How the deck board count works
Deck boards are laid in rows across the joists. The width of the deck decides how many rows fit, and the length of the deck decides how many boards each row needs. Multiply the two and add a little for cuts and bad boards.
Rows = deck width (in) ÷ (board width + gap), rounded up
Boards per row = deck length ÷ board length, rounded up
Boards = rows × boards per row × (1 + waste %)
Example: a 16 × 12 ft deck with 5.5 inch boards and 1/8 inch gaps needs 144 ÷ 5.625 = 25.6, so 26 rows. Each row is one 16 ft board, so that's 26 boards, or 29 with 10% extra.
Joists and screws
The calculator counts the joists needed under the decking at your chosen spacing, including one at each end. Each joist runs the full width of the deck. It doesn't include beams, posts, ledger boards or rim joists, which depend on how your deck is supported and attached.
Joists = deck length (in) ÷ spacing, rounded up, + 1
Screws = rows × joists × 2
Common decking sizes
| Board | Actual width | Typical use |
|---|---|---|
| 5/4 × 6 | 5.5 in | Most wood decks |
| 2 × 6 | 5.5 in | Stiffer wood decking, wider joist spacing |
| 2 × 4 | 3.5 in | Narrow-board look, more fasteners |
| Composite | about 5.5 in | Low-maintenance decks; check the brand's spacing rules |
Planning tips
- Size the deck to standard board lengths when you can. A 16 ft deck uses full 16 ft boards with no joints; a 17 ft deck needs joints in every row.
- Picture-frame borders and stairs need extra boards. Calculate stair treads separately.
- Check your local code before building. Most areas require a permit for decks attached to the house or above a set height.
Frequently asked questions
How many deck boards do I need per square foot?
With standard 5.5 inch wide boards and a 1/8 inch gap, each row covers about 0.47 feet of deck width. That works out to about 2.1 linear feet of decking per square foot of deck, before waste.
What size gap should deck boards have?
For most wood decking, about 1/8 inch between boards lets water drain. Pressure-treated boards that are still wet are often butted tight, because they shrink as they dry. Composite boards usually call for 3/16 inch or whatever the manufacturer specifies.
How far apart should deck joists be?
Joists are commonly spaced 16 inches on center for boards run straight across the joists. Diagonal decking and many composite products need 12 inches on center. Always follow the decking manufacturer's span rating.
How many screws do I need for a deck?
The standard is two screws where each board crosses each joist. The calculator counts every crossing and doubles it, so you can buy the right number of boxes. Hidden fastener systems usually use one clip per crossing.
What board length should I buy?
Pick a length that covers your deck in one run if you can, or that divides evenly into it. If boards must be joined, the joint has to land on a joist, and staggering joints across rows looks better and adds strength.