How many deck boards do I need?
Boards run across the joists, usually parallel to the house. So the number of rows is the depth of the deck divided by the width of one board plus its gap, and each row is as long as the deck is wide.
A 5½-inch board with a ⅛-inch gap covers 5⅝ inches. Twelve feet of depth is 144 inches, which takes 26 rows, the last one ripped a little narrower. Here is what that works out to for common sizes, with the board length that wastes the least:
| Deck, depth × width | Rows | Boards, least waste | Linear ft | Screws |
|---|---|---|---|---|
| 10 × 10 ft | 22 | 11 × 20 ft | 220 | 352 |
| 12 × 12 ft | 26 | 26 × 12 ft | 312 | 520 |
| 12 × 16 ft | 26 | 26 × 16 ft | 416 | 676 |
| 16 × 20 ft | 35 | 35 × 20 ft | 700 | 1,120 |
| 20 × 20 ft | 43 | 43 × 20 ft | 860 | 1,376 |
Those counts are before the 5% of spares, with 5½-inch boards, a ⅛-inch gap and joists 16 inches apart. The calculator handles the rest.
Which board length to buy
This is where most of the waste on a deck comes from, and where the calculator does its real work. On a 12-foot-wide deck, 12-foot boards fit exactly. On a 14-foot deck they don't, and every row becomes a 12-foot board plus a 2-foot piece, which means a joint in every row.
Joints aren't a problem as long as each one lands on a joist, and the joints in neighbouring rows don't line up. The calculator plans it that way. Every other row starts with half a board, every joint falls on a joist, and any offcut long enough to reach from one joist to the next goes back on the pile to be used before a new board is cut.
Then it does the same with every length, 8 to 20 feet, and shows them side by side. Longer boards aren't always the answer. On a 10-foot deck, 20-foot boards cut neatly in two, while 16-foot boards leave a 6-foot piece that has to find a home.
Straight or diagonal
Boards laid at 45° look good and take more work. Every row is a different length, both ends are cut at an angle, and the corners of the deck turn into short pieces. The calculator plans those cuts too, so the extra decking shows up in the count instead of as a surprise at the store.
A board on a diagonal also crosses the joists at a longer stretch, so diagonal decking usually goes on joists 12 inches apart rather than 16. Our guide to joist spacing has the details. If the frame is already built at 16 inches, check your decking's instructions before you go diagonal.
Screws or hidden clips
Wood boards are face-screwed, two screws at every joist, a little in from each edge. That adds up quickly: a 12 by 16 deck with joists at 16 inches takes about 700 screws.
Most composite and PVC boards have a groove down each edge for hidden clips. One clip goes where each board crosses a joist and holds two boards at once, with screws only at the first and last rows. Clips cost more each but leave a surface with no screw heads.
Fair questions
How much extra should I buy?
The cut plan already counts the offcuts, so the extra is only for boards that split, are badly bowed or have ends you'd rather not use. Five percent is a sensible margin for wood. Composite is more consistent, but a spare board or two saves a second trip.
Do the boards overhang the edge?
Usually by an inch or so, and some builders add a fascia board around the rim. The calculator measures from rim to rim. If you overhang a lot, add a few inches to the width and depth.
Does this count the joists?
It counts how many there are at your spacing, so you can check the frame. Their size depends on the span, and the deck cost calculator prices the whole frame. For what goes underneath, the deck footing calculator counts the footings and concrete.
Sources
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide: ledger attachment, joist spans, guards and stairs for wood decks.
- International Residential Code: sections R312 (guards), R311.7.8 (handrails) and R507 (decks). Your city or state adopts and amends it, so check the local version.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
