Decks, Fences & Siding
Deck Stairs Calculator
Measure from the top of the deck boards down to where the bottom step will sit. Enter that height, your riser limit and the tread you plan to use, and the tool lays out the steps.
Worked example
- Risers = 30 in total rise / 7.5 in max riser = 4, round up to 4.
- Riser height = 30 / 4 = 7.5 in (7 1/2 in).
- Treads = risers - 1 = 3 (the top step is the floor or deck). Total run = 3 x 11.25 in = 33.75 in.
- Stringer line = square root of (30^2 + 33.75^2) = 45.16 in; angle 41.6 degrees.
Common sizes at a glance
| Size | Risers (steps up) | Treads | Total run | Stringer length | Stringers |
|---|---|---|---|---|---|
| 18 in deck height | 3 at 6 in | 2 at 11.25 in deep | 1 ft 10 1/2 in | 2 ft 4 13/16 in | 4 |
| 24 in deck height | 4 at 6 in | 3 at 11.25 in deep | 2 ft 9 3/4 in | 3 ft 5 7/16 in | 4 |
| 30 in deck height | 4 at 7 1/2 in | 3 at 11.25 in deep | 2 ft 9 3/4 in | 3 ft 9 1/8 in | 4 |
| 36 in deck height | 5 at 7 3/16 in | 4 at 11.25 in deep | 3 ft 9 in | 4 ft 9 5/8 in | 4 |
| 48 in deck height | 7 at 6 7/8 in | 6 at 11.25 in deep | 5 ft 7 1/2 in | 6 ft 10 13/16 in | 4 |
| 60 in deck height | 8 at 7 1/2 in | 7 at 11.25 in deep | 6 ft 6 3/4 in | 8 ft 3 in | 4 |
Start at the landing, not under the deck
Yards are rarely flat. If you measure straight down from the deck edge, then the ground slopes away, every step comes out wrong. Find where the bottom tread will land, run a straight board and a level out from the deck to that spot, and measure the drop there.
The tool also tells you how far out the stairs reach. In the worked example a 30 in rise lands about 2 ft 9 3/4 in from the deck. Check that spot, then measure again if it lands lower or higher than you thought.
Rules for rise and run
The common U.S. residential code allows a riser up to 7 3/4 in and a tread at least 10 in deep. All risers in a flight should match, because a single odd step is what trips people. That is why the tool divides the total rise evenly instead of using a fixed number.
A comfort check also appears: two risers plus one tread near 24 to 25 in feels natural to most people. Stairs that score well above that feel long and slow; well below feels steep. Local rules can be stricter, and the local code and a licensed professional have the final word.
Why tread depth often comes out to 11.25 in
Many builders make each tread from two deck boards. Two 5.5 in boards with a 1/4 in gap give 11.25 in, so that is a handy default. A 48 in wide stair with three treads then needs six tread boards cut to 48 in.
If you use a single wide board or a composite stair tread, enter its real depth instead. The run, stringer length and landing spot all update from that one number.
Stringers and the bottom pad
The stringer length shown is the sloped line along the stairs, so buy stock a bit longer to leave room for the top and bottom cuts. The tool suggests a stringer count for the width you enter; the stringer size and support details come from your approved plan. For a full cutting layout, see the stair stringer calculator.
The bottom of the stringers needs a firm, level base that will not settle. Many people pour a small pad; the concrete slab calculator gives the bags for it.
Questions
How many steps for a 36 in deck?
With a 7.5 in maximum riser, five risers of about 7 3/16 in and four treads. The top step is the deck itself.
Why are there fewer treads than risers?
The deck surface acts as the last step up, so a flight always has one tread fewer than risers.
Do deck stairs need a railing?
Many codes require a graspable handrail once a stair has a certain number of risers. Plan balusters with the baluster spacing calculator and confirm the rule locally.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.