How many deck boards do I need for a 12 by 12 foot deck?

With boards 5.5 in wide and gaps of 1/8 in there are 144 ÷ 5.625 = 25.6, so 26 rows of 12 ft. That is 26 boards of 12 ft before any allowance, and 29 boards with 10 percent.

Which way should the boards run?

Boards run across the joists. Enter the deck length in the direction of the boards. Turning the boards by 90 degrees changes the number of rows, the length of each row and the direction of the joists, so run the calculator again with length and width swapped.

Does the calculator work for diagonal boards?

Not directly. Diagonal boards are longer, produce more offcuts and can need closer joists. As an estimate, keep the measurements and raise the allowance; TimberTech suggests 30 percent for diagonal and herringbone layouts.

How far apart should the joists be?

That depends on the board and on your building code. The American Wood Council notes that decking products are usually made for joists at 16 or 24 inches, and the code of practice of the UK Timber Decking and Cladding Association allows 300 to 600 mm. Use the value that the manufacturer of your board specifies.

Does the result include the frame, posts and fixings?

No. It includes the surface boards and the joists at the spacing you enter. Rim joists, beams, posts, footings, screws and hidden fasteners are separate.

Decking calculator: deck boards and joists

Enter the size of the deck, the size of the boards, the gap between them and the joist spacing. The calculator counts the rows of boards, the boards to buy and the joists.

Result

This is the result for the example values. Enter your own values and select Calculate.

Boards to buy: 26
Rows of boards: 21
Total board length, without extra: 84 m
Joists: 11
Length of each joist: 3 m

Results are estimates. Measure on site and check the product label before you buy.

What this calculator does

It counts the surface boards of a rectangular deck and the joists that carry them. Boards are counted by total length, so you can see how many boards of a given length to buy.

It does not design the structure. The size of the joists, beams, posts and footings, the fixing to the house and the guards depend on the span, the height and your building code.

How to use it

Choose metric or US units.
Enter the deck length in the direction the boards run and the deck width across the boards.
Enter the real width of one board and the length of the boards you plan to buy.
Enter the gap between boards and the spacing of the joists, measured from centre to centre.
Set the allowance for cuts and defects.
Select Calculate.

How the result is calculated

Rows of boards = deck width ÷ (board width + gap), rounded up.
Total board length = rows × deck length.
Boards to buy = total board length × (1 + allowance ÷ 100) ÷ length of one board, rounded up.
Joists = deck length ÷ joist spacing, rounded up, plus 1 for the joist at the end.
Length of each joist = deck width.

Counting boards by total length assumes that offcuts are used where they fit and that joints between boards are placed on a joist. If your deck is longer than one board and you want every joint in a particular place, draw the layout and count the boards row by row.

The joist count is for joists that run across the boards at a regular spacing. Rim joists, blocking and beams are not included.

Worked example

A deck is 4 m long in the direction of the boards and 3 m wide. The boards are 140 mm wide with 5 mm gaps. Rows: 3000 ÷ 145 = 20.7, rounded up to 21. Total board length: 21 × 4 = 84 m.

With 10 percent allowance: 84 × 1.1 = 92.4 m. With boards of 3.6 m: 92.4 ÷ 3.6 = 25.7, rounded up to 26 boards. Joists at 400 mm: 4000 ÷ 400 = 10 spaces, so 11 joists of 3 m.

In US units: a deck of 12 ft by 10 ft with boards 5.5 in wide and gaps of 1/8 in has 120 ÷ 5.625 = 21.3, so 22 rows. Total board length: 22 × 12 = 264 ft. With 10 percent allowance that is 290.4 ft, and with 12 ft boards 290.4 ÷ 12 = 24.2, rounded up to 25 boards. Joists at 16 in: 144 ÷ 16 = 9 spaces, so 10 joists of 10 ft.

About the example values

The examples follow published guidance. Your boards and your code may require other values.

Board sizes, gaps and joist spacing in published guidance
Subject	Value	Source
Real size of a 5/4 × 6 deck board (United States)	1 in × 5 1/2 in	Southern Forest Products Association
Gap between wood deck boards (United States)	About 1/8 in	American Wood Council, DCA 6
Joist spacing for composite boards (United States)	At most 16 in; 12 in for diagonal boards	Trex installation guide
Board width (United Kingdom)	About 90 mm to 145 mm	Timber Decking and Cladding Association, code of practice
Gap between boards (United Kingdom)	5 mm to 8 mm	Timber Decking and Cladding Association, code of practice
Joist spacing (United Kingdom)	300 mm to 600 mm; span tables based on 400 mm	Timber Decking and Cladding Association, code of practice

The board lengths of 3.6 m and 12 ft are examples of lengths on sale; Trex lists 12, 16 and 20 ft. Composite boards have their own rules for gaps, which depend on the temperature during installation, so follow the instructions of the manufacturer.

Manufacturers give different allowances: Trex suggests 5 percent, and TimberTech 10 to 15 percent for a standard layout and 30 percent for diagonal or herringbone layouts. The calculator starts with 10 percent.

Common mistakes

Entering the nominal board size. A board sold as 6 inches wide is 5 1/2 inches wide, and metric boards are also sold by nominal size in some stores.
Swapping length and width. The length in this calculator is the direction of the boards; the joists run across it.
Forgetting the gap. Without gaps the calculator counts more boards than you need.
Using the joist count as a structural design. Spacing and size of joists depend on the board, the span and the code.
Ordering boards that are shorter than the deck without planning where the joints fall.

Frequently asked questions

How many deck boards do I need for a 12 by 12 foot deck?: With boards 5.5 in wide and gaps of 1/8 in there are 144 ÷ 5.625 = 25.6, so 26 rows of 12 ft. That is 26 boards of 12 ft before any allowance, and 29 boards with 10 percent.
Which way should the boards run?: Boards run across the joists. Enter the deck length in the direction of the boards. Turning the boards by 90 degrees changes the number of rows, the length of each row and the direction of the joists, so run the calculator again with length and width swapped.
Does the calculator work for diagonal boards?: Not directly. Diagonal boards are longer, produce more offcuts and can need closer joists. As an estimate, keep the measurements and raise the allowance; TimberTech suggests 30 percent for diagonal and herringbone layouts.
How far apart should the joists be?: That depends on the board and on your building code. The American Wood Council notes that decking products are usually made for joists at 16 or 24 inches, and the code of practice of the UK Timber Decking and Cladding Association allows 300 to 600 mm. Use the value that the manufacturer of your board specifies.
Does the result include the frame, posts and fixings?: No. It includes the surface boards and the joists at the spacing you enter. Rim joists, beams, posts, footings, screws and hidden fasteners are separate.

Last updated: September 28, 2026