2×12 stringers, at least three of them, spaced no more than 18 inches apart. That means three for stairs 36 inches wide and four for stairs 48 inches wide. Each runs along the slope from the ground to the deck; add about a tread's length for the cuts at each end and round up to the next length lumberyards stock.
Worked example. A 3-foot deck: 36 ÷ 7.75 = 4.6, so 5 rises of 7¼ inches and 4 steps to build. Four steps at 11 inches reach 3 feet 8 inches. The slope is about 57 inches, plus 11 for the end cuts, so three 8-foot 2×12s make the stringers.
📐How it's calculated▼
Rises = deck height (in) ÷ 7.75, rounded up. Each rise = deck height ÷ rises.
Steps to build = rises − 1 (the last rise is onto the deck). Reach = steps × 11 in.
Stringer board ≥ √(height² + reach²) + 11 in, rounded up to a stock 2×12 length (8, 10, 12, 14 or 16 ft). Stringers = width ÷ 18 in, rounded up, + 1, at least 3.
3 ft deck: 4 steps, 7¼ in tall, 3 ft 8 in out, 8-ft 2×12s. 4 ft deck: 6 steps, 6⅞ in tall, 5 ft 6 in out, 8-ft 2×12s. 5 ft deck: 7 steps, 7½ in tall, 6 ft 5 in out, 10-ft 2×12s.
📎Sources:ICC — 2021 International Residential Code, stairways (R311.7),American Wood Council — Prescriptive Residential Wood Deck Construction Guide (DCA 6)
🔍Finding your inputs▼
How high is the deck? Measure straight down from the top of the deck boards to the ground where the bottom step will sit, in feet and inches. If the ground slopes, measure at the spot where the stairs will land, not at the deck's edge.
How wide are the stairs? The width of the steps, side to side, in inches. The code minimum is 36 inches; 48 inches lets two people pass. Wider stairs need more stringers, one at least every 18 inches.
⚠️Special situations▼
My yard slopes where the stairs land
Measure the height at the spot where the bottom step will sit, not at the deck edge. If the ground drops away, that spot is lower, so you may need one more step, and the stairs will reach farther out. Recheck with the new height.
What do the stairs sit on at the bottom?
The deck guide calls for the stringers to bear on a solid base where the stairs meet the ground, such as a concrete pad or footing, rather than on bare soil. Plan the pad to sit just past the reach shown here.
How many deck boards do the steps take?
Two deck boards per step, cut to the stair width. For 36-inch-wide stairs with 4 steps, that is 8 pieces 36 inches long, which you can cut from three 10-foot deck boards with a little to spare.
Why not fewer, taller steps?
Code caps each rise at 7¾ inches. A 2-foot deck could be done in 3 rises (2 steps to build) only if each were 8 inches tall, which is over the limit, so it takes 4 rises of 6 inches, which is 3 steps to build.
❓Common questions▼
How many steps do I need for a 3 foot deck?
Four steps, each about 7¼ inches tall, because code caps a rise at 7¾ inches. That is 5 equal rises from the ground, the last one onto the deck, and the stairs reach about 3 feet 8 inches out from the deck.
How many steps for a 4 foot deck?
Six steps, each about 6⅞ inches tall, reaching about 5 feet 6 inches out. Stringers can be cut from 8-foot 2×12s.
How long should stair stringers be?
At least the slope from the ground to the deck plus about 11 inches for the cuts at each end. For a 3-foot deck that is about 68 inches, so an 8-foot 2×12 is the shortest stock board that works. Use three for stairs 36 inches wide, or four for 48 inches.
What is the code for deck stair rise and run?
The residential code allows each rise up to 7¾ inches and each step at least 10 inches deep, with no more than ⅜ inch of difference between the tallest and shortest step. Stairs must be at least 36 inches wide, and 4 or more rises (3 or more steps to build) need a handrail.
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