Deck Calculator
Estimate decking boards, joists and screws for a deck from its size and board spacing.
// Bill of materials
Costs are rough, materials only — edit the price to match your quote.
How it's calculated
Board rows = deck width (in inches) ÷ (board width + gap), rounded up — each row is one full-length board. Joists = deck length (in inches) ÷ joist spacing, plus one. Screws are two per board at every joist.
Worked example — your inputs
- Deck length
- 12 ft
- Deck width
- 12 ft
- Board width
- 5.5 in
- Gap between boards
- 0.125 in
- Joist spacing
- 16 in on center (standard)
- Price per board
- 15 $
Popular sizes
Tips for buying deck
- Run decking perpendicular to the joists, and stagger butt joints so seams don't line up.
- Leave a small gap between boards (about 1/8 in) for drainage and wood movement.
- Two screws per board at every joist is the standard hold; hidden fasteners change the count.
- Buy 10% extra boards for cuts, culls and a few replacements down the road.
- 16 in joist spacing suits most decks; go 12 in for diagonal decking or heavy loads.
- Use exterior-rated screws and joist hangers, and flash the ledger board against the house.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5.5 in boards and a 1/8 in gap, a 12 ft width needs about 26 rows — so 26 twelve-foot boards, before adding waste.
How many joists for a 12 ft deck?
At 16 in on center, a 12 ft length needs about 10 joists (length in inches ÷ 16, plus one). Closer spacing means more joists.
How many screws for a deck?
Two screws per board at each joist. A 12×12 deck at 16 in spacing works out to roughly 520 screws — buy a little extra.
Which way should deck boards run?
Perpendicular to the joists. Enter deck length as the direction the boards run and width as the span across them.