Deck stair calculator

Total rise for a deck stair is the vertical distance from the top of the decking boards down to the surface the stair lands on. If the stair ends on bare grade, measure straight down with a story pole or a level and a tape, not along the slope.

If the stair lands on a concrete pad, patio, or paver landing, measure to the top of that surface instead of to raw dirt. That landing surface is your true bottom reference point, and it needs to be reasonably level and set below any expected frost heave in your area, which a local contractor or inspector can confirm.

Grade is rarely perfectly flat under a deck. Measure at the actual location the stair will sit, not at the nearest corner post, since a few inches of slope changes the riser count.

Most residential deck stairs use open risers, meaning there's no solid board closing off the back of each step. That's why the calculator's open riser toggle matters: with open risers, IRC-style reference values commonly limit the opening to a maximum of 4 in so a sphere that size can't pass through.

The deck surface itself is almost always the top tread, so the flush-top rule applies: number of treads equals number of risers minus one. If you calculate 6 risers, you'll cut 6 stringers steps but only build 5 physical treads below deck level, because the deck itself is the sixth 'step'.

• Open risers: check the gap against your local code's sphere-passage rule
• Flush top: deck surface = top tread, so treads = risers − 1
• Closed risers (solid boards) are sometimes required near grade in windy or exposed sites

The horizontal run of the stringers has to land on something solid. A poured concrete pad, a paver patio, or compacted gravel with a paver base are all common choices; the calculator's total run tells you how far out from the deck the bottom of the stringers will sit.

Add a few extra inches of landing surface beyond the last tread's edge so anyone stepping off the stair lands on solid, level ground rather than the very edge of the pad. Many builders pour the landing pad at least as wide as the stair and 36 in deep.

Pressure-treated 2x lumber, cedar, and composite decking (like Trex) all work as deck stair treads, but they aren't the same thickness or span the same distance without support, which changes how you frame the stringers.

Composite boards typically need tighter stringer spacing than solid wood, often 12 in on center instead of 16 in, because composite is more flexible and can deflect under foot traffic on an unsupported step.

The top of each stringer needs a solid connection to the deck's rim joist, usually with a stair stringer hanger or a ledger/cleat and structural screws, not just angled nails. The bottom of the stringer bears on the landing pad or a pressure-treated sill plate anchored to it.

Use at least three stringers on stairs wider than about 36 in, and check spacing against your local span table, since stringer spacing depends on both the tread material and the stringer's own width and thickness.

In colder climates, footings or landing pads that support a deck stair are sometimes required to sit below the local frost depth, similar to deck post footings; check with your building department since frost depth varies widely by region.

IRC-style residential reference values commonly cited for deck stairs are a maximum 7.75 in riser, minimum 10 in tread, no more than 0.375 in variation between the largest and smallest riser or tread in a flight, and a minimum 36 in width. These are commonly cited reference numbers, not a guarantee of compliance; your local authority having jurisdiction makes the final call, and permit requirements for deck stairs vary by municipality.

If you're cutting the stringer boards themselves and want the layout marks for a framing square, the deck stair stringer calculator on this site walks through the drop cut and rim joist attachment in more detail.

Building on a sloped yard instead of a flat pad? The outdoor stair calculator is tuned for shallower, more comfortable landscape-stair proportions rather than the steeper ratios typical of framed deck stairs.