Deck Calculator
Size, board type and joist spacing in; a lumber list out. Boards are counted row by row at the lengths you can actually buy, with joists and fasteners to match.
How deck framing turns into a lumber list
Deck boards sit across joists, and every crossing is where fasteners go. Change the spacing and the joist count moves. Turn the boards to 45° and each board crosses joists at a longer effective span, which is why diagonal decks usually need joists closer together.
How this calculator works
- Rows of boards = deck width ÷ (board width + gap). Pieces per row = deck length ÷ board length, rounded up; butt joints must land on a joist.
- Diagonal layouts are sized by area with 15% waste, because every row ends in an angled cut.
- Joists = deck length × 12 ÷ spacing + 1. Each board crosses every joist and gets two screws or one hidden clip there.
Deck questions
How many deck boards do I need for a 12×12 deck?
With 5.5 inch boards and a 1/8 inch gap, 12 ft of width takes 26 rows. With 12 ft boards that is 26 boards, and about 28 with 5% spare.
What is the gap between deck boards?
Typically 1/8 inch for dry wood and 3/16 inch for composite, or as set by the hidden fastener. Wet pressure-treated boards are often installed tight.
How many screws per deck board?
Two at every joist crossing, about an inch from each edge. A 16 ft board over joists at 16 inches crosses 13 joists, so 26 screws.
Composite or wood?
Composite costs more up front and often needs closer joists, but does not need staining or sealing. Pressure-treated pine is cheapest to build and needs a finish every couple of years.