Deck Calculator
Boards, joists and screws for a deck - worked out from the board width and gap, not rough square-foot guesses.
How it works
- Rows of boards = deck width / (board width + gap), rounded UP
- Boards = rows x boards per row (deck length / board length, rounded UP), plus waste
- Joists = deck length / joist spacing + 1, running across the width
- Screws = about 2 per board at every joist crossing
Worked example
A 12 ft x 10 ft deck with 5.5 in boards and a 1/8 in gap: each row takes 5.625 in, so the 120 in width needs 22 rows. Boards run the 12 ft length, so 22 boards at 12 ft - with 10% waste buy 25 boards (about $450 at $18). Joists at 16 in spacing: 10 joists across, and roughly 440 screws.
Assumptions to know
- Board width is the actual dressed width - a nominal 2 x 6 is really 5.5 in wide.
- Diagonal decking needs a higher waste allowance, usually 15 to 20 percent.
- Railing, stairs, beams and posts are not included.
Results are planning estimates only. Always check the coverage and instructions on the actual product you buy, and measure your own space carefully before ordering. When in doubt, a little extra material is cheaper than a second trip or a stalled job.
Frequently asked questions
How many deck boards do I need for a 12 x 10 deck?
With standard 5.5 inch boards and a 1/8 inch gap, a 10 ft wide deck needs 22 rows. If the boards run the 12 ft direction and you buy 12 ft boards, that is 22 boards before waste - about 25 with a normal allowance.
What gap should I leave between deck boards?
About 1/8 to 1/4 inch for drainage and expansion. Pressure-treated lumber shrinks as it dries, so many builders butt it tight or use a small gap; composite boards follow the maker spacing guide.
How many screws does a deck need?
Roughly 2 screws at every joist crossing for every board. This calculator totals them from your joist spacing, so you can buy the right box size instead of guessing.
Does this include the deck frame?
It estimates joist count and joist lumber length from your spacing. Beams, posts, footings, ledger and railing are separate - use the concrete calculator for post footings.