Deck Calculator
Estimate deck boards, joists, and support posts needed for a deck based on dimensions, board size, and joist spacing.
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Extended More scenarios, charts & detailed breakdown ▾
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Professional Full parameters & maximum detail ▾
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Material Counts
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Cost Estimate
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How to Use This Calculator
- Enter the deck length and width in feet.
- Enter the deck board width and gap in inches.
- Enter joist spacing and post spacing in inches.
- Results show boards, joists, and posts needed.
Formula
Boards = ceil(Deck width × 12 ÷ (board width + gap)) × (1 + waste%)
Joists = floor(Deck length × 12 ÷ joist spacing) + 1
Posts = ceil(Deck length ÷ post spacing) × ceil(Deck width ÷ post spacing)
Example
Example: 16 × 20 ft deck, 5.5 in boards, 0.25 in gap, 16 in joist spacing → 37 boards, 16 joists, 12 posts.
Frequently Asked Questions
- Calculate the deck area (length × width in sq ft), then divide by the net coverage per board. Net coverage per board = (board width in inches − gap in inches) ÷ 12 × board length. For a 5/4 × 6 composite board at 1/8-inch gap, net width = (5.5 − 0.125) ÷ 12 = 0.448 ft. A 12-foot board covers 0.448 × 12 = 5.38 sq ft. For a 400 sq ft deck: 400 ÷ 5.38 = 75 boards before waste. Add 10% for a straight pattern = 83 boards. For diagonal patterns add 15% for edge cuts. This calculator handles the per-board coverage calculation automatically.
- Joist spacing depends on the decking material. For 5/4 pressure-treated wood decking, 16 inches on center is standard. Composite decking usually requires 12 or 16 inches on center — check the manufacturer specification, as some brands require 12-inch spacing for diagonal installations. Joists spaced at 24 inches are only appropriate for 2× solid wood decking and result in more noticeable flex underfoot. Tighter joist spacing increases material cost but improves rigidity, reduces bounce, and is required for diagonal or picture-frame decking patterns.
- Joist count = (Deck length ÷ Joist spacing in feet) + 1 end joist. For a 20 ft deck at 16-inch spacing: 20 ÷ (16 ÷ 12) + 1 = 20 ÷ 1.333 + 1 = 16 joists. You also need doubled joists at each end (rim joists) and at any beam locations, and additional joists around stair openings or hot tub cutouts. The rim joists frame the perimeter and are included in the joist count. Always add 2 extra joists to account for doubled header requirements and any blocking needed mid-span for boards wider than 8 inches.
- 4×4 posts (actual 3.5 × 3.5 in) are adequate for decks up to 6 feet above grade with post spans of 6 feet or less. 6×6 posts (actual 5.5 × 5.5 in) are required for elevated decks above 6 feet, for spans over 6 feet between posts, or when local building codes mandate it (common in seismic and high-wind zones). Post length equals the desired deck height above grade plus 12 inches buried in the footing. Always anchor posts to concrete piers with post base hardware rather than setting wood directly in concrete to prevent rot.
- Add 10% waste for a straight-pattern deck — this covers end cuts at the house wall and the outer rim, plus boards with defects. For a diagonal (45-degree) pattern, add 15% because every board is cut at both ends at an angle. For a herringbone or picture-frame pattern, add 20%. Always round up to the nearest bundle or pack size. Keep a few extra boards from the same production run for future repairs: composite decking colors can change slightly between manufacturing runs, making exact matches difficult years later. The calculator includes an adjustable waste percentage field.