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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.
1
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.
2
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.
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
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
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 rolls
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.
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.
