Decking Calculator
Board rows and stock lengths for a rectangular deck.
How it works
Boards run along the deck length. Count rows using actual board width plus the side gap, then stock boards for each row. Add your allowance at the end. This conservative estimate does not reuse offcuts between rows or count the frame, fasteners or stairs.
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How the Decking Calculator works
Estimate deck boards for a rectangular surface with boards running along its length. Enter actual board width and the side gap specified for the chosen decking product.
Count rows, then stock boards
A 12 × 10 ft deck with 12 ft boards, 5.5 in actual width and a 3/16 in side gap needs 22 rows. One stock board spans each row, so 22 boards are needed before allowance and 25 with a 10% allowance. The displayed dimensions are an example, not a specification for every product.
Gaps and offcuts
Rows are rounded up from (deck width + gap) / (board width + gap). Board count per row is rounded up from deck length / stock length. This conservative method does not reuse one row’s leftover stock in another row.
Manufacturer instructions distinguish side gaps, butt joints and clearance to fixed objects; temperature and product system may affect them. The source from Trex explains those distinctions. The calculator uses only the entered side gap, so check the final layout and end joints before ordering.
Surface boards versus framing
The count covers the deck surface only. Joists, beams, posts, fasteners, fascia and stair material require separate takeoffs. Use the lumber calculator for known framing sizes and stair geometry for a separate straight flight.
FAQ
Can I enter nominal board width? expand_more
Use actual exposed width. A nominal designation may be wider than the surface installed.
Does it minimize cutting waste? expand_more
No. Each row is supplied from whole stock boards; offcuts are not shared across rows.
Does it select joist spacing? expand_more
No. The support layout comes from the deck design and the product installation instructions.