This deck board calculator estimates how many deck boards and screws you need for a rectangular deck surface, based on the deck size, board width, gap between boards, board length, joist spacing and a waste allowance. It covers the walking surface only, so you can price decking for a new build or for replacing old boards on an existing frame.
How to Use the Deck Board Calculator
- Enter the deck length in feet. This is the direction the boards run, which is across the joists.
- Enter the deck width in feet. This is the direction you count rows of boards, usually out from the house.
- Choose the board width. A standard 5/4 x 6 or 2 x 6 deck board is about 5.5 inches wide, and a 2 x 4 is about 3.5 inches.
- Choose the gap between boards. Many builders use 3/16 inch for wood, but follow the board maker’s instructions, especially for composite.
- Choose the board length you plan to buy and the joist spacing of your frame.
- Pick a waste allowance and read the board count, linear feet and screw estimate in the results.
How the Calculation Works
The calculator converts the deck width to inches and divides it by the board width plus the gap, then rounds up. That gives the number of rows of boards. It then divides the deck length by the board length and rounds up to find how many boards make up each row. Rows times boards per row, plus the waste allowance, rounded up, is the number of boards to buy. Linear feet is that board count times the board length.
For screws, the calculator counts the joists the boards cross: deck length in inches divided by joist spacing, rounded up, plus one for the end joist. Each board gets 2 screws at every joist, so screws equal rows times joists times 2.
Worked example with the default values: a 16 x 12 foot deck, 5.5 inch boards, 3/16 inch gap, 16 foot boards, 16 inch joist spacing and 10% waste.
- Deck area: 16 x 12 = 192 square feet
- Width per row: 5.5 + 0.1875 = 5.6875 inches
- Rows: 12 x 12 = 144 inches, and 144 / 5.6875 = 25.3, rounded up to 26 rows
- Boards per row: 16 / 16 = 1 board
- Boards with waste: 26 x 1 x 1.10 = 28.6, rounded up to 29 boards
- Linear feet: 29 x 16 = 464 feet
- Joists crossed: 16 x 12 = 192 inches, 192 / 16 = 12, plus 1 = 13 joists
- Screws: 26 rows x 13 joists x 2 = about 676 screws
Deck Boards for Common Deck Sizes
These results use the calculator’s defaults (5.5 inch boards, 3/16 inch gap, 16 inch joists and 10% waste) with a board length chosen to suit each deck length.
| Deck size (length x width) | Board length | Rows | Boards to buy | Linear feet | Screws (about) |
|---|---|---|---|---|---|
| 10 x 10 ft | 10 ft | 22 | 25 | 250 | 396 |
| 12 x 12 ft | 12 ft | 26 | 29 | 348 | 520 |
| 16 x 12 ft | 16 ft | 26 | 29 | 464 | 676 |
| 16 x 16 ft | 16 ft | 34 | 38 | 608 | 884 |
| 20 x 12 ft | 20 ft | 26 | 29 | 580 | 832 |
| 20 x 16 ft | 20 ft | 34 | 38 | 760 | 1,088 |
| 24 x 12 ft | 12 ft | 26 | 58 | 696 | 988 |
Tips and Common Mistakes
- Leave room to trim the ends. Board ends are often checked or out of square, and many decks overhang the rim slightly. If your deck length matches the board length exactly, consider the next longer board or use a slightly higher waste allowance.
- Plan butt joints on joists. When rows need more than one board, every joint has to land on a joist, and it is common to stagger joints from row to row. That can mean extra cutoffs, so lean toward 15% waste on long decks.
- Composite has its own rules. Composite and PVC boards often need specific gaps, end gaps for expansion, and sometimes 12 inch joist spacing, especially for diagonal layouts. Hidden fastener systems replace face screws, so ignore the screw count if you use them.
- Diagonal decking needs more. The calculator assumes boards run straight across the joists. A 45 degree pattern produces more angled cuts and usually needs 15% or more waste.
- Stairs, railings and fascia are separate. Count stair treads, rail caps and fascia boards on their own, since they are often a different size or product.
- Buy screws by the box. Deck screws are sold by count or by pound. Round up to the next box size, and choose screws rated for treated lumber or the screws your board maker specifies.
Frequently Asked Questions
How many deck boards do I need for a 12 x 16 deck?
With 5.5 inch boards running the 16 foot direction, a 3/16 inch gap and 10% waste, you need about 29 boards that are 16 feet long. That is 26 rows plus a few spares.
What gap should I leave between deck boards?
For dry wood boards, about 1/8 to 1/4 inch is common, and 3/16 inch is a popular middle choice. Wet pressure treated boards are often laid tight because they shrink as they dry. Composite gaps should follow the manufacturer’s instructions.
How many screws per deck board?
The usual pattern is 2 screws per board at every joist. On 16 inch joist spacing, a 16 foot board crosses 13 joists and takes about 26 screws.
Does this include the deck frame?
No. The calculator only covers the surface boards and their screws. Joists, beams, posts, footings, hangers and hardware need their own plan, and a deck frame usually needs a permit and must meet local code.