Deck Calculator
Enter your deck's length and width, joist spacing, and decking board width to estimate the number of joists and total linear feet of decking material needed.
Joists needed
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Enter your deck dimensions to calculate.
How the Deck formula works
Joist count and decking material estimates:
Joists = ceil( (Length × 12) / Joist spacing ) + 1 Deck area = Length × Width Decking linear feet ≈ Deck area / (Board width in feet)
Step-by-step calculation
- Convert deck length to inches and divide by joist spacing to find the number of joist bays, rounding up.
- Add 1 to get the total joist count (a run with 10 bays needs 11 joists).
- Divide the total deck area by the decking board's width (in feet) to estimate total linear feet of decking boards needed.
Worked example
A 16 ft × 12 ft deck with joists 16 inches on center, using 5.5-inch-wide decking boards: Joists = (16×12)/16 + 1 = 13. Deck area = 192 ft². Decking linear feet ≈ 192 / (5.5/12) ≈ 419 linear feet.
Frequently asked questions
What's a standard joist spacing?
16 inches on-center is the most common spacing for residential decks, though 12-inch spacing is sometimes used for added strength, particularly with certain decking materials or heavier loads.
Should I add extra material for waste?
Yes — adding roughly 10% extra for decking boards accounts for cuts, staggered seams, and mistakes, similar to other material estimates like flooring or roofing.