Deck Material Calculator — Free Online Tool
Don't guess at the lumber yard. This deck material calculator estimates how many decking boards and joists your project needs from the deck's length, width, and board size.
What is the Deck Material Calculator?
Board count comes from simple division: deck width in inches divided by board width plus the gap between boards (typically 1/4 inch). A 12-foot-wide deck with 5.5-inch boards needs about 25 boards — each board spanning the deck's length. Joists usually run 16 inches on-center, which is why the joist count is length-based.
These are the two big-ticket items, but a real build also needs beams, posts, concrete footings, ledger hardware, and fasteners — budget those separately. And always add about 10% extra for cuts, mistakes, and the occasional warped board the yard slips into the stack.
How to use this calculator
- Enter deck length and width in feet.
- Choose your board width and the gap between boards.
- Click Calculate Materials for boards, area, and joists.
Formula
Frequently asked questions
How many deck boards do I need?
Divide the deck width (in inches) by board width plus gap, then round up — that's your board count. Each board spans the deck's length.
What gap should I leave between deck boards?
1/4 inch is standard for wood and most composites — it allows drainage and expansion. Check your specific product's instructions.
How far apart should deck joists be?
16 inches on-center is standard for residential decks; go 12 inches for diagonal decking patterns or heavy loads.
Should I order extra material?
Yes — order about 10% more than this deck material calculator suggests, for cuts, waste, and defective boards.