Deck Board Calculator

Plan Your Deck Down to the Last Board

Few things stall a weekend deck build like running three boards short on a Sunday afternoon. This deck board calculator turns your deck's footprint into a precise shopping list: how many deck boards to buy, how many joists frame the surface, how many screws to grab, and what it all costs in your currency. Enter the length, width, board size, and gap, and you get whole-unit quantities you can take straight to the lumber yard.

The Formulas Behind the Estimate

The tool builds your estimate from four simple relationships:

Deck area = Length × Width Rows = ⌈(Width × 12) ÷ (Board width + Gap)⌉ Linear feet = Area ÷ (Board width ÷ 12) Joists = ⌈(Length × 12) ÷ Joist spacing⌉ + 1

Boards run across the width, so each row's coverage is the board's face width plus the gap. Joists run the other way, spaced along the length, with a "+1" because a run of N spaces needs N+1 supports. Screws come last: two at every board-over-joist crossing.

A Worked Example: A 12 × 20 ft Deck

Say you're building a 12 ft by 20 ft deck with standard 5.5" boards, a 1/8" (0.125") gap, and joists 16" on-center.

• Area: 12 × 20 = 240 ft²
• Rows across the width: (12 × 12) ÷ (5.5 + 0.125) = 144 ÷ 5.625 = 25.6, rounded up to 26 rows
• Boards per row: each 20 ft row takes two 16 ft boards → 26 × 2 = 52 boards
• Linear feet: 240 ÷ (5.5 ÷ 12) = 240 ÷ 0.458 = about 524 linear ft
• Joists: (20 × 12) ÷ 16 + 1 = 15 + 1 = 16 joists
• Screws: 52 boards × 16 joists × 2 = 1,664 screws

At $18 per board that's a $936 decking bill before fasteners — and you can see instantly how dropping to a 5.25" board or a tighter gap changes the row count.

Why Joist Spacing Changes Everything

Joist spacing is the input most people underestimate. Tightening from 16" to 12" on-center adds roughly a third more joists and screws, but it's non-negotiable for diagonal layouts where boards cross the framing at an angle and span a longer unsupported distance. If your boards meet the joists at 45°, drop to 12" on-center and rerun the numbers.

Practical Buying Tips

• Round up, then add waste. The calculator already rounds to whole boards; add another 10-15% for end cuts and defects, more for picture-frame borders.
• Buy boards that beat your span. A 20 ft run from two 16 ft boards leaves a seam — stagger seams between rows so they don't line up, or buy 20 ft boards where available.
• Match the gap to the wood. Wet pressure-treated lumber shrinks, so it can go in tighter; kiln-dried and composite need the full 1/8".
• Order a spare box of screws. Stripped heads and drops are inevitable on a deck this size.

Beyond the Surface

This tool estimates the decking surface, its joists, and the fasteners — the parts that make up most of your bill. For the structure underneath, size your footings with the concrete calculator, and if you're skirting or cladding the deck frame, the brick calculator can help with masonry around it. Bookmark this page and rerun it any time you tweak the dimensions, and you'll never make a second trip for "just a few more boards" again.