Deck Post Spacing Calculator

Maximum tabulated post spacing from IRC 2021 Table R507.5(1) — the beam span cell read as on-center post spacing

Starting point only — not a substitute for code, the grade stamp, or an engineer. These spacings come from the standard IRC 2021 prescriptive table for deck beams: Table R507.5(1) at 40 psf live / 10 psf dead, L/360, No. 2, wet service. Maximum beam span between posts is maximum post spacing for that beam. Local adopted code, amendments, snow load (Tables R507.5(2) through R507.5(4) are not in this tool), actual species/grade on the stamp, ply nailing, bearing, and anything outside those rows still control.

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Layout Gear For Post Bays

Post spacing is a table cell. Caps, hangers, screws, and bases are how that cell becomes a frame. The square and level still have to match the stamp and the bay you are actually building.

Simpson Strong-Tie BC6 galvanized post cap for 6x

Simpson BC6 Post Cap

Locks a 6x beam to the post after the table sets the bay. Hardware does not change the R507.5(1) cell.

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Simpson Strong-Tie LUS210Z ZMAX joist hanger for 2x10

Simpson LUS210Z Joist Hanger

Face-mount hanger for 2x10 joists. Clean hangers still sit on a beam whose post spacing came from the table.

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Simpson Strong-Tie SDWS 6 inch timber screws pack of 50

Simpson SDWS 6" Timber Screw

Exterior timber screws for built-up beams and cap connections. A tight ply stack is not a longer tabulated bay.

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Simpson Strong-Tie ABU66Z adjustable standoff post base for 6x6

Simpson ABU66Z Post Base

6x6 standoff base, uplift-rated. The pad still has to match tributary from the post spacing you actually framed.

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16×24 Framing Square

Layout post bays after the table says the beam reaches. Same square for the deck stairs off that frame.

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48" Box Beam Level

Existing sag at midspan shows up here before you argue with a span table. Sight the beam, then compare to the tabulated post spacing.

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How to Use This Deck Post Spacing Calculator

Size the joists first. Then tell this tool which species group you are buying, how many 2x plies you will nail up, the size of each ply, and the effective deck joist span that beam has to carry. The result is the maximum tabulated post spacing from IRC 2021 Table R507.5(1) — the same cell the beam span calculator returns as maximum beam span. Optionally enter the bay you already have. If that number is longer than the cell, the yellow/red flag means the prescriptive table does not cover it.

Max post spacing = IRC 2021 Table R507.5(1) beam span cell · Same data as the beam span calculator

Method: post spacing guide. When to add posts vs upsize the beam: decision article. Bookmarkable grid: IRC deck beam span tables.

Deck Post Spacing Calculator