NOVORY

Deck Calculator

Calculate how many deck boards you need for any deck size, with waste allowance for cuts and breakage.

Quick answer

48 sixteen-foot boards. No butt joints needed.

Calculator

ft
ft
in
in

How it works

Deck length × Deck width = Deck area

The calculator multiplies the deck's length by its width to get the total surface area that needs to be covered with boards.

Board width + Board gap = Board coverage width

Each board occupies its own width plus the gap between it and the next board. Adding these together gives the coverage width per board. Board and gap dimensions are entered in inches (imperial) or centimeters (metric) and converted to match the deck dimensions.

Deck width ÷ Board coverage width = Boards required

Dividing the deck width by the board coverage width gives the number of boards needed to span the deck. The result is rounded up to the nearest whole board, then increased by the waste percentage.

Worked example

A 16 ft × 12 ft deck with 5.5 in boards and 0.25 in gaps:

5.5 + 0.25 = 5.75 in = 0.479 ft

12 ÷ 0.479 = 25.05 → 26 boards

26 × 1.10 = 28.6 → 29 boards (with 10% waste)

Purchase 29 deck boards for this project.

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Deck board math has one decision that matters more than the arithmetic: which way the boards run. Boards spanning the short dimension need no butt joints and waste almost nothing; boards spanning the long way need a joint every row and 10% extra disappears fast.

This calculator divides the deck width by the board's coverage width (board plus gap) and rounds up to whole boards, then adds waste. Match the board length you buy to the deck's other dimension — 12-ft boards for a 12-ft span — and every board lands clean.

How to measure for deck boards

  1. Measure the deck's length and width in feet. Decide which dimension the boards will span — the shorter one wastes less.
  2. Enter the actual board width in inches (a '5.5-inch' deck board is really 5.5) and your gap: ⅛ inch for dry lumber, ¼ inch for wet pressure-treated that will shrink.
  3. The calculator counts boards across the width you entered. Buy boards in the length matching the other dimension to avoid butt joints.
  4. Keep 10% waste for cutoffs and the occasional warped board — decking is never perfect.

Boards by deck size (5.5-in boards, ⅛-in gap)

DeckBuy this lengthBoards+ 10% waste
12 × 12 ft12 ft2629
12 × 16 ft12 ft3539
16 × 20 ft16 ft4348

Boards = ceil(second-dimension inches ÷ 5.625). Buy boards in the length of the first dimension.

Worked example: boards for a 16×20 deck

A 16 ft × 20 ft deck, 5.5-in boards with a ⅛-in gap, 16-ft boards spanning the 16-ft dimension:

  1. Coverage per board: 5.5 + 0.125 = 5.625 in.
  2. Boards across the 20-ft width: (20 × 12) ÷ 5.625 = 42.67 → 43 boards.
  3. With 10% waste: 43 × 1.10 = 47.3 → 48 boards.

48 sixteen-foot boards. No butt joints needed.

Pro tips for decking

  • Run a picture-frame border: it hides every cut end and makes the deck look built, not assembled.
  • Gap wet pressure-treated at ¼ inch — it shrinks as it dries. Kiln-dried or composite gets ⅛ inch.
  • Crown every board up. Sight down each board before fastening; crowns down collect water.
  • Joists at 16 inches on-center for wood, 12 for composite — check your decking's span rating before you frame.

How to use the deck calculator

1. Choose your unit system — Imperial (feet and inches) or Metric (meters and centimeters).

2. Enter the length and width of your deck.

3. Enter the width of a single deck board — in inches for imperial or centimeters for metric.

4. Enter the gap between boards — typically 1/8 to 1/4 inch.

5. Select a waste factor — 10% is recommended for most deck projects.

6. Click Calculate to see how many boards you need, with and without waste.

How deck board quantity is calculated

The calculator first determines the deck area by multiplying the deck length by the deck width. It then calculates the board coverage width by adding the board width and the gap between boards.

Dividing the deck width by the board coverage width gives the number of boards needed to span the deck. The result is rounded up to the nearest whole board, then increased by the waste percentage.

Why add a waste factor for deck boards?

Deck board installations always produce waste. Boards must be cut to fit at edges and around posts, stairs, or railings. Some boards may split or crack during installation. A waste factor ensures you have enough material to finish without a second trip to the lumberyard.

Keep a few extra boards for future repairs — replacing a damaged board later is much easier when you have matching material on hand.

Deck calculator example

Suppose your deck is 16 feet long and 12 feet wide, using 5.5-inch boards with a 1/4-inch gap.

Board coverage width: 5.5 + 0.25 = 5.75 inches = 0.479 feet.

Boards required: 12 ÷ 0.479 = 25.05 — rounded up to 26 boards.

With 10% waste: 26 × 1.10 = 28.6 — rounded up to 29 boards.

You should purchase 29 deck boards for this project.

Frequently Asked Questions

How do I calculate how many deck boards I need?+
Divide the deck width by the combined board coverage width (board width plus the gap between boards). Round up to the nearest whole number to get the base count of boards, then add a waste factor — typically 10% — for cuts, mistakes, and future repairs.
How wide should the gap be between deck boards?+
A gap of 1/8 to 1/4 inch is standard for most decking. The gap allows for drainage, debris clearing, and seasonal expansion. Pressure-treated lumber may need a larger gap (up to 3/8 inch) because it shrinks as it dries. Composite decking typically uses 1/8 inch gaps.
What board width should I use?+
Standard deck boards are typically 5.5 inches wide (nominal 6-inch boards). Some composite boards are 5.25 or 5.5 inches. Always measure the actual board width, not the nominal size, since planed lumber is smaller than its labeled dimension.
How much waste should I add for deck boards?+
A 10% waste factor is standard for most deck projects. For decks with multiple angles, diagonal layouts, or curved edges, use 15%. For simple rectangular decks with few obstacles, 5% may be sufficient. Always keep a few extra boards for future repairs.
Do I need to account for board length?+
This calculator assumes each board runs the full length of the deck. If your deck is longer than your boards, you will need additional boards to cover the extra length, plus extra blocking or joists at the seams. Plan your board layout before purchasing materials.

Related Guides

How Many Deck Boards Do I Need for a 12x16 Deck?How Many Deck Boards Do I Need for a 16x20 Deck?

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