Building a deck involves four separate categories of lumber, and each one gets calculated differently. The decking boards that make the surface are easy. The framing underneath is where most people undercount, because the joist and beam layout depends on span tables and load requirements, not just the deck footprint.
Here is how to estimate each category accurately.
The four lumber categories
1. Decking boards (the walking surface) 2. Joists (the interior framing that supports the deck boards) 3. Beams (the larger members that support the joists) 4. Posts (the vertical supports from ground to beam)
Estimate each separately. Buying all four from the same supplier in one order usually gets you a better price per board foot.
1. Estimating decking boards
Deck boards run perpendicular to the joists. The most common sizes are 5/4 x 6 and 2 x 6 pressure treated. A 5/4 x 6 board has a nominal face width of 5.5 inches but installs at 5 to 5.25 inches with a 1/4-inch gap for drainage.
Number of rows = deck length / (board face width + gap in feet)
For a 16-foot-long deck using 5/4 x 6 boards with 1/4-inch gaps:
- Board width + gap = 5.5 in + 0.25 in = 5.75 in = 0.479 ft
- Rows = 16 / 0.479 = 33.4, so 34 rows
Board length: Choose a length that covers the deck width with minimal waste. For a 12-foot-wide deck, 12-foot boards work perfectly. For a 14-foot-wide deck, you need 16-foot boards (the next standard length up) and cut 2 feet of waste from each.
Add 15 percent waste to account for board ends that need trimming, knots, and the occasional split board. So 34 rows x 1.15 = 40 boards for a 12-foot deck.
2. Estimating joists
Joist spacing depends on the deck board thickness. Standard guidance:
- 5/4 x 6 deck boards: 16-inch joist spacing
- 2 x 6 deck boards: 24-inch joist spacing
Number of joists = (deck width in inches / spacing in inches) + 1
For a 12-foot-wide deck (144 inches) at 16-inch spacing:
- Joists = (144 / 16) + 1 = 9 + 1 = 10 joists
Joist length equals the deck depth minus the thickness of the rim joists on each end. For a 16-foot-deep deck: joists run 16 ft minus 3 inches on each side = 15 ft 6 in. Use 16-foot lumber and cut to fit.
Add 2 rim joists that run the full width of the deck on each end.
3. Estimating beams
Beams span parallel to the deck boards, perpendicular to the joists. The number of beams depends on the joist span. Typical 2x10 joists can span up to 12 feet. For a 16-foot-deep deck, you need an interior beam at 8 feet, creating two joist spans of 8 feet each, plus the ledger board attached to the house.
Beam size depends on the tributary area each beam carries. For residential decks, 3-ply 2x10 beams are common for most spans. A structural engineer or your local building department can confirm if your load or span requires something heavier.
4. Estimating posts
Posts go from the footing piers to the underside of the beams. Count the number of beam-to-post connections on your plan, then measure from grade to the underside of the beam at each location. Posts are typically 4x4 for decks under 8 feet high and 6x6 for taller decks or heavier loads.
A complete worked example
Deck dimensions: 12 feet wide by 16 feet deep. Ground-level deck, 5/4 x 6 decking, 16-inch joist spacing.
| Category | Quantity | Size |
|---|---|---|
| Decking boards | 40 pieces | 5/4 x 6 x 12 ft |
| Interior joists | 8 pieces | 2 x 10 x 16 ft |
| Rim joists | 2 pieces | 2 x 10 x 12 ft |
| Ledger board | 1 piece | 2 x 10 x 12 ft |
| Interior beam (3-ply) | 3 pieces | 2 x 10 x 12 ft |
| Posts (24 in above grade) | 4 pieces | 4 x 4 x 4 ft |
Pressure treatment: what grade to specify
All lumber in contact with the ground or within 6 inches of grade requires ground-contact pressure treated lumber, rated UC4A or UC4B depending on your region. Framing that is above ground but covered by decking, like the joists, uses above-ground rated lumber at UC3B. Decking boards exposed to the weather use UC3A or composite alternatives.
Do not use interior-rated or untreated lumber anywhere in a deck. It will rot within 5 to 10 years and compromise the structure.
Use the lumber calculator to calculate board feet for any deck dimension and lumber size. For full board-foot pricing, the board foot calculator converts any lumber dimension to board feet so you can compare quotes from different suppliers accurately.