Deck Board Calculator

Tip: buy one or two boards beyond the count. A single warped or split board can hold up the whole job on a Saturday afternoon.

Worked example

A 16 ft by 12 ft deck with boards running the 16 ft direction covers 192 square feet. Standard 5/4 x 6 deck boards are 5.5 inches wide, and a 1/8 inch gap is common for pressure-treated lumber.

Each row of boards takes 5.5 + 0.125 = 5.625 inches. The deck is 144 inches wide, so 144 / 5.625 = 25.6, which rounds up to 26 rows. 26 rows x 16 ft = 416 linear feet. Add 10% for cuts and defects and you need 457.6 linear feet.

With 16 ft boards, 457.6 / 16 = 28.6, so you buy 29 boards. The frame needs 13 joists at 16 inches on center, each 12 ft long, and about 700 deck screws.

How many deck boards do I need?

Deck boards run in rows across the joists. The number of rows depends on how wide the deck is in the direction you lay them side by side, and on the width of one board plus the gap. Divide the deck width in inches by board width plus gap and round up. That is how many rows you need.

Multiply rows by the deck length to get linear feet of decking. Add an allowance for cutting and for boards you reject, then divide by the length of the boards you plan to buy and round up. The calculator does each of these steps and shows them under Show the math.

Choosing board length

The best board length matches the deck length, so each row is one piece with no joints. Common lengths are 8, 10, 12, 16 and 20 feet. If the deck is longer than your boards, you will have butt joints, and each joint must land on a joist. Stagger the joints from row to row so they do not line up in a visible seam.

For a deck slightly longer than a standard length, such as 17 feet, many builders buy 20 ft boards and trim them, or add a breaker board across the middle that boards on each side butt into. Either way the linear footage from this calculator stays the same. Enter the board length you will actually buy so the board count is right.

Board widths and gaps

• 5/4 x 6 pressure-treated: 5.5 inches wide. Gap of about 1/8 inch. Wet lumber shrinks as it dries, so some builders butt it tight and let the gap open up.
• 2 x 6 lumber: 5.5 inches wide, same gap guidance as 5/4.
• Composite and PVC boards: usually 5.5 inches wide. Follow the maker's gap chart, often 3/16 to 1/4 inch side to side, and leave end gaps that depend on temperature.
• Hardwoods like ipe: often 3.5 or 5.5 inches wide with a 3/16 inch gap.

Narrower boards mean more rows, more boards and more screws for the same deck. A 3.5 inch board needs almost 60% more rows than a 5.5 inch board.

Joists, screws and hidden fasteners

Joists are the framing members the boards sit on. At 16 inches on center, the standard spacing for most decking, a 16 ft deck is 192 inches long, so it needs 192 / 16 + 1 = 13 joists. Many composite boards and boards laid on a diagonal need 12 inch spacing. Check the decking manufacturer's span rating before you buy framing lumber.

Face screwing uses two screws where each board crosses each joist. A good rule of thumb is about 350 screws per 100 square feet, and screws are sold in boxes by count or by pound. If you use hidden clips, buy one clip per board crossing instead and a few extra screws for the starter and finish boards.

This calculator covers the deck surface and joist count. It does not size beams, posts or footings, which depend on spans, loads and local code. Your building department can tell you what a permit requires.

How much extra to allow

Ten percent is a sound default for a straight-laid rectangular deck. Use 15% for decks with angles, picture framing around the edge or lots of cuts around posts. Pressure-treated boards vary in quality, so if you are picking boards from a pile at the store, you can stay near 10%. If they are delivered and you cannot choose, lean higher.

More calculators

Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes.