What it does
Plan the shopping list for a deck. Type the deck’s size, the board width and gap, the board lengths your timber yard sells and the joist spacing, and the calculator works out how many rows of boards you need, which board lengths to buy for each row, the joists, and the screws or hidden clips.
It works in metric or imperial and runs entirely in your browser. Span tables and permits are local, so check your building code before you build. For the footings use the concrete calculator, and for steps down to the garden the stair calculator.
How to use it
- Choose Metric or Imperial. Switching loads the example in those units.
- Type the deck length along the boards and the width across them.
- Type the board width, the gap and the lengths you can buy, separated by commas.
- Set the joist spacing and how the boards are fixed. Reset example brings the example back; Clear empties the sizes.
What each setting does
- Deck length, along the boards: the direction the boards run. Joists run the other way.
- Board width and gap: rows = (deck width + gap) ÷ (board width + gap), rounded up, because there is one gap fewer than boards.
- Board lengths you can buy: for each row the calculator picks the combination of up to 4 boards with the least total length; joints must sit on a joist. Offcuts are not reused.
- Joist spacing: 400 or 600 mm (16 or 24 in) centres are common. Joists = deck length ÷ spacing, rounded up, plus one.
- Fixing: face screws (screws per board at each joist, 2 by default) or hidden clips, counted as one per board at each joist.
Worked example
A deck 4 m long and 3 m wide with 140 mm boards and 5 mm gaps: 3005 ÷ 145 = 20.7, so 21 rows. The yard sells 3.6 m and 4.8 m boards, so each 4 m row takes one 4.8 m board: 21 boards of 4.8 m. Joists at 400 mm across 4 m: 4000 ÷ 400 = 10, plus one, so 11 joists of 3 m. Screws: 21 × 11 × 2 = 462.
FAQs
Does it count beams, posts and rim joists? No. It counts the decking, the joists under it and the fixings.
How many spare boards should I buy? A board or two extra covers split ends and bad cuts.
Where the numbers come from
- Board, joist and fastener counts are fibub's own working from the sizes you type.
- Two screws per board at each joist and a 5 mm (3/16 in) gap are common rules of thumb; board makers give their own fixing and gap instructions. Span tables and permits are set by your local building code.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.