How many deck boards do you need?

How do I calculate how many deck boards I need?

Find the deck area, add 10% for cuts, and divide by the area one board covers: its width plus the gap, times its length. A 12 by 16 ft deck is 192 square feet, or 211.2 with 10% extra. A 5/4 × 6 board is 5½ inches wide; with a ⅛ inch gap it covers 5⅝ inches, so a 16 ft board covers 7.5 square feet — 211.2 ÷ 7.5 = 28.2, so buy 29 boards.

How wide is a 5/4 × 6 or 2 × 6 deck board?

Both are 5½ inches wide; the names are nominal sizes from before the lumber was planed. A 5/4 board is about 1 inch thick and a 2 × 6 is 1½ inches. Most composite boards are also about 5½ inches wide. Enter the actual width if yours differs.

How much gap should I leave between deck boards?

Composite makers such as Trex call for about 3/16 inch side to side, and their hidden clips set it for you. Kiln-dried wood usually gets about ⅛ inch. Wet pressure-treated boards are often laid tight, since they shrink as they dry — enter 0 for those.

How many deck screws do I need?

Two screws where each board crosses each joist. With 5½ inch boards and ⅛ inch gaps on joists 16 inches apart, that is about 1.6 crossings per square foot — a 192 square foot deck takes about 616 screws, or 308 hidden clips instead.

Should I buy 12, 16 or 20 ft boards?

Pick lengths that span the deck with the fewest butt joints, and make every joint land on a joist. A 16 ft deck is easiest with 16 ft boards; a 20 ft deck with 20 ft boards or staggered 12 and 8 ft pieces. Enter the prices to see the cheapest way to buy.

How much extra should I add for a diagonal pattern?

Diagonal and herringbone patterns cut a triangle off the end of every board, so the waste is higher than for straight boards; many installers allow 15% or more instead of 10%. The calculator takes any waste percentage you enter. Boards laid on a diagonal also need joists closer together, and the IRC allows 5/4 wood decking on a diagonal only on joists 12 inches apart.

Is the deck area the same as the area of the boards?

The boards add the gaps between them to their own width, so the area per linear foot of board is slightly more than the board width. A 5½ inch composite board with a 3/16 inch gap covers (5.5 + 0.1875) ÷ 12 = 0.4740 square feet per linear foot, and the 280 square feet of the example needs 591 linear feet before waste.

How the board count is worked out

The area to cover, plus the waste allowance, is divided by the area one linear foot of board covers: the board width plus the gap, in feet. The result in linear feet is divided by the board length and rounded up. Sold lengths are usually 12, 16 and 20 ft, so the calculator compares the ways to buy; a length that fits the deck with fewer butt joints and less waste is usually cheaper than the cheapest board per foot. Fasteners are counted at each place a board crosses a joist.

Worked example: a 14 by 20 ft composite deck

The deck is 280 square feet, 308 with 10% extra. Each linear foot of board covers 0.4740 square feet, so 650 linear feet of board are needed: 33 boards of 20 ft, or 41 boards of 16 ft. With joists 16 inches apart, each board crosses a joist at 443 points on the whole deck, so that is 443 hidden clips, or 886 face screws at two per crossing. With joists 12 inches apart the crossings rise to 591.

Common mistakes

Buying exactly the area with no waste leaves you short once the end cuts and a few damaged boards are taken out. Ignoring the gap between boards understates the number of boards slightly. Another is forgetting that composite boards expand and contract with temperature, so manufacturers specify the gap and an end gap; follow their installation guide. And check the span: the joist spacing must suit the decking. A 5/4 board is intended for 16 inch joists in straight layout, and composite products have their own limits.

Sources: 2021 International Residential Code, Section R507.7 (decking: 5/4 boards on joists 16 inches apart straight, 12 inches apart on a diagonal); Trex installation guidance for gap and hidden-clip use. Related: deck joist calculator, deck beam span calculator, deck footing calculator and square footage calculator.