How it works
The formula
boards = rows × boards per row × (1 + waste), round up
joists = length (in) ÷ spacing, round down, + 1
screws = rows × joists × 2 × 1.05, ÷ 400 per box, round up
stain gallons = area × coats ÷ coverage, round up
Assumptions
- Boards run along the length. Joists run across the width, spaced along the length, so every row of boards crosses every joist.
- Boards are 5.5 in wide (5/4×6 and 2×6 are the same width) with a 0.1875 in (3/16 in) gap. Each row is one 12 ft board, or more joined end to end and staggered when the deck is longer. Boards are priced as 12 ft boards; for other lengths, enter your price. 2×6 is priced with the 5/4×6 price, so enter yours.
- Two screws where each board meets a joist, plus 5% extra. A 5 lb box of #10 × 2-1/2 in deck screws holds about 400: retailer listings show 370 to 455 depending on brand, so check the box.
- Joists are counted for any deck. We price them only as 12 ft treated 2×8s, and only when the width is 12 ft or less. For anything wider, price your own lumber. We never suggest a joist size.
- Stain covers 200 sq ft per gallon per coat by default. Olympic's listings say 250 to 350 sq ft on smooth wood and less on rough or weathered wood. Change it to match your can. Railings and stairs are not included.
- We don't size joists, beams, spans, ledgers or footings. Those depend on your local code and loads.
Worked example
A 12 × 16 ft deck, boards along the 12 ft side, joists 16 in apart, 10% waste
- Rows: 192 in ÷ 5.6875 in = 33.76, round up to 34
- Boards: 34 rows × 1 board per row × 1.10 = 37.4, round up to 38
- Joists: 144 in ÷ 16 in = 9.0, + 1 = 10
- Screws: 34 × 10 × 2 × 1.05 = 714. ÷ 400 = 1.78, round up to 2 boxes
- Stain: 192 sq ft × 2 coats ÷ 200 = 1.92, round up to 2 gallons
- At the middle of our typical prices, boards, screws and stain come to about $651.75, before joists
Common deck sizes
| Deck | Boards | Screw boxes | Stain gallons |
|---|---|---|---|
| 10×10 ft | 25 | 2 | 1 |
| 12×12 ft | 29 | 2 | 2 |
| 12×16 ft | 38 | 2 | 2 |
| 16×20 ft | 95 | 4 | 4 |
Boards run along the first number. 5/4×6 or 2×6 boards, 16 in joists, 10% waste, 2 coats at 200 sq ft per gallon.
Questions people ask
How many deck boards do I need?
For a 12 × 16 ft deck, 38 boards of 12 ft: 34 rows, plus 10% for cuts and mistakes. A diagonal pattern wastes more, so use 15%: that makes 40. A deck longer than 12 ft needs more than one board per row, as in a 20 × 12 ft deck with 2 per row.
How many screws per deck board?
Two screws at every joist the board crosses. A 12 ft board over joists 16 in apart crosses 10 joists, so about 20 screws per board. The whole deck takes about 714 screws, or 2 boxes of 5 lb with about 400 in each.
How much stain do I need for a deck?
A 192 sq ft deck with 2 coats at 200 sq ft per gallon takes 2 gallons. One coat takes 1. Rough or old wood soaks up more, and railings and stairs add area, so check the coverage on your can.
How big should the gap between deck boards be?
We use 0.1875 in (3/16 in). Wet treated lumber shrinks as it dries, so many builders lay it with little or no gap. Follow the board maker's instructions, especially for composite, which can need a different gap.
What joist spacing should I use?
16 in centre to centre is common for wood decking, and composite boards often call for 12 to 16 in depending on the maker, or closer for diagonal laying. Check your board's instructions and local code. At 12 in, your 12 ft deck has 13 joists instead of 10. We count joists but don't size them.