Decking Calculator (UK)
Last updated: 12 August 2026
Work out how many decking boards you need for your deck, plus the joists and screws to build the frame. Covers standard UK board widths and joist centres.
How the calculation works
Board rows = deck width ÷ (board width + expansion gap), rounded up. Linear metres = rows × deck length, plus 10% for cuts and wastage. Joists = deck length ÷ joist centres, plus one for the final joist.
Screws assume two per board at every joist crossing, which is standard practice for timber decking. Composite boards often use hidden clips instead — check the manufacturer fixing schedule, as clip counts differ.
Joist spacing for decking
| Deck type | Joist centres | Notes |
|---|---|---|
| Standard timber, boards at 90° | 400mm | Most common UK spec |
| Composite decking | 300–400mm | Follow manufacturer — composite sags at wide centres |
| Boards laid diagonally | 300mm | Diagonal spans further between supports |
| Light-duty timber | 600mm | Only with thick boards and low loading |
Do not forget the base
A deck is only as good as what it sits on. Allow for a weed membrane, a compacted sub-base or concrete pads under the bearers, and a fall of about 1:100 away from the building so water runs off. Ground-contact timber must be use-class 4 treated.
Frequently asked questions
How many decking boards do I need?
Divide the deck width by the board width plus the expansion gap. For a 3m wide deck using 144mm boards with 5mm gaps, that is 3000 ÷ 149 = 21 rows. Multiply by the deck length for linear metres, then add 10% for cuts.
What gap should I leave between decking boards?
Around 5mm is standard for kiln-dried timber, allowing for drainage and movement. Green or wet timber shrinks as it dries, so fit tighter at 3mm. Composite decking follows the manufacturer specification, often 5 to 8mm.
How far apart should decking joists be?
400mm centres is the standard for timber decking with boards running at 90 degrees. Reduce to 300mm for composite boards or diagonal layouts, since both need more support.
How many screws per decking board?
Two screws per board at every joist crossing. A 4m x 3m deck at 400mm joist centres typically needs around 220 screws — buy a box of 250.
What size deck do I need for a table and chairs?
Allow roughly 3m x 3m for a four-seat table with room to pull chairs out, and 4m x 4m for six seats plus circulation. Add space for any steps or planters.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.