How it works
Enter the deck size and board details. Boards run parallel to the house, so the number of rows comes from the deck depth divided by board width plus gap, and boards per row from the deck width divided by board length.
Joists run perpendicular to the house at the spacing you choose, with one extra to close the end. Composite decking usually requires 16 in or 12 in spacing (12 in for diagonal layouts); always follow the manufacturer's span rating.
Face screwing uses two screws per board at every joist. A 5 lb box of 3 in deck screws holds roughly 350 to 400 screws, though the count varies by gauge. Hidden fastener systems use one clip per board per joist instead.
Formula
Rows = depth × 12 ÷ (board width + gap). Boards = rows × ⌈width ÷ board length⌉ × (1 + waste). Joists = width × 12 ÷ spacing + 1. Screws = rows × joists × 2.
Questions people ask
What joist spacing do I need?
Wood decking is commonly installed on joists 16 in on center. Many composite boards allow 16 in for straight layouts and require 12 in for diagonal layouts. Check the product's installation guide.
How big of a gap between deck boards?
About 1/8 to 3/16 inch for kiln-dried or composite boards. Wet pressure-treated lumber is often laid tight because it shrinks as it dries.
How many deck screws do I need?
Two per board at each joist. A 16 × 12 deck with 5.5 in boards and 16 in joists needs about 26 rows × 13 joists × 2 = 676 screws.