Decking Calculator

Calculating decking materials isn't as simple as dividing square footage by the nominal board size. Use the actual board width, the installation angle, and waste from cuts. A gap between boards is for drainage. It doesn't change the board count.

1. 1

   Calculate Your Deck Area

   Multiply length × width in feet or meters. For a 12×20 ft deck, thats 240 sq ft. In metric: 3.6m × 6m = 21.6 m². Irregular shapes? Break it into rectangles and add them up.

2. 2

   Determine Board Coverage

   Each board covers its actual width × length. A 5.5 inch board (5/4×6) that's 12 feet long covers 5.5 sq ft. Leave a gap for drainage when you install. Don't add that gap into the board count.

3. 3

   Add Waste Factor

   Multiply by 1.05 for 5% waste (perpendicular 90° install) or 1.10 for 10% (diagonal 45° install). This accounts for cuts and unusable pieces.

4. 4

   Round Up

   Always round up to the next whole number. You can't buy half a board, and having extras is better than falling short.

When you buy a "2×6" board, you are not getting 2 inches by 6 inches. Nominal sizes refer to the rough-cut dimensions before planing. The actual dimensions are smaller—and these are what matter for your calculations.

Pro tip: 5/4 deck boards are specifically designed for decking. They are thick enough to span standard joist spacing but lighter and more affordable than 2× lumber.

Standard deck board lengths are 8, 10, 12, 16, and 20 feet (2.4m, 3m, 3.6m, 4.9m, and 6m). The right choice depends on your deck dimensions and how you want seams to fall.

• 12-foot boards (3.6m)Most popular length. Easy to transport and handle. Works well for decks up to 12 feet (3.6m) wide.
• 16-foot boards (4.9m)Great for wider decks. Fewer seams mean a cleaner look but harder to transport in a standard pickup.
• 20-foot boards (6m)Best for large decks. Typically special order. Minimizes waste on long runs.
• Mixing lengthsYou can combine lengths strategically to reduce waste. Just make sure seams are staggered and fall over joists.

The gap between deck boards isn't just aesthetic—it's functional. Proper spacing allows water to drain, debris to fall through, and wood to expand and contract with temperature changes.

• 1/8" gap (3mm): Tight spacing, good for composite boards that don't shrink
• 3/16" gap (5mm): Standard for most installations, recommended for pressure-treated
• 1/4" gap (6mm): Wider spacing, helps in wet climates or with dry lumber
• Butt joints: No gap needed at board ends over joists

If you're using wet pressure-treated lumber, you can install boards touching each other. They'll shrink as they dry, naturally creating the gap. Kiln-dried lumber should be gapped at installation. Either way, the gap doesn't change how many boards you order.

Deck screws are sold by the pound or box. Here's how to estimate what you need:

• Standard screw method: 2 screws per board at each joist crossing
• Hidden fasteners: Approximately half the screw count (e.g., 435 fasteners for 870 screws)
• 2.5" or 3" deck screws work for most applications
• Stainless steel or coated screws prevent rust stains on your deck

A typical 5-lb box (2.2kg) contains roughly 350 deck screws. For a 200 sq ft deck with 12" joist spacing at 90°, you'll need approximately 870 screws—so grab three boxes. At 16" spacing, you'll need about 650 screws (two boxes).

Once you've calculated your materials, the next step is creating a professional estimate or invoice for the project. Whether you're a contractor bidding on a job or a homeowner tracking costs, proper documentation saves headaches later.

Invoice Mama helps contractors create professional estimates and invoices in seconds. Our AI-powered system can turn your material calculations into polished documents that help you win jobs and get paid faster. Try it free and see why deck builders love our simple approach to paperwork.