WeekendCalc

Deck Board Calculator

Enter the size of your deck and the boards you plan to use. You get the number of decking boards to buy, the joists underneath, and the screws to fasten them.

Your project

Usually the side along the house.

The direction joists run.

%

Covers cuts, bad boards and angled ends. Use 15% for diagonal decking.

Results

Deck boards to buy
29 boards
At your chosen board length, with waste
Rows of decking across the width
26 rows
Total decking length (with waste)
458 linear ft
Deck area
192 ft²

Framing underneath

JoistsEach as long as the deck width
13 joists

Fasteners

Screws (2 per board at each joist)
676 screws
  • If your boards are shorter than the deck length, plan for butt joints to land on a joist (use a double joist there), and add boards for the extra cuts.

How this calculator works

Decking boards run across the joists. We work out how many rows of boards fit across the deck's width (each board plus its gap), multiply by the deck length to get total linear feet, add waste, and divide by the board length you will buy. Joists are counted at your spacing along the length, plus one for the end. Screws are two per board at every joist it crosses.

rows = ⌈width ÷ (board width + gap)⌉ boards = rows × length × (1 + waste %) ÷ board length

Worked example: a 16 × 12 ft deck in 5.5 in boards with 1/8 in gaps: 144 in ÷ 5.625 = 25.6, so 26 rows. That is 416 ft of decking, 458 ft with 10% waste, or 29 sixteen-foot boards. With joists at 16 in you need 13 joists and about 676 screws.

Tips before you buy

  • Buy boards in the longest length that suits your deck to cut down on butt joints, which are weak points and need to land on a joist.
  • Gap size matters: pressure-treated lumber shrinks as it dries, so lay it close to flush (1/8 in) while composite is gapped per the maker's instruction.
  • Fasteners: use screws rated for ground-contact treated lumber (coated or stainless), since cheap screws corrode and stain boards.
  • A deck that is attached to your house or high off the ground usually needs a permit and an inspection. The calculator only covers the surface boards and the joists.

Quick answers by size

8 × 8 ft deck14 boards of 12 ft8 × 10 ft deck17 boards of 12 ft10 × 10 ft deck21 boards of 12 ft10 × 12 ft deck24 boards of 12 ft12 × 12 ft deck29 boards of 12 ft12 × 16 ft deck39 boards of 12 ft12 × 20 ft deck48 boards of 12 ft14 × 14 ft deck39 boards of 12 ft16 × 16 ft deck52 boards of 12 ft16 × 20 ft deck64 boards of 12 ft

Tools & materials for this project

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Deck screws (exterior)Composite deckingJoist tapeHidden deck fastenersDeck stain / sealer

Frequently asked questions

How many deck boards do I need for a 12x12 deck?

With 5.5 in boards and 1/8 in gaps, a 12 × 12 deck has 26 rows. That is about 312 ft of decking, or 343 ft with 10% waste. In 12 ft boards that is 29 boards.

How far apart should deck joists be?

Sixteen inches on center is standard for most decking. Composite and diagonal layouts often need 12 in. Span tables in your building code give the exact limit.

How many screws do I need per deck board?

Two per joist crossing. That works out to roughly 350 to 400 screws per 100 ft² with 16 in joist spacing.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.