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- A 16 × 12 ft deck with 5.5 in boards and a 1/8 in gap needs 26 rows of boards.
- With 10% waste and 16 ft boards, that is 29 boards.
- You also need about 13 joists at 16 in on center and about 676 deck screws.
- At $25 a board the boards cost about $725.
How to use the decking calculator
Enter the deck size
Length is the direction the boards run. Width is the direction across the boards.
Choose board width and length
Pick the board you plan to buy, such as 5/4 × 6 (5.5 in wide) and the length that fits your deck.
Set the pattern and waste
Straight boards need about 10% extra. Diagonal layouts need about 15%.
Review joists and screws
Choose your joist spacing to see how many joists and screws the deck needs.
How the decking count is calculated
Decking formula
Rows = deck width in inches ÷ (board width + gap), rounded up
Linear feet = rows × deck length
Boards = linear feet × (1 + waste %) ÷ board length, rounded up
Joists = deck length in inches ÷ spacing + 1
Screws = rows × joists × 2
Always check your local building code and your decking maker’s instructions for spacing and fastening.
Worked example: a 16 × 12 ft deck
The deck uses 5.5 in boards with a 0.125 in gap, 16 ft boards, 16 in joist spacing, 10% waste and $25 per board.
- Deck area 192 sq ft
- Rows (144 ÷ 5.625 = 25.6) 26
- Linear feet (26 × 16) 416
- With 10% waste 457.6
- Boards (457.6 ÷ 16 = 28.6) 29
- Joists (192 ÷ 16 + 1) 13
- Screws (26 × 13 × 2) 676
- Cost of boards (29 × $25) $725
Using longer boards cuts down on joints, but make sure the board length fits the deck so you avoid short offcuts.
Decking calculator FAQ
How many deck boards do I need?
Divide the deck width by the board width plus gap to get the number of rows, multiply by the deck length, add waste, and divide by the board length.
How much waste should I allow?
About 10% for straight boards and 15% for diagonal patterns or decks with many cuts.
What gap should I leave between boards?
Typically about 1/8 inch for wood boards, but follow the manufacturer’s guidance for your product and climate.
How many screws do I need?
Use two screws at each point where a board crosses a joist. The calculator multiplies rows by joists by two.
What joist spacing is typical?
16 inches on center is common. Some composite products need 12 inches. Check your board manufacturer’s requirements.
Does this include framing lumber?
It counts joists but not posts, beams, ledger boards or footings. Plan those separately.