How many steps does a deck need?
Take the height of the deck in inches and divide by 7¾, the tallest a step is allowed to be under the residential code most of the US uses. Round up. That's your number of risers, and the height divided by that number is how tall each one is.
| Deck height | Risers | Height of each riser | Treads | Run, with 10½ in treads |
|---|---|---|---|---|
| 14 in | 2 | 7 in | 1 | 10 1/2 in |
| 21 in | 3 | 7 in | 2 | 21 in |
| 30 in | 4 | 7 1/2 in | 3 | 31 1/2 in |
| 36 in | 5 | 7 3/16 in | 4 | 42 in |
| 48 in | 7 | 6 7/8 in | 6 | 63 in |
| 60 in | 8 | 7 1/2 in | 7 | 73 1/2 in |
| 96 in | 13 | 7 3/8 in | 12 | 126 in |
There is always one fewer tread than risers, because the deck itself is the top step.
The two numbers the code cares about
A riser can be no taller than 7¾ inches, and a tread no shallower than 10 inches. Those are the limits, not targets. Most people find a riser around 7 inches with a tread of 10½ or 11 the most comfortable to walk.
The third rule catches more people than the first two: every step has to match. The tallest riser in a flight can't differ from the shortest by more than ⅜ of an inch. Your foot learns the rhythm on the first step, and one odd step is where people fall.
Run: how far the stairs stick out
Multiply the number of treads by the tread depth. A deck 48 inches up needs 7 risers and 6 treads, and with 10½-inch treads the stairs reach more than 5 feet into the yard. Check that you have the room before you settle on where they go.
The stairs also need somewhere solid to land. A concrete pad or a row of pavers at the bottom keeps the stringers out of the dirt and gives you an even last step.
Stringers
Stringers are the notched boards that carry the treads, normally cut from 2×12s. We count one at each side and enough in between that none are more than 16 inches apart, which is why a 36-inch stair gets four.
The estimate gives the length of each stringer along its slope. Buy the next board length up.
When you need a handrail
Once a stair has four or more risers, it needs a handrail on at least one side, and it has to be one you can close your hand around. A flat 2×6 on top of the railing doesn't count. The calculator flags it when your stairs reach that point.
These are the model code's numbers. Your city or state can change them, and stairs are inspected closely, so confirm the limits with your building department before you cut anything.
Fair questions
What if the ground slopes?
Measure the height at the spot where the stairs will land, not at the edge of the deck. On a slope those can be a foot apart, and the stairs are built for the landing.
What does a step cost?
We use $150 a step in wood and $170 in composite for stairs 36 inches wide, built by a contractor, and scale that for wider stairs.
Are stairs included in the deck calculator?
Yes, at the same price per step. The deck cost calculator takes a number of steps, and this page tells you what that number should be.
Sources
- International Residential Code: sections R312 (guards), R311.7.8 (handrails) and R507 (decks). Your city or state adopts and amends it, so check the local version.
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide: ledger attachment, joist spans, guards and stairs for wood decks.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
