Deck board calculator
Deck boards usually run parallel to the house, across the joists. The count comes from three numbers:
- Rows = deck depth in inches ÷ (board width + gap). Standard 5/4×6 and 2×6 boards are 5-1/2 in wide; wood gets a 1/8 in gap, most composite 3/16 in.
- Boards per row = deck length ÷ board length, rounded up. Buy boards as long as the deck so you have no butt joints, or plan where the joints fall over a doubled joist.
- Waste — 10% for a straight layout, 15% for diagonal boards or a picture-frame border.
| Deck size | Board length | Boards (5/4×6 wood) | Joists at 16 in | Screws |
|---|---|---|---|---|
| 10 × 10 ft | 12 ft | 25 | 8 | ~350 |
| 12 × 12 ft | 12 ft | 29 | 10 | ~520 |
| 16 × 12 ft | 16 ft | 29 | 13 | ~680 |
| 16 × 16 ft | 16 ft | 39 | 13 | ~910 |
| 20 × 12 ft | 20 ft | 29 | 16 | ~830 |
| 20 × 16 ft | 20 ft | 39 | 16 | ~1,120 |
| 24 × 12 ft | 2 × 12 ft | 58 | 19 | ~990 |
The first number is the deck length along the house.
How many deck boards do I need? A worked example
Take a 16 ft wide, 12 ft deep deck in pressure-treated 5/4×6. The depth is 144 in; each board plus its gap takes 5-5/8 in, so you need 144 ÷ 5.625 = 25.6, rounded up to 26 rows. Sixteen-foot boards cover each row in one piece, and 10% on top makes 29 boards. Thirteen joists at 16 in on center means each board is fastened 13 times — two screws each, about 680 screws, or about 340 hidden clips if you use a clip system.
One caveat for wet pressure-treated lumber: many builders butt the boards tight, because they shrink as they dry and open up the gap on their own. Kiln-dried and composite boards keep the gap you set.
Deck joist spans: check before you buy
The calculator compares your deck depth with the maximum joist span in the 2021 IRC (Table R507.6), for joists running from the ledger to a single beam with no cantilever:
| Joist | Southern pine 12 in | 16 in | 24 in | Doug fir / hem-fir / SPF 12 in | 16 in | 24 in |
|---|---|---|---|---|---|---|
| 2×6 | 9′11″ | 9′0″ | 7′7″ | 9′6″ | 8′8″ | 7′2″ |
| 2×8 | 13′1″ | 11′10″ | 9′8″ | 12′6″ | 11′1″ | 9′1″ |
| 2×10 | 16′2″ | 14′0″ | 11′5″ | 15′8″ | 13′7″ | 11′1″ |
| 2×12 | 18′0″ | 16′6″ | 13′6″ | 18′0″ | 15′9″ | 12′10″ |
This is the mistake we see most in online deck plans: a 12 ft deep deck framed with 2×8s at 16 in exceeds the code span (11′10″ for southern pine). Go up to 2×10s or tighten the spacing to 12 in. Your local code may differ, and snow loads above 40 psf need shorter spans.
What this calculator doesn't size
Beams, posts, footings, stairs and railings depend on your deck height, soil and frost depth. Posts sit on concrete footings below the frost line — the footing calculator and post hole calculator give the concrete for them. A deck attached to the house also needs its ledger bolted through the house rim joist with flashing above it, never nailed.
When the boards are down, wait for new pressure-treated wood to dry out before you finish it, then use the deck stain calculator for gallons. Pricing cedar or hardwood decking by volume? The board foot calculator converts board-foot quotes into a per-board price.
