DIY Calculators OnlineHow the deck is counted
Decking runs parallel to the house, across the joists. The number of rows is the deck depth divided by one board width plus one gap. Each row needs the deck length divided by the board length, rounded up.
rows = ⌈depth × 12 ÷ (board + gap)⌉ · joists = ⌈length × 12 ÷ spacing⌉ + 1
Joists run from the ledger on the house out to the rim. The joist count includes both end joists. Each joist gets one hanger at the ledger. Screws are two per board at every joist it crosses.
Choosing joist spacing
| Decking | Typical max spacing |
|---|---|
| 5/4 wood boards, square to joists | 16" o.c. |
| 2× wood boards | 24" o.c. |
| Composite boards | 12–16" o.c., per the maker's install guide |
| Any board laid diagonally | 12" o.c. |
Joist size depends on span, species and load. This planner doesn't size joists or beams: use the span tables in the AWC deck guide (DCA 6) or your local code. Most decks need a permit, and inspectors check ledger attachment closely.
Stain coverage
Most deck stains cover 150–300 square feet per gallon per coat. New smooth boards are at the high end. Weathered or rough wood soaks up more, so use 150. Stain the deck surface, then add a little extra if you also do the stairs and railing.
Questions
How many deck boards for a 12×12 deck?
With 5 1/2" boards and a 3/16" gap you need 26 rows. Using 12 ft boards that is 26 boards, 28 with 5% extra.
Should I buy longer boards?
Boards that span the whole deck avoid butt joints, which look better and need doubled joists underneath. On decks longer than the longest board, stagger the joints so they don't line up on one joist.
How many screws per box?
Boxes vary from about 350 to 1,750 screws. Divide the screw count above by the box size and round up.