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.

Stairs

Deck stair calculator

Enter the total rise from the top of the decking down to the landing, and the total run if you already have one. This lays out the deck stairs: riser count, the rise and run of each step, the stair angle, total run, and stringer length, with an IRC code check that marks each row pass or fail. It is the same math the CutOnce app runs, checked against the same IRC limits as an interior stair.

Updated 2026-11-06

Total rise (deck surface to landing)

Type it the way your tape reads: 3 ft 9 in, 45, or 3′ 9″

Total run (optional)

Leave blank for the ideal tread. Enter the run you have and the tread depth comes from it.

NosingOpen risers

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°

IRC code check

Every row this page can check passes the IRC default.

Check	Limit	This stair	Section	Result
Riser height	max 7-3/4 in	7-1/2 in	IRC R311.7.5.1	Pass
Tread depth	min 10 in	10 in	IRC R311.7.5.2	Pass
Rise between landings	max 12 ft 7 in	3 ft 9 in	IRC R311.7.3	Pass
Nosing	3/4 in to 1-1/4 in on solid risers, treads under 11 in	not required	IRC R311.7.5.3	Note
Headroom	min 6 ft 8 in	in the app	IRC R311.7.2	App
Riser variation	max 3/8 in	0 in, equal risers	IRC R311.7.5.1	Note
Guard triangle, open side	6 in sphere	in the app	IRC R312.1.3	App

Every riser here is the same height, so designed variation is zero; keep the built risers within 3/8 in of each other. Headroom and the guard triangle need the stairwell opening, floor thickness, and rail height, so the CutOnce app checks those. The app also runs every check in metric.

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. The calculator above checks riser height, tread depth, total rise between landings, and nosing, and marks each row pass or fail. It starts with open risers checked, since most deck stairs have them, and open risers need no nosing; uncheck it for solid risers. Headroom and the guard triangle need the rail and stairwell details, so the app checks those. 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

How we verify this math

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.