What this estimate does and does not do
This calculator estimates decking surface materials and a simple joist count. It does not design a safe deck. Deck structure depends on span tables, loads, beam sizes, posts, footings, ledger attachment, frost depth, guardrails, stairs, lateral bracing, and local building code. Use this for early material planning only.
Formula used
board rows = ceil(deck width × 12 / (board width + gap))
net linear feet = board rows × deck length
order linear feet = net linear feet × (1 + waste %)
boards needed = ceil(order linear feet / board length)
joists = ceil(deck length × 12 / joist spacing) + 1
screws = ceil(board rows × joists × screws per crossing × 1.10)Worked example
For a 16 ft by 12 ft deck, 5.5 inch boards, 1/8 inch gaps, 12 ft boards, 16 inch joist spacing, and 10% waste:
rows = ceil(144 / 5.625) = 26
linear feet = 26 × 16 = 416 ft
order linear feet = 457.6 ft
boards = ceil(457.6 / 12) = 39 boardsAssumptions
- Boards run along the deck length.
- Joists are counted along the board length.
- Two screws per joist crossing is the default face-screw rule.
- Hidden fasteners, diagonal layouts, picture framing, stairs, and borders need extra material.
Frequently asked questions
How do you calculate deck boards?
Divide deck width by the board width plus gap to get board rows, multiply by deck length for linear feet, add waste, then divide by board length and round up.
How many screws does a deck board need?
A common face-screw rule is two screws at every joist crossing for each deck board, with extra for dropped, stripped, or damaged screws.
How do you estimate joist count?
Divide the deck length in inches by joist spacing and add one for the end joist. This is a layout estimate, not a structural span design.
Can this calculator design deck framing?
No. It estimates surface boards, joists, and screws. Structural framing, beams, footings, guards, stairs, spans, loads, and permits require code-compliant design.