What joist spacing for decking?

16 inches on center is standard; 12 inches on center for diagonal layouts and some composite brands — check the brand's installation guide.

Deck Board Calculator

Boards and screws for your deck — counted, not guessed.

Your deck

Deck length (ft) Parallel to the boards.

Deck width (ft)

Board

Gap Wet pressure-treated shrinks — hence the wider gap.

Board stock length

Joist spacing 12" OC for diagonal decking or where your composite brand requires it.

Deck framing (joists, beams, posts) is out of scope here — see the Stair Calculator and Fence Post Concrete Calculator for the rest of the project.

Materials list

Generated by AllUnits.io — allunits.io/calculators/decking

How this calculator figures it

Board count divides deck width by board face width plus gap — 5.5 inches plus 1/8 inch for standard 5/4×6 — with 10% added for cuts and culls. Fasteners follow the standard two-screws-per-joist-crossing rule, which works out to roughly 350 screws per 100 square feet on 16-inch joist spacing.

How it works

rows = ceil(width ÷ (board width + gap)) · boards = ceil(rows × length × 1.10 ÷ stock length)

joists crossed = floor(length ÷ spacing) + 1 · screws = rows × joists × 2

Worked example — a 16 × 12 ft deck in 5/4×6 with a 1/8" gap on 16" centers: 144 ÷ 5.625 = 25.6 → 26 rows; 26 rows × 16 ft × 1.10 ÷ 16 ft stock = 29 boards. The boards cross floor(192 ÷ 16) + 1 = 13 joists, so screws are 26 × 13 × 2 = 676.

Deck board FAQs

What gap between deck boards?

1/8" for kiln-dried lumber and composite; 1/4" for wet pressure-treated, which shrinks; 3/16" for hardwoods.

How many screws per board?

Two per joist crossing, set 3/4–1 inch from each edge — about 350 screws per 100 sq ft at 16" OC.

What joist spacing?

16" OC standard; 12" OC for diagonal layouts and some composite brands — check the brand's installation guide.

Why 10% waste?

End cuts, staircase notches, and culling bowed boards.

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Related calculators & sources

Finish the job: Deck Stain Calculator for the gallons, Stair Calculator for the steps, Board Foot Calculator for lumber tallies, and the Fence Post Concrete Calculator for footing posts. Fastener practice is cited at deck installation sources; convert measurements with the area and length converters.

Estimates for planning and purchasing. Not engineering or code-compliance advice — confirm structural work with your local building department.