Decking Board Calculator

Decking board calculator: deck area ÷ 5.5

This decking board calculator counts the surface boards for a deck. The formula is boards = (length × width) ÷ 5.5, rounded up to a whole board. The 5.5 is the square-foot coverage of one 5.5 inch wide, 12 ft long board. Tick the 10% option for end trims and cuts; it multiplies the count by 1.10 before rounding. Cost is boards × price per board.

Why one board covers 5.5 square feet

A board’s face is its width times its length. 5.5 inches is 0.458 ft. Multiply by 12 ft and you get 5.5 sq ft. That coincidence makes the math easy for standard 12 ft boards: area ÷ 5.5.

Most wood deck boards (nominal 5/4 × 6 and 2 × 6) and many composite boards are about 5.5 inches wide. Some composite lines are narrower or wider, so check the actual width.

Worked examples

1. Default deck: 11 × 10 ft, $25 per board

• Area: 11 × 10 = 110 sq ft
• Boards: 110 ÷ 5.5 = 20 boards, costing $500.00
• With 10% on: 20 × 1.10 = 22 boards, costing $550.00

2. A 16 × 12 ft deck, $25 per board

• Area: 16 × 12 = 192 sq ft
• Boards: 192 ÷ 5.5 = 34.9, rounded up to 35 boards ($875.00)
• With 10% on: 38.4, rounded up to 39 boards ($975.00)

Boards running the 16 ft direction are cleaner as 16 ft lengths, while 12 ft boards would need butt joints over doubled joists. The square-foot count stays the same either way, so buy the board lengths that suit the layout.

Decking reference tables

Board size	Coverage per board	Boards per 100 sq ft
5.5 in × 12 ft	5.50 sq ft	19
5.5 in × 16 ft	7.33 sq ft	14
5.5 in × 20 ft	9.17 sq ft	11
3.5 in × 12 ft	3.50 sq ft	29

Deck size (12 ft boards)	Area	Boards	With 10%
10 × 10 ft	100 sq ft	19	21
12 × 12 ft	144 sq ft	27	29
20 × 12 ft	240 sq ft	44	48
24 × 16 ft	384 sq ft	70	77

The calculator is fixed at 5.5 sq ft per board. For other sizes, use the coverage column.

How to measure a deck for boards

• Measure the finished surface. Include any overhang past the rim joist, often about an inch.
• Measure along the boards. Note which direction boards run; that sets which board length to buy.
• Split odd shapes. Treat an L-shaped deck as two rectangles and add the board counts.
• Treat stairs separately. Each tread usually takes two boards across its width.
• Count picture-frame borders separately if you want an exact list; otherwise use about 15% extra.

Choosing board lengths to cut waste

Square-foot math gives the decking quantity, and board length determines how much ends up as offcuts. If your deck runs 12 ft in the board direction, 12 ft boards fit with no joints. If it runs 14 ft, 16 ft boards leave a 2 ft offcut each, while a mix of 12 ft and 16 ft boards might fill the run with a staggered joint pattern.

Lay out the joints so they land on joists and do not line up in neighboring rows. Some builders use a breaker board, a perpendicular board across the deck, to avoid butt joints entirely on long runs.

Common decking mistakes

• Ignoring board length. Ordering 12 ft boards for a 14 ft run leaves 2 ft gaps to fill with offcuts.
• Skipping the joist spacing check. Many composite boards require closer joist spacing than wood. Check the board maker’s span chart.
• No allowance for bad boards. Wood boards can arrive split or badly bowed. The 10% covers that.
• Mixing lots. Composite colors can vary slightly between production runs.
• Forgetting fascia and stair risers. These are trim pieces, not decking, and need their own count.
• Forgetting the screws or clips. Hidden-fastener systems are sold by the deck area they cover, so keep your square footage handy when you shop.

Related calculators

For a full material list, use the deck calculator. The composite decking calculator covers composite-specific sizes, and the deck joist calculator sizes the framing underneath.