How many boards for a 12 × 20 deck?
A 12x20 deck is 240 sq ft of surface. With standard 5.5-inch boards, a 1/4-inch gap, and 10% waste, that's 35 sixteen-foot boards.
| Board length | Boards needed (incl. 10% waste) |
|---|---|
| 12 ft | 46 |
| 16 ft | 35 |
| 20 ft | 28 |
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
Planning the rest of the 12x20 deck
Boards are the visible quarter of a deck project. Under them you'll frame joists at 16 inches on center (12 for composite on a diagonal), and the whole structure needs footings sized to your frost depth. For the full budget picture, see what a deck really costs, and if you're weighing this against a ground-level alternative, the comparison lives on our sister site: deck vs. patio costs. Stairs? The stair stringer calculator turns your deck height into riser and tread counts.
Frequently asked questions
How many deck boards do I need for a 12x20 deck?
Using standard 5.5 inch wide boards with a 1/4 inch gap and 10% waste, a 12x20 deck takes about 35 16-ft boards, 46 12-ft boards, or 28 20-ft boards. Match board length to your deck dimension to minimize butt joints.
How much does decking for a 12x20 deck cost?
Surface boards only: pressure-treated runs about $960 to $1,920 installed ($4 to $8/sq ft), composite about $2,880 to $6,000 ($12 to $25/sq ft). Framing, footings, railings, and stairs are separate.
What board length should I buy for a 12x20 deck?
At 20 ft, single boards can't span the long dimension, so plan staggered butt joints over doubled joists, or run boards across the 12 ft dimension.