How Much Do I Need?

Deck Board Calculator

How many deck boards do I need?

Enter the size of the deck, the width and length of your boards and the gap between them. The calculator works out how many rows of boards fit, the boards to buy and the screws to fix them to the joists.

Units
Deck
Boards and fixings

A "6-inch" board is actually 5.5 in wide. Use the real width.

3/16 in = 0.1875 in. Follow the board maker’s spacing.

Centre to centre. 16 in (40 cm) is common.

For cuts, ends and boards with defects. Diagonal patterns need more.

Cost (optional)

Add the price to see an estimated total.

Results update as you type.

Your estimate

Decking to buy

29 boards

16 ft boards, plus about 744 screws, including 10% extra.

Estimated amount to buy

Boards (16 ft long)
29
Deck screws
744

Calculated quantity (no waste)

Deck area
192 ft²
Rows of boards
26
Total board length
416 ft
Boards
26
Joists crossed by each row
13
Screws
676
How this was calculated
  1. Rows = 12 ft ÷ (5.5 in + 0.187 in gap) = 25.32, rounded up to 26
  2. One board per row, because 16 ft boards cover the 16 ft length: 26 boards
  3. With 10% waste = 26 × 1.1 = 28.6, rounded up to 29
  4. Joists = 16 ft ÷ 16 in spacing, rounded down, + 1 = 13
  5. Screws = 26 rows × 13 joists × 2 = 676; with 10% extra = 744
  • When the deck is longer than a board, the count assumes offcuts are reused in other rows. Joints must land on a joist, so plan the layout before buying.
  • Hidden-fastener systems use clips instead of face screws; follow the maker’s count.

How the deck board calculation works

  1. Rows = deck width ÷ (board width + gap), rounded up. The last row can be ripped narrower to fit.
  2. Boards = rows × (deck length ÷ board length), with at least one board per row, rounded up. Then the waste allowance is added and rounded up again.
  3. Joists = deck length ÷ joist spacing, rounded down, plus one.
  4. Screws = rows × joists × screws per board at each joist, plus the extra.

Use the board's actual width, not its nominal name: a "5/4 × 6" deck board is about 5.5 in (140 mm) wide.

Gaps and spacing

Deck boards need gaps for drainage and movement. Wood that is still wet from treatment shrinks as it dries, so it is often laid tight, while kiln-dried and composite boards are usually spaced at around 1/8–1/4 in (3–6 mm). Composite makers publish exact gap and joist-spacing requirements, and they must be followed for the warranty.

Rounding rules

  • Rows and boards are rounded up, before and after the waste allowance.
  • Joists are counted by rounding the spacing division down and adding the end joist.
  • Screws are rounded up to a whole screw. They are usually sold by the box or by weight.