How to Calculate Deck Boards
Building a deck is one of the most rewarding DIY projects, but running out of decking boards mid-project is a nightmare. Our deck board calculator takes the guesswork out of your material list by converting your total square footage into exact linear feet, and then dividing by your chosen board length to tell you exactly how many boards to buy.
The Math Behind the Calculator
First, we calculate the total square footage of your deck (Length ร Width). Next, we convert your board width from inches to feet. By dividing the total square footage by the board width in feet, we get the total linear feet of decking required. Finally, we apply a waste factor and divide by the length of the boards you are purchasing to get your final board count.
Joist Spacing: Why It Changes Your Board Layout
Decking boards must be supported by joists spaced no more than 16" on-center for most composite and 5/4 lumber decking when run perpendicular, and as tight as 12" on-center for boards run diagonally or for thinner composite products โ always check your specific decking manufacturer's span chart. Joist spacing doesn't change your total board count, but it does determine where every butt joint must land, which affects how you stagger board lengths to avoid two seams lining up next to each other.
Composite vs. Wood: Board Count Differences
Composite decking is typically sold in fixed lengths (12, 16, or 20 ft) with no custom cutting at the mill, while pressure-treated wood is more flexible in length but prone to more waste from culling bowed or cracked boards. If you're still deciding between materials, see our Composite vs. Wood Decking Guide for a full 15-year cost comparison.
How many deck boards do I need?
A nominal 5/4 × 6 deck board is 5.5 in wide, and with a 1/8 in gap covers 5.625 in — so every square foot of deck takes about 2.13 linear feet of decking. Counts below exclude waste; add 10% for a straight layout and 15% for diagonal.
| Deck area | Linear feet of decking | 12 ft boards | 16 ft boards |
|---|---|---|---|
| 100 sq ft | 214 lin ft | 18 | 14 |
| 200 sq ft | 427 lin ft | 36 | 27 |
| 320 sq ft | 683 lin ft | 57 | 43 |
| 500 sq ft | 1067 lin ft | 89 | 67 |
Frequently Asked Questions
What is the standard width of a deck board?
The most common decking board is a "5/4 x 6" board, which actually measures 5.5 inches wide. Standard 2x4s measure 3.5 inches wide, and 2x8s measure 7.25 inches wide.
How much gap should I leave between deck boards?
You should leave a 1/8 to 1/4 inch gap between boards for water drainage. Our calculator assumes standard spacing. If you are using hidden fastener clips, the clip itself automatically sets the correct gap.
How deep do deck footings need to be?
Deck footings must extend below your local frost line, which varies by region. This calculator covers the decking boards themselves โ for the concrete volume needed for footings, posts, or sonotube piers, use our Concrete Footing & Sonotube Calculator.
What is standard deck railing spindle spacing?
Most building codes require baluster (spindle) gaps no wider than 4 inches, so a sphere that size can't pass through. For a typical 8-foot railing section with 2-inch wide balusters, that usually works out to 16-18 spindles, but always verify exact spacing against your local code before installing.
How many deck boards do I need for a 12x12 deck?
For a 144 sq ft deck using standard 5.5" wide boards run the short way, you'll need roughly 314 linear feet of decking (144 รท 0.458 ft board width), plus 10% waste โ about 26 boards if buying 12-foot lengths.