Deck calculator (boards, joists and screws)

Deck calculator (boards, joists and screws): what it is for

Enter the length and width of the deck and pick the decking board (5.5 in for 5/4×6 and 2×6, 3.5 in for 2×4), the gap between boards, the joist spacing and the length of the boards you'll buy. The calculator shows the deck area, the number of rows and boards with waste, the number of joists and the screws or hidden clips, plus the cost from your prices.

The estimate covers the decking surface and joists. Beams, posts, footings, ledger, hardware and railing depend on the design and your local code.

How to use

1. Enter the deck size.
2. Choose the board, gap, joist spacing and length.
3. Read the boards, joists, screws and cost.

When to use it

• Planning a new deck.
• Replacing old deck boards.
• Comparing wood and composite costs.

Frequently asked questions

How do you calculate how many deck boards you need?

Divide the deck width by the board width plus the gap (a 5.5 in board with a 3/16 in gap covers about 5.69 in), round up to get the rows, and multiply by the number of boards each row needs in the length you buy. Add 5 to 10% for cuts and bad boards.

What joist spacing should I use?

16 inches on center is standard for most wood decking laid straight. Many composite boards and diagonal layouts need 12 inches on center. Always follow the decking manufacturer's span table and your local code.

How many screws per board?

Use two screws at every joist for each board. With joists 16 inches on center, that is about 350 screws per 100 sq ft of 5.5 in decking. Hidden fastener systems use one clip per board per joist.

Is it really free?

Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.

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