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Decking Calculator

Deck area divided by the coverage of a single board, plus a waste allowance for cuts.

Deck area (sq ft)

Board width (in)

Board length (ft)

Waste factor (%)

Boards needed

Account for gaps between boards separately — this estimates coverage from board face width, not including expansion gaps.

What you entered

Deck area (sq ft): 200Board width (in): 5.5Board length (ft): 12Waste factor (%): 10

1
Coverage area of one board
(5.5/12) × 12= 5.5 sq ft
2
Deck area with 10% waste
200 × 1.10= 220 sq ft
3
Number of boards
220 ÷ 5.5= 41 boards

Result

Boards needed: 41

A 200 sq ft deck needs about 41 boards at 5.5"×12'.

Deck boards needed for common deck sizes, by board length, at 10 percent waste

All counts assume the standard 5.5 inch wide deck board and a 10 percent waste allowance. Each board covers its face width times its length, and the calculator divides the deck area plus waste by that coverage and rounds up.

Deck size (ft)	Deck area (sq ft)	8 ft boards	12 ft boards	16 ft boards
8 x 10	80	24	16	12
10 x 12	120	36	24	18
12 x 12	144	44	29	22
12 x 16	192	58	39	29
12 x 20	240	72	48	36
14 x 20	280	84	56	42
16 x 20	320	96	64	48
16 x 24	384	116	77	58
20 x 24	480	144	96	72

Decking only: joists, beams, posts, footings, ledger, flashing, hangers and fasteners are all separate and are the parts that carry the load. Coverage is taken from the board face, so the 1/8 to 1/4 inch drainage gap between boards is absorbed by the waste allowance rather than counted. Choose a board length that spans the deck with the fewest end joints, since every joint must land on a joist and adds a cut. Raise the waste factor to 15 percent for diagonal, herringbone or picture-frame layouts. Composite boards use the same math but come in fixed lengths, so buy to the length the manufacturer stocks.

This doesn't include gaps between boards

Decking boards are installed with a small gap (typically 1/8 to 1/4 inch) between them for drainage and expansion — this calculator estimates from the board's face width, so actual coverage per board is slightly less once gaps are factored in.

Diagonal or pattern layouts increase waste

A simple straight layout parallel to the house wastes the least material — diagonal, herringbone, or picture-frame border patterns need more cuts and a higher waste allowance.

Frequently asked questions

My deck is 12x16 ft — how many 5.5-inch-wide, 8-ft boards do I need?

Each 8-ft board covers 3.67 sq ft (0.458 ft wide x 8 ft). For 192 sq ft of deck: 192 / 3.67 = about 53 boards. Add 10% waste: order 58 boards. Two boards span the 16-ft length with one joint per row.

Does this calculator account for the gaps between deck boards?

It estimates from the board's face width, so actual coverage is slightly less once standard 1/8-inch to 1/4-inch gaps are factored in. For most deck sizes, this difference is small enough that the waste allowance absorbs it.

What's a standard deck board size?

Most common: 5.5 inches wide (nominal 6-inch or 1x6), in 8, 12, or 16-foot lengths. Composite boards are typically the same dimensions. Choose board length to minimize end-to-end joints — 16-ft boards reduce seams on most decks.