Deck board calculator

Enter the deck size and board details to see how many boards, joists and screws to buy.

By the Calcist team · Checked against known answers · Updated October 27, 2026

Private: calculated on your device. Nothing you type is sent anywhere.

Next: Want boards, joists, screws and footing concrete on one list? Use the project planner. Need the footings too? Use the footing calculator.

How decking is calculated

Count the rows of boards across the deck, then how many boards each row needs along its length.

Rows = (deck width in inches + gap) ÷ (board width + gap), rounded up
Boards per row = deck length ÷ board length, rounded up
Joists = deck length (in) ÷ joist spacing, rounded down, + 1

Worked example

A 16 × 12 ft deck with 16 ft boards, 5.5 in wide with 3/16 in gaps: (144 + 0.1875) ÷ 5.6875 = 25.4, so 26 rows of one board each. With 10% extra, buy 29 boards. Joists at 16 in along the 16 ft side: 192 ÷ 16 = 12, + 1 = 13 joists, so 26 rows × 13 joists × 2 screws = 676 screws.

Board sizes

Lumber is sold by its nominal size. A "5/4 × 6" or "2 × 6" deck board is really about 5½ in wide, and composite boards are usually about 5½ in too. Measure your boards or check the maker's spec.

Limitations

Joints: where a row needs two boards, the joint must fall over a joist. Choose a board length that divides your deck neatly to cut waste.
Diagonal or patterned decks need 15% or more extra.
Gaps: follow the maker's gap for composite decking, which allows for expansion.
Framing: joist size and span, beams and footings must follow your local code.

Sources

American Wood Council: DCA 6, Prescriptive residential wood deck construction guide

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

16 × 12 ft deck, 5.5 in boards, 3/16 in gaps: 29 boards with 10% ✓
Same: 26 rows, 13 joists, 676 screws ✓
20 ft deck with 12 ft boards: 2 boards per row ✓
Edge: board width 0 is refused ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

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Questions people ask

How many deck boards do I need for a 12 × 16 deck?

With 5.5 in boards and 3/16 in gaps running the 16 ft way, 26 rows of 16 ft boards, or about 29 boards with 10% extra.

How far apart should deck boards be?

About 1/8 to 3/16 inch for wood, so water drains; follow the manufacturer's gap for composite boards.

How many screws per deck board?

Two at every joist the board crosses. A 16 ft board over joists at 16 inches crosses 13 joists, so 26 screws.

How wide is a deck board?

Most are sold as 6-inch boards but measure about 5½ inches wide.

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