Deck Calculator

US & Canada

Enter your deck size to get the decking boards, joists and screws to buy, with your board size, gap and joist spacing.

Your deck

1. Deck size

ft
ft

Decking runs along the house, joists run out from it.

2. Boards

in
%

A 1/8 in gap is common for kiln dried and composite boards. Follow the board maker's spacing.

3. Framing

screws

Composite and diagonal decking often need 12 in spacing. Joist size depends on span, species and your local code.

Result

0

The rest of the list

Joist count includes both ends. Beams, posts, ledger, hangers and footings depend on the design and are not included.

How the deck maths works

  • Rows of boards = deck depth ÷ (board width + gap), rounded up
  • Boards = rows × deck length × (1 + waste) ÷ board length, rounded up
  • Joists = deck length ÷ joist spacing + 1
  • Screws = rows × joists × screws per crossing

Boards are measured by their actual face width: a 5/4 × 6 or 2 × 6 board is 5 1/2 in wide, not 6.

Materials for common deck sizes

5 1/2 in boards, 1/8 in gap, 12 ft boards, joists at 16 in, 10% waste, 2 screws per crossing.

Deck boards, joists and screws for common deck sizes
Deck Area 12 ft boards Joists Screws
10 × 10 ft 100 sq ft 21 8 352
12 × 12 ft 144 sq ft 29 10 520
12 × 16 ft 192 sq ft 39 10 700
16 × 16 ft 256 sq ft 52 13 910
16 × 20 ft 320 sq ft 64 13 1,118
20 × 20 ft 400 sq ft 79 16 1,376

Building in Canada? Joist sizes depend on span and species. See the Canadian deck joist span chart, and check footing depth with your municipality.

Before you buy

  • Get a permit first. Most municipalities need one for a raised or attached deck.
  • Plan board joints over joists. Stagger butt joints, and add a joist where two boards meet if your design calls for it.
  • Composite needs closer joists. Many composite boards need 12 in centres for diagonal layouts. Check the maker's guide.
  • Not included here: beams, posts, ledger board, hangers, footings, stairs and railings.

Common questions

How many deck boards do I need for a 12x12 deck?
With 5 1/2 in boards and a 1/8 in gap, a 12 × 12 ft deck has 26 rows of boards. That is 312 linear feet, or 29 boards 12 ft long with 10% waste.
How many deck boards for a 12x16 deck?
For a deck 16 ft along the house and 12 ft deep: 39 boards 12 ft long with 10% waste, on 13 joists at 16 in centres. 16 ft boards would avoid butt joints along the length.
How many deck boards for a 16x16 deck?
Using 16 ft boards, a 16 × 16 ft deck takes 39 boards with 10% waste: 35 rows, one board per row plus spares.
How many joists do I need for a deck?
Divide the length along the house by the joist spacing and add one. A 16 ft deck at 16 in centres needs 13 joists; a 20 ft deck needs 16. Joist size depends on how far they span, the lumber species and your local code.
How many screws do I need for a deck?
Two screws at every point a board crosses a joist. A 12 × 12 ft deck at 16 in centres has 260 crossings, so about 520 screws. Check the screw box for how many it holds, and follow the board maker's fastening schedule for composite.
What gap should deck boards have?
It depends on the board. Kiln dried and composite boards are usually gapped at about 1/8 in to 1/4 in, while wet pressure treated boards are often laid tight because they shrink as they dry. The board maker's instructions govern, and the calculator lets you change the gap.
How far can deck joists span?
It varies with size, species, spacing and snow load. In Canada, see our deck joist span chart; elsewhere, use your local code's span table. Your building department has the final say.