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Deck Screw Calculator

Estimate the screws and boxes for your deck build.

How it is calculated

Each place where a deck board crosses a joist takes screws, so the tool counts those crossings. Area is in square feet, board width and joist spacing are in inches. crossings = area × 144 ÷ (board width × joist spacing), because each board-width by joist-spacing rectangle of deck holds one crossing. Then screws = ceil(crossings × screws per crossing) and boxes = ceil(screws ÷ screws per box).

All five fields must be greater than zero. The result is rounded up to whole screws and whole boxes. It adds no waste allowance and ignores the gap between boards, so it gives the minimum for a plain layout. Extra screws are needed for rim boards, stair treads, breaker boards, mis-drives and cut boards at the edges, so many builders add a margin on top.

Worked example

A 200 sq ft deck with 5.5 in boards on joists 16 in apart, 2 screws per crossing and 350 screws per box.

Area in square inches: 200 × 144 = 28,800. One crossing covers 5.5 × 16 = 88 sq in, so crossings = 28,800 ÷ 88 ≈ 327.27. Screws: 327.27 × 2 = 654.5, rounded up to 655 screws. Boxes: 655 ÷ 350 = 1.87, rounded up to 2 boxes.

Try this example →

Common mistakes

  • Entering the area in square metres or the board width in centimetres. The tool expects square feet for area and inches for the board width and joist spacing.
  • Entering the joist spacing as centre-to-centre in feet. It must be in inches (16 or 24 are typical values).
  • Using the nominal board width instead of the real one. A board sold as a 6 is narrower, about 5.5 in, so the nominal figure undercounts crossings.
  • Treating the result as the full purchase. There is no waste allowance, so a cut-heavy or complex deck needs more screws than shown.
  • Forgetting that hidden fasteners or clips are counted differently. The screws-per-crossing field should match your actual method, such as 1 for clips with one screw or 2 for face-screwing.

About this calculator

The estimate converts the deck area from square feet to square inches (× 144) and divides it by board width × joist spacing (both in inches) to count the board-joist crossings, multiplies by the screws-per-crossing count you specify (typically 2 per board-joist intersection) — then divides by the screws-per-box count you enter (boxes commonly hold 100 or 350) to round up to whole boxes.

DIY deck builders and contractors use it before a lumber-yard run to avoid the classic mid-project problem of running short on fasteners, and it's especially useful for estimating exterior-grade or coated deck screws, which cost noticeably more per box than general-purpose screws and aren't worth over-buying.

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Comments (1)

  • Editorial team

    Look at how much the box rounding leaves you. In the 655-screw example, two boxes of 350 give 700 screws, so only about 45 are spare, roughly 7%, which may not cover mis-drives and cut boards. Compare that spare with the extras your deck needs before deciding on a third box. Do you drive two screws per crossing or use hidden clips? Tell us your usual method.

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