Deck Board Calculator

Divide the deck width by one board width plus the gap, and round up — that is the number of rows. Multiply by how many boards each row needs along the deck length. A 12-by-16-foot deck with 5.5-inch boards and a 1/8-inch gap needs about 26 rows, so roughly 26 sixteen-foot boards plus waste.

About 1/8 inch is typical for kiln-dried wood decking, which shrinks as it dries — start at 1/8 and it settles wider. Wet or green lumber can be butted tight because it shrinks on its own. Composite decking has a manufacturer-specified gap; follow that figure exactly.

Standard wood decking laid square to the joists is usually fine at 16 inches on centre. Composite boards and any decking laid diagonally generally require 12-inch spacing for adequate support. Always confirm against the decking manufacturer’s instructions and your local code.

Face-screwed decking takes two screws per board at every joist it crosses. Multiply the number of board rows by the number of joists, then by two. A 26-row deck on 13 joists needs about 676 screws. Hidden-fastener clip systems use roughly one clip per board per joist instead.

Pick a stock length that matches your deck length so each row is a single board with no butt joint — a 16-foot deck is cleanest with 16-foot boards. If your deck is longer than available stock, stagger the butt joints over joists so they do not line up row to row.

Boards have defects, ends get trimmed, and cuts around stairs or posts produce offcuts. The calculator adds about 10 percent so your delivery is not one board short. For a simple rectangular deck you can trim that lower; for a deck with angles or multiple levels, add more.