Deck stair calculator

Every stair calculator will tell you a riser is under 7.75 inches. Very few tell you whether your risers match each other, and that is the requirement that fails finished stairs at inspection.

There are two ways it goes wrong on real jobs, and both are avoidable. The first is measuring the rise to the dirt instead of to the finished landing surface, so the bottom riser ends up short or tall by whatever the pad turns out to be. Pour or set the pad first, or decide its finished height before you cut anything.

The second is forgetting to drop the stringer. Treads sit on top of the notches, so unless you trim the bottom of the stringer by one tread thickness, the first step is a full tread thicker than the rest — usually about an inch and a quarter, which is more than three times the tolerance. The layout on the results panel above assumes uniform risers; the drop is what makes that true once the treads are on.

The IRC section numbers above are the 2021 IRC’s — the edition this engine’s stair constants are recorded against. Editions renumber, so the section that carries a rule in your adopted code may not be the one printed here. Residential values, and your Authority Having Jurisdiction still governs — some jurisdictions amend guard height, and commercial work follows a different code entirely. Two rows above cite the deck guide rather than an IRC subsection. DCA 6’s stair text requires an intermediate landing once total stair rise passes 12'-0" (DCA6-2015 p.20, within the IRC’s stair provisions at R311.7), and this engine breaks a straight run at exactly that rise. Its Figure 28 lets a cut stringer span no more than 6'-0" horizontally, and the calculator above counts the support lines a longer run needs. Confirm how your adopted edition states the flight limit. Inside a saved design, turned runs split every 6 feet of rise so each landing stays a sane size.

The riser count comes first. Divide the measured rise by your target — 7 inches is a comfortable step — and round to a whole number. Then divide the rise by that count again: that quotient is the riser height every step will actually be. If it lands above 7.75 inches, add a riser and divide once more. The calculator does exactly this, and it tells you when it added one.

Treads are one fewer than risers, because the deck surface is the top tread and the landing pad is the bottom one. That off-by-one is the most common arithmetic mistake in stair layout, and it is why a stair ordered from a riser count sometimes arrives with one tread too many.

Stringers are cut from 2x12 stock, spaced for a composite tread: no more than 12 inches apart and never fewer than four, the Trex stair chart's value — 4 of them for a 36-inch stair, 5 for 48 inches. Wood treads can sit on stringers up to 18 inches apart with a minimum of three (DCA 6-2015 p.20), and some composite treads need 9 inches; check your tread's own stair guide. A notched stringer has far less material left than the board it started as, which is why they sit closer together than the joists above them. A cut stringer may span no more than 6'-0" horizontally (DCA 6-2015 Figure 28): past that it needs a post under it, or a landing. The stock length shown includes 5% for the cut.

The deck those stairs hang off needs sizing too — start with the joist span calculator and the beam and footing calculator.