Deck Stair Calculator

Exterior treated 2x12 stringers — rise, run, step count, length, and a typical IRC flag

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Deck Stair Layout Gear

Same square and gauges as an interior stringer. The difference is treated 2x12, hangers at the rim, and a landing that is not dirt.

Swanson Speed Square

Fast 90 and 45 checks on the plumb cut at the deck rim and the seat cut at the landing. Pair it with a rafter square for the full rise/run walk.

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Johnson Stair Gauges

Clamp unit rise and unit run on a framing square and walk the treated 2x12. Same layout as interior — wet PT just needs a darker pencil.

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16×24 Framing Square

Unit rise on the tongue, unit run on the body. This calculator is written for that walk on a treated 2x12.

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7-1/4" Circular Saw

Stop short of the crotch on PT 2x12 the same way you would indoors. Finish the corner with a handsaw so you keep the 5" throat.

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48" Box Beam Level

Prove the first tread and the landing pad before you hang the rest of the flight. A long level also checks the deck rim is not rolling.

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Mark and Cut Walkthrough

Story pole, square walk, and the overcut rule. Use this calculator for deck numbers, then the interior guide before the first kerf.

Read the Layout Guide →

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How to Use This Deck Stair Calculator

Measure from the finished deck walking surface down to the finished landing (concrete, pavers, or compacted pad — not dirt). Enter the horizontal run you actually have in front of the deck. Pick 2 stringers (36" stair) or 3 (center stringer, typical over 36"). Read step count, rise per step, run per step, treated 2x12 stringer length, remaining throat, and the typical IRC flag.

unit rise = total rise ÷ risers · unit run = total run ÷ treads · length = √(rise² + run²)

Same core math as the interior Stair Stringer Calculator. How to mark and cut: stair stringer layout guide. Uneven risers and callbacks: stair troubleshooting.

Landing pads: concrete volume calculator. This tool does not size guards, posts, or a steel header.

Deck Stair Calculator