Deck Board Calculator
Deck size in, board count out. It counts the gap between boards and adds waste, which is where most hand calculations come up short.
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26 rows across a 12 ft width, plus 10% waste. Deck surface only, no framing, railing or stairs.
Get the deck quoted properlyThe arithmetic
A 2x6 measures 5.5 inches, not 6. Add a quarter inch gap and each board covers 5.75 inches of deck, so a 12 foot width needs 26 rows rather than the 24 the nominal size suggests. That gap is the whole reason hand counts fall short.
Rows multiplied by deck length gives linear feet. Divide by the board length you are buying, round up, then add waste for the cuts.
Why the gap is not optional
Pressure treated timber arrives wet and shrinks as it dries, so boards laid tight in April have gaps by August anyway. Composite moves less and usually wants 3/16, though the manufacturer's instructions override anything on this page.
What this leaves out
Joists, beams, posts, footings, railing, stairs and fasteners. This is the walking surface only.
For the whole build, the deck cost calculator takes square footage to an installed range, and the decking material guide covers what to put on top. Staining it after? Try the deck stain calculator.
Common questions
How many deck boards do I need?
Work out the rows first. Deck width in inches, divided by the board width plus the gap. A 12 foot deck in 2x6 with a quarter inch gap needs 26 rows. Multiply rows by deck length for linear feet, then divide by the board length you are buying.
What gap should I leave between deck boards?
A quarter inch for pressure treated timber, because it shrinks as it dries. Composite moves far less and usually calls for 3/16. Check the instructions that come with the boards, because a wrong gap can void the warranty.
How much waste should I allow on a deck?
Ten percent on a plain rectangle. Fifteen on anything with angles, a picture frame border or a diagonal layout, because every cut leaves an offcut too short to use.
Is a 2x6 actually 6 inches wide?
No. A 2x6 is 5.5 inches and a 2x4 is 3.5. Nominal sizes describe the timber before it was dried and planed, and every deck calculation has to use the real figure.
Should I buy longer boards?
Where the deck allows it, yes. Fewer butt joints looks better and wastes less, and every joint needs blocking underneath it, which is extra framing.
Do I need more boards for a diagonal deck?
Yes. A 45 degree layout adds roughly 15 percent because every board is cut at an angle at both ends and the offcuts rarely fit anywhere else.
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