Deck & Floor Joist Span Calculator

Maximum tabulated span from standard IRC 2021 tables — a starting point, not a final design

Starting point only — not a substitute for code, the grade stamp, or an engineer. These spans come from standard IRC 2021 prescriptive tables (floor: Table R502.3.1(2) at 40 psf live / 10 psf dead, L/360, No. 2; deck: Table R507.6 at 40 psf live, wet service, No. 2, no cantilever). Local adopted code, amendments, snow load, actual species/grade on the stamp, notching, bearing, and anything outside those rows still control. For long spans, heavy loads, or non-standard framing, use a licensed engineer.

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Framing Layout Gear

Span tables tell you how far a joist may run. The square, tape, and level still have to match the stamp and the bay you are actually framing.

16×24 Framing Square

Layout the joist bays after the table says the size works. Same square you will use on the deck stairs off that same frame.

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Swanson Speed Square

Fast 90s when you are checking joist crowns and hanger seats. Not a substitute for reading the grade stamp.

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48" Box Beam Level

Existing sag shows up here before you argue with a span table. Sight the bay, then compare to the tabulated number.

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7-1/4" Circular Saw

Cut to the span you actually have, not to a wish. If the table does not reach, change size, spacing, or call an engineer — do not sneak an extra foot.

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Johnson Stair Gauges

When the same deck gets stairs, lock rise and run on the square. Joist span and stringer layout are different jobs.

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How Span Tables Work

Species, grade, spacing, load, and the stamp. Read this before you treat a 2x10 as one number everywhere.

Read the Joist Span Guide →

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How to Use This Joist Span Calculator

Choose floor or deck, then species/grade, joist size, and on-center spacing. The result is the maximum tabulated span from the matching IRC 2021 cell. Read the yellow flag on the result: it is a starting point. Confirm the stamp on the lumber, the adopted code in your jurisdiction, and whether your loads actually match 40 psf live / 10 psf dead.

Floor = IRC 2021 Table R502.3.1(2) · Deck = IRC 2021 Table R507.6 · Grade assumed: No. 2

How the tables work and how to read a grade stamp: joist span guide. Bounce, sag, and inherited framing: troubleshooting. Why floor and deck numbers diverge: floor joists vs deck joists. Bookmarkable grid: IRC joist span tables.

Deck stairs still need their own layout: Deck Stair Calculator. Interior stringers: Stair Stringer Calculator. Commons: Rafter Length & Roof Pitch Calculator. Buying the bunk: Board Foot Calculator.

Deck & Floor Joist Span Calculator