The deck stair calculator lays out a straight stair from the ground to a deck: how many risers, how tall each one is, how many treads, how far the stair runs and how long its stringer is. It checks the risers, treads, nosing, width and flight against the stair figures of the 2024 International Residential Code, which Austin adopts unchanged and which California's residential code, the one Los Angeles builds to, prints the same.
It lays out one flight and decides nothing about a permit. It does not size stringers, posts or footings; those come from a prescriptive guide, the manufacturer or an engineer.
How to use it
- 01
Measure the total rise
From the ground or pad where the bottom step will land, straight up to the top of the deck surface. Measure at that spot: on sloping ground the height under the deck's edge is different, and the difference ends up in the bottom riser.
- 02
Pick a tread depth
Nosing to nosing, at least 10 inches. At 11 inches or more the code needs no nosing; below that, set one of ¾ to 1¼ inches, or more if the risers are open.
- 03
Set the width and the risers
Enter the clear width, at least 36 inches above the handrail. The calculator starts from the fewest risers the code allows; add one for shorter steps, and the stair grows one tread longer.
- 04
Read the checks
A check mark means the layout meets a code figure, an exclamation mark that it does not, and an arrow a code line the layout brings in, such as a handrail or a guard. None of them is a permit decision.
How the math works
Risers
Risers = the total rise ÷ 7¾, the code's tallest riser, rounded up; each riser = the total rise ÷ the risers. A 36-inch rise is 4.6 of the tallest risers, so it takes 5 risers of 7.2 inches.
Treads and run
The deck is the top landing, so treads = risers − 1, and the total run, from the face of the top riser under the deck's edge to the face of the bottom riser, = treads × the tread depth. 5 risers take 4 treads; at 11 inches that is a 44-inch run.
Stringer
Stringer = √(total rise² + total run²): the straight line from the deck surface above the top riser to the foot of the bottom riser. For a 36-inch rise and a 44-inch run it is about 56.9 inches, and a board that long covers the layout before the top and bottom cuts.
Angle
Angle = arctan(riser ÷ tread). The code sets no angle; it follows from the riser and tread limits. At the steepest the code allows, a 7¾-inch riser on a 10-inch tread, it is 37.8 degrees.
Where the number goes wrong
- Measuring the rise under the deck's edge on sloping ground. The bottom riser takes the difference, and the code keeps every riser within ⅜ inch of the others.
- Counting the deck as a tread. It is the top landing: 5 risers take 4 treads.
- Forgetting the foot of the stair. The code wants a floor or landing at the bottom of each flight, 36 inches deep on a straight run; ask what your department accepts there.
- Using the deck's guard figures on the stair, or the other way round. Along a stair's open side a guard may be 34 inches above the nosings with gaps under 4⅜ inches; on the deck it is at least 36 inches, 42 in California, with gaps under 4 inches.
- Treating the layout as structure. Stringer size, spacing and support come from a prescriptive guide, the manufacturer or an engineer, not from this count.
Questions people ask
How many steps for a 30-inch deck?
At least 4 risers: 30 ÷ 7¾ is about 3.9, rounded up to 4. 4 equal risers are 7½ inches each, with 3 treads between the ground and the deck.
How many steps for a 36-inch deck?
At least 5 risers of 7.2 inches, and 4 treads; with 11-inch treads they cover 44 inches.
How do you work out the length of a stair stringer?
Find the diagonal of the total rise and the total run: √(rise² + run²). For a 36-inch rise with 4 treads of 11 inches, the run is 44 inches and the stringer about 57 inches, from the top of the top riser to the foot of the bottom riser.
What is the tallest riser allowed on deck stairs?
7¾ inches under the 2024 International Residential Code, measured between the leading edges of adjacent treads, and California's code prints the same. Treads are at least 10 inches deep.
Do deck stairs need a handrail?
A flight of 4 or more risers needs one on at least one side, 34 to 38 inches above the nosings. A 30-inch deck, with 4 risers, needs one.