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Carpentry · boards · joists
How many decking boards do I need?
Boards including the gap between them, the frame under it and the fixings. The gap matters: 5 mm across a 4 m deck is two boards saved or wasted.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Cover per board =
board width + gap - Rows =
deck width ÷ cover, boards per row =deck length ÷ board length - Screws =
rows × joists × 2— two into every joist crossing
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Screws per crossing | 2 no. | |
| Post spacing | 1.8 m | |
| Membrane roll | 25 m² |
A worked example
With the defaults the page opens on:
- How long is the deck? 5 metres
- How wide is it? 3.5 metres
- Which board width? 144 mm
- What gap between the boards? 5 millimetres
- What length do the boards come in? 3.6 metres
- What centres for the joists? 400 mm
- How much spare for cuts? 10 per cent
Boards
- Deck area: 17.5 m²
- Cover per board: 149 mm — 144 mm + 5 mm gap
- Rows: 24 rows — deck width ÷ cover
- Boards to order: 53 no. — rows × 2 per row, +10%
- Total board length: 190.8 m — 53 boards × 3.6 m
- Decking screws: 672 no. — two into every joist crossing
Frame
- Joists: 14 no. — at 400 mm centres
- Joist timber: 49 m
- Perimeter frame: 17 m
- Posts / pads: 12 no. — at 1.8 m grid
- Weed membrane: 1 roll
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How many decking boards do I need?
Rows = deck width ÷ (board width + gap); boards per row = deck length ÷ board length; then waste. A 5 m × 3.5 m deck in 144 mm boards with a 5 mm gap needs 24 rows.
What gap between decking boards?
5 mm on dry timber, 3 mm if it is already wet, because it shrinks as it dries. Across a 4 m deck the gap is worth two boards, so the calculator asks for it.
How many decking screws?
Two into every joist crossing: rows × joists × 2. The joists are counted at your centres; 400 mm for a solid deck, because 600 mm bounces.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.