What does this tool do?
Deck boards are counted in rows across the deck width, then multiplied by the number of boards needed to span the length. Joists are counted from the length and the joist spacing and checked against the deck width.
The Formula
Rows = ⌈Deck width × 12 ÷ (Board width + 1/8 in gap)⌉; Boards = Rows × ⌈Deck length ÷ Board length⌉ × 1.05Frequently Asked Questions
How many boards do I need for a 12 × 10 ft deck?
With 5.5 in boards and a 1/8 in gap you need 22 rows; at 12 ft long that is 22 boards, or 24 with a 5% cut allowance.
Should joists run the short or long way?
Joists usually span the shorter dimension and are spaced 12–16 in apart. Deeper joists or closer spacing are needed for heavy loads or wide spans.