How to use the deck board calculator
- Enter the deck size. Length is the direction the boards run; width is across the boards.
- Pick the board width and the length you will buy. The table in the result compares every common length.
- Choose screws or hidden clips.
- Check the assumptions: gap, joist spacing and waste. Diagonal decks need more waste.
- Add prices for a decking and fastener total.
The deck board formulas
Rows = deck width in inches ÷ (board width + gap), rounded up
Boards per row = deck length ÷ board length, rounded up
Boards = rows × boards per row × (1 + waste), rounded up
Joists = deck length in inches ÷ joist spacing (rounded down) + 1
Screws = rows × joists × screws per crossing × (1 + extra)
Worked example
A 12 × 16 ft deck with 5 1/2 in boards, 1/8 in gap, 16 ft boards
- Rows = 144 in ÷ (5.5 + 0.125) = 25.6, so 26 rows.
- Boards per row = 16 ÷ 16 = 1, so 26 boards; with 10% waste 28.6, so 29 boards.
- Joists = 192 in ÷ 16 + 1 = 13.
- Screws = 26 × 13 × 2 = 676; with 5% extra 710 screws, or 3 boxes of 350.
Board coverage
| Board | Actual width | Covers with 1/8 in gap | Rows per 10 ft of width |
|---|---|---|---|
| 2 × 4 | 3 1/2 in | 3 5/8 in | 34 |
| 2 × 6 or 5/4 × 6 | 5 1/2 in | 5 5/8 in | 22 |
| Typical composite | about 5.4 in | about 5.5 in | 22 |
Common mistakes to avoid
- Using nominal width. A "6 inch" board is 5 1/2 in wide. Using 6 in under-counts rows.
- Ending on a sliver. Check the last-board width in the result and rip the first and last boards evenly.
- Joints between joists. Every butt joint must land on a joist, often with extra blocking.
- Ignoring the maker's span. Many composites need 12 in joist spacing for diagonal layouts.
- No fascia or stair treads. Rim boards, stair treads and picture framing are extra boards.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5 1/2 in boards and a 1/8 in gap, a 12 ft wide deck needs 26 rows. Using 16 ft boards along the 16 ft length, that is 26 boards, or 29 with 10% extra for cuts and defects.
How many screws per deck board?
Two screws per board at every joist is the normal pattern. A 12 × 16 deck with joists at 16 in on center has 13 joists and 26 rows of boards, so about 676 screws, or 710 with 5% extra.
Which way should deck boards run?
Deck boards usually run at right angles to the joists, often parallel to the house. Diagonal boards look great but need closer joist spacing and more waste.
What gap should I leave between deck boards?
Commonly 1/8 to 1/4 inch for drainage and airflow. Wet pressure-treated wood shrinks as it dries, so it is often laid tight. Composite and PVC boards need the gap the manufacturer specifies, including at the ends.
Should I buy longer boards to avoid joints?
Boards that cover the full length avoid butt joints and look cleaner, but long boards cost more and are harder to handle. If the deck is longer than your boards, stagger the joints and land every joint on a joist.
Does this calculator size my joists and beams?
No. It counts joists for the surface at the spacing you choose. Joist size, beams, posts, footings and ledger attachment are structural and must follow your local code or a deck design.