Deck stair calculator
Enter one measurement, the total rise from the top of the decking down to the landing, and this lays out the deck stairs: riser count, riser height, tread depth, total run, and stringer length. It is the same math the CutOnce app runs, checked against the same IRC limits as an interior stair.
Updated
Type it the way your tape reads: 3 ft 9 in, 45, or 3′ 9″
- Risers
- 6
- Riser height
- 7-1/2 in
- Treads
- 5
- Tread depth
- 10 in
- Total run
- 4 ft 2 in
- Stringer length
- 5 ft 7-1/4 in
- Stair angle
- 36.9°
Riser height is within the IRC R311.7.5.1 maximum of 7.75 in, and tread depth meets the R311.7.5.2 minimum of 10 in.
Measure the rise the deck way
A deck stair's total rise is measured straight down from the top of the finished decking to the top of the landing, held plumb, not sloped down the hill. Run a level out from the deck to a stake over where the bottom step will land, then measure down to the pad or grade. Measuring along the slope, or to the dirt at the deck's edge instead of out where the stair actually ends, is what leaves the bottom riser tall or short.
A worked example
Say your deck sits 3 ft 9 in above the landing pad. Here is the whole stair and where each number comes from.
| Total rise | 3 ft 9 in | given (deck to landing) |
| Risers | 6 | round(45 / 7.5), cap check ceil(45 / 7.75); larger wins |
| Riser height | 7-1/2 in | 45 / 6 |
| Treads | 5 | 6 - 1 (top tread is the deck) |
| Tread depth | 10 in | 25 - 2 x 7.50, floored at 10 |
| Total run | 4 ft 2 in | 5 treads x tread depth |
| Stringer | 5 ft 7-1/4 in | sqrt(rise squared + run squared) |
| Angle | 36.9° | atan(riser / tread) |
How many stringers across the width
Rise and run size one stringer. How many you cut is set by the stair's width and what the treads can span, not by this calculator. Space them close enough that the tread material never sags underfoot.
| 5/4 (1 in) deck boards | 12 to 16 in on center |
| 2x6 treads | up to 24 in on center |
| Composite / PVC treads | per the maker's span rating, often 12 in |
These are working rules of thumb. Confirm the spacing against your decking's published span rating and the prescriptive tables in DCA 6 (the AWC deck guide) before you cut. A 36 in wide stair on 2x6 treads is usually three stringers; drop to 12 in on center under thin composite.
The bottom needs a landing
Deck stairs cannot dump straight onto loose dirt. The IRC wants a floor or landing at the bottom of the flight, as wide as the stair and at least 36 in deep in the direction of travel, on a stable pad, gravel, pavers, or concrete, that sheds water away from the house. Set the landing height first, because it is the surface your total rise measures down to.
What the code requires
Deck stairs follow the same IRC stair geometry as any other stair. These are the 2021 IRC residential defaults the app ships; this page checks the three geometric limits. Your jurisdiction can differ, so confirm locally before you cut.
| Maximum riser height | 7-3/4 in | IRC R311.7.5.1 |
| Minimum tread depth | 10 in | IRC R311.7.5.2 |
| Maximum rise between landings | 151 in | IRC R311.7.3 |
| Landing at top and bottom | 36 in min deep | IRC R311.7.6 |
| Open riser gap, rise over 30 in | blocks a 4 in sphere | IRC R311.7.5.1 |
Common questions
- Do I measure the rise to the grade at the deck or out where the stair lands?
- Out where the stair lands. The ground usually falls away from a deck, so the drop at the deck's edge is shorter than the drop where the bottom step actually sits. Level out to the stair's foot, then measure straight down to the landing surface. That vertical is your total rise.
- How many stringers does a deck stair need?
- Enough that the treads never span too far. A 36 in wide stair on 2x6 treads is commonly three stringers; thinner 5/4 or composite decking wants them at 12 in on center, so a 36 in stair goes to four. Follow the tread maker's span rating and the DCA 6 tables rather than a single rule.
- Can deck stairs have open risers?
- Yes, and most do. Where the flight's total rise is more than 30 in, IRC R311.7.5.1 still limits the open gap between treads so a 4 in sphere cannot pass through it, which usually means a riser board, a sub-riser, or treads spaced tight. On a low deck under 30 in of rise the gap is unrestricted, but the taller the flight the more it matters, because that is where a child could get through.
- Why is my tread depth exactly 10 in?
- That is the IRC minimum. On a steeper flight the comfort rule (two risers plus a tread near 25 in) would drop the tread under 10 in, so the calculator holds it at the code floor instead of returning something you cannot build.
- Is the stringer length the board I should buy?
- No. It is the diagonal of the flight. Add for the top and bottom cuts, the connection to the deck, and the overhang past the bottom riser, then buy the next stock length up. A notched 2x12 also loses depth to the throat, which the app tracks.
Cut the whole deck, not just the stair
This page lays out the stair. The app does the rest of the deck: joist and beam spans, footings, decking board counts and the diagonal picture-frame, plus a printable stair cut plan with the notches marked.
CutOnce runs this stair math on the jobsite, checks it against IRC R311.7, then prints the cut plan with the notches marked, and it frames the rest of the deck too.
See also