How the deck board count works
On most decks the boards run parallel to the house and the joists run out from it. The calculator first works out how many rows of boards fit across the depth of the deck, using the board's real width plus the gap. Then it finds how many boards each row takes, based on board length, and adds waste.
Joists are counted along the length of the deck at the spacing you pick, plus one at the end. Screws are figured at two per board at every joist it crosses.
Worked example: a 16 × 12 deck
- Board plus gap: 5.5 + 0.1875 = 5.69 in
- Rows: 144 in ÷ 5.69 = 25.3, so 26 rows
- 16 ft boards, one per row: 26 boards
- Add 10%: 29 boards
- Joists at 16 in: 13
- Screws: 26 rows × 13 joists × 2 = 676
Board length matters
When the deck is longer than your boards, you get butt joints, and every joint has to land on a joist. On a 16-foot deck, 16-foot boards give clean single runs. On a 20-foot deck, many builders mix 12s and 8s and stagger them so the joints don't line up in one row.
Joist spacing
Sixteen inches on center is standard for wood decking. Many composite boards call for 12 inches, and closer still for diagonal layouts. Follow the board maker's installation guide, since it often affects the warranty.
This tool counts joists but doesn't size them. Joist size depends on span, wood species and load. Your local building department has span tables, and attached decks usually need a permit.
Building a railing or privacy screen on the same project? The fence calculator handles posts and pickets.
Frequently asked questions
How many deck boards do I need for a 12 by 12 deck?
With 5.5 inch boards and a 3/16 inch gap, you need 26 rows. Using 12-foot boards that's 26 boards, or 29 with 10 percent extra.
What gap should I leave between deck boards?
For dry wood, about 1/8 to 3/16 inch is common. Wet pressure-treated boards are often laid tight because they shrink as they dry. For composite, use the spacing in the manufacturer's guide.
How many screws do I need per deck board?
Two screws at each joist the board crosses. A 16-foot board on joists 16 inches apart crosses 13 joists, so it takes 26 screws.
What joist spacing should I use for composite decking?
Many composite brands call for 12 inches on center for a straight layout and less for diagonal patterns. Check the installation guide for your exact board.
Results are estimates for planning. Product coverage, site conditions and installation methods vary, so check manufacturer guidance before you buy.