Decking Calculator
Use this decking calculator to plan a whole deck, not just the surface. Enter the deck length and width, board size and gap, joist spacing, beam lines and post spacing. It estimates boards with waste, joists, rim and band length, posts and footings, and fasteners, with an optional cost. If you only need boards, try the deck board calculator, or use the deck material calculator for boards plus a joist count.
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Unit prices (optional)
Area
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Board rows across the width
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Boards needed
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Boards with waste
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Joists (count × length = total)
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Rim and band boards (perimeter)
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Posts
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Footings (one per post)
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Fasteners with waste
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Fastener boxes
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Estimated cost
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Boards run along the deck length: rows = width ÷ (board width + gap), rounded up, times boards per row (length ÷ board length, rounded up). Joists run along the width and are spaced along the length, plus one at the end so spacing never exceeds your value. Posts = beam lines × (length ÷ post spacing, rounded up + 1), with one footing per post. Fasteners = board rows × joists × fasteners per crossing (2 is typical), plus the waste percentage. Joist and beam sizes, allowed spans, post spacing, footing size and ledger details come from span tables, your plans and local code. The 8 ft post spacing and 16 in joist spacing are adjustable placeholders, not recommendations.
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How it works
- How many deck boards do I need?
- Divide the deck width by the board width plus the gap and round up to get rows, then multiply by the boards per row (length ÷ board length, rounded up). A 16 ft × 12 ft deck with 5.5 in boards and a 1/8 in gap has 26 rows, so with 16 ft boards that is 26 boards, or 29 with 10% waste.
- How many joists does a deck need?
- Joist count is the deck length ÷ spacing, rounded up, plus one. At 16 in on center, a 16 ft deck has 12 spaces, so 13 joists. Here joists run along the width, so the total is count × width: 13 × 12 ft = 156 ft. Allowed joist spans depend on size, species and spacing, so check span tables or local code.
- How many posts and footings does a deck need?
- Each post normally has its own footing. This calculator counts posts as beam lines × (length ÷ post spacing, rounded up + 1), which is 3 posts for one 16 ft beam at 8 ft spacing. Allowed post spacing depends on beam size, species and joist span, and footing size depends on load and soil, so use span tables, your plans and local code. 8 ft is an adjustable placeholder.
- How many deck screws do I need?
- Plan on two screws at each board-to-joist crossing. For 5.5 in boards on 16 in centers that works out to about 3.5 screws per sq ft, or 350 per 100 sq ft. The 16 ft × 12 ft deck is 26 rows × 13 joists × 2 = 676, or 744 with 10% extra. Enter screws per box to get a box count.
- How is this different from the deck material and deck board calculators?
- The deck board calculator counts boards only, and the deck material calculator adds boards and a joist count. This decking calculator covers the whole deck: boards with waste, joists and their length, rim and band length, posts and footings, fasteners and an optional cost.