How to Use the Decking Board Calculator
Whether you're building with pressure-treated lumber or composite decking, getting the board count right before you order saves you a trip back to the lumber yard mid-project and keeps leftover material to a minimum. This calculator takes your deck dimensions and board size, then works out how many rows you need and the total board count, including a waste allowance.
Decking boards typically run 5.5 in wide (the actual width of nominal 2x6 decking) and come in stock lengths of 12, 16, or 20 ft. A gap of about 1/8 to 3/16 in between boards is standard for both wood and composite decking, allowing for drainage and seasonal movement without trapping debris.
A default 5% waste allowance covers end cuts and the occasional defective board, but picture-frame borders, angled cuts around posts, and diagonal patterns will increase actual waste — bump the allowance to 10% or more for those layouts. Always double-check manufacturer specs, since actual board width can vary slightly between wood species and composite brands.
Frequently Asked Questions
Wood and composite decking boards expand and contract with temperature and moisture, so a 1/8 to 3/16 inch gap between boards is standard to prevent warping and allow drainage. A wider gap means fewer boards fit in the same area.
Yes. This calculator assumes boards run across the width dimension you enter. If your joist layout requires boards to run the other direction, simply swap the length and width values before calculating.