How the deck calculator works
Rows of boards = deck width (in) ÷ (board width + gap), rounded up
Linear feet = rows × deck length
Boards = linear feet × (1 + waste) ÷ board length, rounded up
Joists = deck length (in) ÷ spacing + 1
Screws = rows × joists × 2
Worked example
A 16 by 12 foot deck with 5.5 inch boards and 1/8 inch gaps needs 26 rows (144 ÷ 5.625). That is 416 linear feet, or 29 boards of 16 feet with 10% extra. Joists 16 inches apart along 16 feet: 13 joists, 12 feet long. Screws: 26 × 13 × 2 = 676.
What this does not cover
Beams, posts, footings, the ledger board, railings and stairs carry the deck's load and are set by your local building code and span tables. Most areas require a permit for a deck. Use the stairs calculator for steps and the concrete calculator for footings.
Board choices
| Board | Actual width | Notes |
|---|---|---|
| 5/4x6 pressure-treated | 5.5 in | Most common, lowest cost |
| 2x6 pressure-treated | 5.5 in | Stiffer, heavier |
| Composite | About 5.5 in | Follow the maker's gap and joist spacing |
Tips
- Choose board length so joints land on joists and are staggered from row to row.
- Use screws rated for treated lumber or the composite brand you use.
Common questions
How many deck boards do I need for a 12x16 deck?
With 5.5 inch boards and 1/8 inch gaps, 12 feet of width needs 26 rows. Running 16 foot boards along the 16 foot side, that is 26 boards, or 29 with 10% extra.
How far apart should deck joists be?
16 inches on center is standard for wood decking laid straight. Diagonal decking and many composite boards need 12 inches. Always check the board maker's instructions.
How many screws do I need for a deck?
Two screws per board at every joist. A 16 by 12 deck with 13 joists and 26 rows needs about 676 screws.
What gap should I leave between deck boards?
About 1/8 inch for dry or composite boards. Wet pressure-treated boards shrink, so they are often laid tight. Follow the product's guide.