Outdoor & gardenFree calculator
Work out how many deck boards, joists and screws you need for a rectangular deck. Enter the deck size, board size, gap and joist spacing to get a complete shopping list.
Your shopping list
How to measure
- Measure the deck in the direction the boards will run (length) and across (width).
- Check the real board width. A 6 in board is actually about 5.5 in wide.
- Leave a small gap for drainage: about 1/8 in (3–5 mm).
The formula
Rows = deck width ÷ (board width + gap) Boards = rows × (deck length ÷ board length) + 5% Screws = rows × joists × 2
Worked example
A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps needs 26 rows. With 16 ft boards that is 26 boards, or 28 with 5% extra.
Tips before you buy
- Use stainless or coated deck screws so they do not rust or stain the timber.
- Stagger board joints over different joists for a stronger, neater deck.
- Composite boards often need closer joist spacing; check the maker's guide.
Frequently asked questions
Which way should deck boards run?
Usually across the joists, parallel to the house, so water drains off the ends.
Screws or hidden fasteners?
Screws are cheaper and faster. Hidden clips look cleaner and are common with composite boards.
What joist spacing do I need?
16 in (40 cm) centres is standard for most deck boards. Diagonal boards or thinner boards may need 12 in (30 cm).
Results are estimates for planning only. Always check quantities with your supplier before you buy.