How the decking calculator works
A timber deck has three main parts: the decking boards you walk on, the joists they're screwed to, and the fixings. The calculator works out each one from the size of your deck:
rows of boards = (deck width + gap) ÷ (board width + gap), rounded up
boards = rows × deck length × (1 + extra %) ÷ board length, rounded up
joists = deck length ÷ joist spacing, rounded up, + 1
joist timber = joists × deck width + 2 × deck length (rim joists)
screws = rows × joists × 2
The "deck length" is the direction the boards run. The joists run the other way, at right angles to the boards, and a rim joist along each long side completes the frame. The calculator doesn't include posts, bearers or a sub-frame for a raised deck, which depend on your ground and height.
Figures we use
- Board width: the default is 144mm, a common width for UK softwood decking. The Timber Decking and Cladding Association (TDCA) advises that timber deck boards should be no wider than 145mm.
- Gap between boards: 5mm. The TDCA recommends a gap of 5 to 8mm for drainage and seasonal movement.
- Joist spacing: 400mm centres. TDCA guidance gives a maximum of 400mm for thin 22mm softwood boards and up to 600mm for thicker boards on domestic decks, so 400mm is a safe default that gives a stiff deck. Composite boards have their own spacing rules, so check the manufacturer's guide.
- Fixings: two screws every time a board crosses a joist, set at the quarter points of the board's width, as the TDCA recommends.
- Extra for cuts: 10%, to allow for staggered joints, trimming ends and rejecting the odd twisted board.
Worked example: a 4m × 3m deck
A deck 4m long (the boards run this way) and 3m wide, using 144mm boards 3.6m long with 5mm gaps:
- Each board plus gap takes up 149mm, so 3,005 ÷ 149 = 20.2, rounded up to 21 rows
- Board run: 21 × 4m = 84m. Add 10%: 92.4m
- Boards: 92.4 ÷ 3.6 = 25.7, rounded up to 26 boards
- Joists at 400mm centres: 4,000 ÷ 400 = 10, plus 1 = 11 joists of 3m
- Joist timber: 11 × 3 + 2 × 4 = 41m
- Screws: 21 × 11 × 2 = 462 screws
The last row will usually need ripping down to width, which is part of the reason for the extra 10%.
A bigger deck
A 4.8m × 3.6m deck using 4.8m boards needs 25 rows, 28 boards, 13 joists of 3.6m (56.4m of joist timber with the rims) and 650 screws.
Decking boards per square metre
With 144mm boards and 5mm gaps, each metre of deck width takes 6.7 rows of boards, so one square metre of deck uses about 6.7m of board before wastage. In 2.4m boards that's about 2.8 boards per m², or 1.9 boards per m² in 3.6m lengths.
Tips for building a deck
- Choose a board length that suits your deck. If the deck is 3.6m long, 3.6m boards mean no joints at all. Otherwise stagger the joints and make sure every board end lands on a joist.
- Use treated timber for joists, rated for ground contact if they're near the soil, and treat any cut ends.
- Lay a weed membrane and gravel under a low deck to stop weeds and help drainage. The gravel calculator works out how much you need.
- Allow for a slight fall away from the house so rainwater runs off.
- Pre-drill hardwood boards near the ends to avoid splitting.
Need concrete pads or post holes for the frame? The concrete calculator works out bags of ready-mix. For an irregular deck, measure it with the square metre calculator first.
Frequently asked questions
How many decking boards do I need for 4m × 3m?
With 144mm boards, 5mm gaps and the boards running the 4m way, you need 21 rows. That's 26 boards of 3.6m including 10% extra for cuts.
How far apart should decking joists be?
400mm centres is the usual choice. TDCA guidance allows up to 600mm for thicker timber boards on domestic decks, but thinner 22mm softwood boards need joists no more than 400mm apart.
How many screws do I need for decking?
Two screws each time a board crosses a joist. A 4m × 3m deck with 21 rows of boards and 11 joists needs 21 × 11 × 2 = 462 screws.
What gap should I leave between decking boards?
Leave 5 to 8mm between boards for drainage and seasonal swelling, as the Timber Decking and Cladding Association recommends.
How many decking boards per square metre?
About 6.7m of 144mm board per m² with 5mm gaps. That's around 2.8 boards of 2.4m or 1.9 boards of 3.6m per square metre, before adding 10% for cuts.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.