Stair Stringer Calculator for Deck Stairs

How to use the stair stringer calculator

1. Measure the total rise. Measure from the top of the deck boards down to where the bottom step will land, usually a concrete pad or compacted gravel. On sloping ground, measure where the stair will actually end, a few feet out from the deck.
2. Pick how to set the steps. “Max riser height” divides the rise into the fewest equal steps that stay under the height you enter. “Number of steps” forces a count and shows the riser height that results.
3. Enter the tread depth. This is the level cut on the stringer. Two 2×6 deck boards with a ¼ in gap give 11¼ in, which is the default. Use a chip or type your own.
4. Enter the stair width. Width sets how many stringers you need. The minimum is three, spaced no more than 18 in apart.

If any value breaks a code limit, a warning replaces the normal note under the results.

Stair stringer formula: rise, run and length

A deck stair has one more riser than it has treads, because the deck itself is the top step. That is why a 36 in rise with five risers needs only four treads cut into the stringer. Miscounting this makes the stair one tread too long.

Every riser in the flight comes out the same height because the calculator divides the total rise evenly. That matters because the IRC allows only ⅜ in difference between the tallest and shortest riser in a flight. Measure the rise after the landing pad is poured, not before.

The stringer length is the straight-line distance from the top of the rise to the end of the run, the hypotenuse of a right triangle. A cut stringer also needs extra wood beyond that. At the top it needs room to bear on or hang from the rim, and at the bottom it needs a level seat cut. The calculator adds one step’s diagonal for that and rounds up to the next standard even-foot length, starting at 8 ft. That allowance is our own rule of thumb, so lay out your first stringer on the board before you buy the rest.

The “wood left under cuts” figure is the depth of 2×12 remaining square to the bottom edge after the notches are cut. It comes from 11¼ in minus the depth of each notch, which is riser × tread ÷ the step diagonal. That thinner section is why cut stringers have shorter allowed spans.

Worked examples

36 in deck, 11¼ in treads

36 ÷ 7.5 = 4.8, rounded up to 5 risers of 7.2 in. There are 4 treads, so the total run is 4 × 11.25 = 45 in. Stringer length = √(36² + 45²) = 57.6 in, at a 32.6° angle. Each stringer fits on one 8 ft 2×12. For a 36 in wide stair you need three stringers. Five risers is more than three, so the stair also needs a handrail.

Low step-down, 22 in rise

22 ÷ 7.5 = 2.93, so 3 risers of 7.33 in and 2 treads. At 11¼ in treads the run is 22.5 in and the stringer about 31.5 in, so one 8 ft 2×12 makes several short stringers. With fewer than four risers, the DCA 6 handrail requirement does not apply. A deck this low is also under the 30 in height where a guard is needed.

Tall deck, 106 in rise, 10½ in treads

106 ÷ 7.75 = 13.7, so 14 risers of 7.57 in and 13 treads. Total run = 136.5 in, more than 11 ft out from the deck. Stringer length = √(106² + 136.5²) = 172.8 in, or 14.4 ft, so you need 16 ft stock once the trim allowance is added. A stringer that long almost certainly spans farther than DCA 6 allows for a cut 2×12, so plan a 4×4 support post or an intermediate landing.

Matching an existing count

For a 33 in rise, forcing 4 risers gives 8.25 in, over the 7¾ in maximum, so a warning appears. Five risers gives 6.6 in.

Code limits for deck stairs

The International Residential Code (IRC) sets the geometry, and most U.S. towns adopt it with few changes. The figures below are quoted from the IRC in a Minnesota city’s stair handout, and DuPage County’s summary of the 2024 IRC gives the same riser and tread limits. Local amendments can differ, so check with your building department.

• Risers: 7¾ in maximum (R311.7.5.1), measured from the leading edge of one tread to the next. The tallest and shortest riser in a flight may not differ by more than ⅜ in.
• Treads: 10 in minimum depth (R311.7.5.2), measured nosing to nosing. The same ⅜ in variation rule applies.
• Nosing: stairs with solid risers need a ¾ to 1¼ in nosing, unless the treads are at least 11 in deep (R311.7.5.3).
• Handrail: four or more risers need a handrail on at least one side, 34 to 38 in above the nosings (R311.7.8).

The American Wood Council’s prescriptive deck guide, DCA 6, adds the structural rules many permit offices hand out for decks:

• Stringers must be at least 2×12.
• A stair built only with cut stringers needs at least three of them.
• For stairs wider than 36 in, stringers can be at most 18 in on center.
• Stairs must be at least 36 in wide.
• A stair taller than 12 ft in total needs an intermediate landing.
• Stringers have a maximum span. If yours is longer, add a 4×4 post bolted to the stringer on its own footing, or build a landing.

The decking at the top of the stairs is part of the deck calculator, which also covers joists, beams, footings and fasteners. Price the concrete for the landing pad and post footings with the concrete calculator.

Laying out and cutting the first stringer

Mark the riser and tread on a framing square with stair gauges, or with tape on the blade and tongue. Step the square down the 2×12, tracing each notch. At the bottom, cut off one tread thickness so the first step comes out the same height as the rest once the treads are on. Skipping that cut makes the bottom step taller by the thickness of your tread boards, usually 1 to 1½ in.

Cut the notches with a circular saw, stopping short of the inside corners. Finish the corners with a handsaw or jigsaw so you don’t overcut into the throat. Test the first stringer in place, then use it as the pattern for the rest.

Quick reference: steps for common deck heights

Calculated at a 7½ in maximum riser with 11¼ in treads. Run is horizontal distance from the deck face.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Risers = total rise ÷ maximum riser, rounded up; every riser is the same height (total rise ÷ risers). Treads cut into each stringer = risers − 1, because the deck is the top step.
• Stringer length = √(rise² + run²). The 2×12 stock length adds one step diagonal for the top and bottom cuts and rounds up to the next even foot (minimum 8 ft). That allowance is a CalcTheJob rule of thumb, not a code figure.
• Code checks use the IRC and AWC DCA 6 figures cited below. Local amendments can differ.
• Wood left under the cuts is geometry only and does not replace the DCA 6 stringer span limits.

Sources

1. City of Austin, Minnesota, Building Department, Residential Stairways, Handrails & Guardrails. Quotes IRC R311.7.5.1–.3: riser max 7¾ in, tread min 10 in, ⅜ in variation, nosing unless treads are 11 in or deeper.
2. DuPage County Building & Zoning (Illinois), Exterior Basement Stairwell. Same riser and tread limits under the 2024 IRC.
3. American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015 IRC). 2×12 stringers, at least 3 cut, max 18 in apart; 36 in min width; landing over 12 ft of rise; handrail at 4+ risers; guards over 30 in.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.