CalculateY
Quick Answer
A 12 x 16 foot deck using 5.5-inch wide, 12-foot boards with 16-inch joist spacing requires approximately 36 decking boards, 13 joists, and 8 posts.
Standard deck board: 5.5"
Common Examples
| Input | Result |
|---|---|
| 12 x 16 ft, 5.5" boards, 12 ft long, 16" joist spacing | 192 sq ft, 36 boards, 13 joists, 8 posts, 2 beams |
| 10 x 12 ft, 5.5" boards, 12 ft long, 16" joist spacing | 120 sq ft, 22 boards, 10 joists, 8 posts, 2 beams |
| 14 x 20 ft, 5.5" boards, 16 ft long, 16" joist spacing | 280 sq ft, 38 boards, 16 joists, 10 posts, 3 beams |
| 12 x 12 ft, 5.5" boards, 12 ft long, 12" joist spacing | 144 sq ft, 27 boards, 13 joists, 8 posts, 2 beams |
How It Works
The Formulas
Decking Boards
Board rows = ceil(Deck Width in inches / (Board Width + 0.125))
The 0.125-inch addition accounts for the standard 1/8-inch gap between boards for drainage and expansion. Total boards = rows x ceil(Deck Length / Board Length). Each row may require more than one board if the deck length exceeds the available board length.
Joists
Number of joists = ceil(Deck Length in inches / Joist Spacing) + 1
Joists run perpendicular to the decking boards and are spaced at regular intervals. The “+1” accounts for the end joist. Standard joist spacing is 16 inches on center for most residential decks. Closer spacing (12 inches) provides a stiffer deck surface, while 24-inch spacing may be acceptable for certain composite decking products.
Posts
Posts are placed approximately every 6 feet around the perimeter of the deck. The calculation counts posts along each side and subtracts the four corners that would otherwise be counted twice. Posts support the beams and transfer the deck load to the footings below.
Beams
Standard decks use 2 beams running along the length, one near each edge. Decks wider than 12 feet typically require a third center beam for adequate support. Beam sizing (2x8, 2x10, or doubled lumber) depends on the span between posts and the total load, which this calculator does not determine.
Material Considerations
Pressure-treated lumber is the most common and affordable decking material. Composite decking (such as Trex or TimberTech) offers lower maintenance but costs more. Cedar and redwood provide natural rot resistance at a premium price. Board lengths are typically available in 8, 10, 12, 16, and 20-foot increments.
Waste and Extras
This calculator provides the net material count without a waste factor. Adding 10% to the decking board count is advisable to account for cuts, defects, and miscuts. Additional materials not included here are rim joists, joist hangers, screws or hidden fasteners, post anchors, concrete footings, and railing components.
Worked Example
For a 12 x 16 foot deck with 5.5-inch wide boards, 12-foot board lengths, and 16-inch joist spacing: Deck area = 12 x 16 = 192 sq ft. Board rows = ceil((12 x 12) / (5.5 + 0.125)) = ceil(144 / 5.625) = ceil(25.6) = 26 rows. Boards per row = ceil(16 / 12) = 2. Total boards = 26 x 2 = 52 boards. Joists = ceil((16 x 12) / 16) + 1 = ceil(12) + 1 = 13 joists. Posts along length = ceil(16 / 6) + 1 = 4. Posts along width = ceil(12 / 6) + 1 = 3. Total posts = (2 x 4) + (2 x 3) - 4 = 10 posts. Beams = 2 (width is 12 ft, so no center beam needed).