How to Calculate and Build Deck Stairs
Deck stairs intimidate a lot of DIYers, but the math is simpler than it looks. Once you know your total rise, everything else follows. Here’s how to lay out safe, code-friendly deck stairs.
Start with the total rise
The total rise is the vertical distance from the lower landing (ground or pad) to the top of your deck surface. Measure it carefully — every other number depends on it.
Step 1: Find the number of risers
Divide the total rise by a comfortable target riser height (around 7 to 7.5 inches), then round to a whole number:
risers = total rise ÷ target riser height (rounded)
For a 36-inch rise and a 7.5-inch target: 36 ÷ 7.5 = 4.8, which rounds to 5 risers.
Step 2: Find the exact riser height
Because every riser must be equal, divide the total rise by your whole riser count:
riser height = total rise ÷ number of risers
So 36 ÷ 5 = 7.2 inches per riser. The Deck Stairs Calculator does this rounding for you and flags if the result falls outside code.
Step 3: Count the treads
There is always one fewer tread than risers, because the deck surface itself acts as the top step:
treads = risers − 1
Five risers means 4 treads.
Step 4: Calculate the total run
Multiply the number of treads by your tread depth (commonly 10 to 11 inches):
total run = treads × tread depth
With 4 treads at 11 inches, the stairs project 44 inches out from the deck.
Step 5: Find the stringer length
The stringer is the diagonal board the steps attach to. Use the Pythagorean theorem:
stringer length = √(rise² + run²)
For a 36-inch rise and 44-inch run: √(36² + 44²) = √(1296 + 1936) ≈ 56.9 inches. Buy stringer stock a bit longer to allow for cuts and attachment.
Code limits to hit
In most areas, under the IRC:
- Riser height: maximum 7¾ inches
- Tread depth: minimum 10 inches
- Uniformity: risers must be within 3/8 inch of each other
- Stringer spacing: typically every 16 inches on center (a 48-inch-wide stair uses four stringers)
Always confirm with your local building department — handrail and graspability rules also apply once you have a certain number of steps.
Putting it together
- Measure total rise.
- Calculate risers, riser height, treads, total run, and stringer length.
- Cut or buy stringers, attach securely to the deck, and set the bottom on a solid pad.
- Add treads, then railings and handrails as required.
Run your exact numbers through the Deck Stairs Calculator first — it gives you the full layout and warns you if your riser height drifts outside the safe range.