Sixteenth
Deck Calculator
Enter the deck length along the house and how far it comes out. You get the deck boards to buy, the joists, the rim joist and the screws.
Boards run along the house; joists span out from the ledger. Plain numbers are feet, gap is in inches.
How the deck is counted
rows = deck width ÷ (board width + gap) · linear feet = rows × deck length × (1 + waste)
joists = deck length ÷ spacing + 1 · screws = rows × joists × 2
The joist count includes both end joists. The rim joist runs along the outside edge, and the ledger along the house, each the length of the deck. Joist size depends on the span: check a span table or your local code (for example, the DCA 6 guide from the American Wood Council).
Plan your board lengths so butt joints land on a joist and are staggered between rows. On decks longer than your boards, a picture-frame border or a breaker board can avoid butt joints entirely.
Questions
How many deck boards do I need?
Divide the deck width by the board width plus the gap to get the rows, multiply by the deck length for the linear feet, add waste, and divide by the board length. A 5/4×6 board is 5-1/2" wide; with a 3/16" gap each row covers 5-11/16".
What joist spacing should I use?
16" on center is standard for wood decking laid square to the joists. Many composite boards, and any boards run on a diagonal, need 12". Check the decking maker's instructions.
How many screws for a deck?
Two screws where each board crosses each joist. The calculator multiplies rows × joists × 2.