Decking Calculator UK · metric

Boards, linear metres and cost for softwood or composite decks. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Deck area is length × width. Boards are assumed 3.6 m long and 144 mm wide, so linear metres = area ÷ 0.144 with 10% waste, then boards = linear metres ÷ 3.6. Costs cover DIY materials and a professionally installed range per m².

The waste allowance

This calculator adds 10% to the board count, and 15% for a diagonal layout. That is more generous than the 5% a manufacturer usually quotes, and the gap is deliberate. A manufacturer's figure covers ordinary offcuts on a simple rectangle where every board runs the full width. It does not cover the second cut on every board round a step, the crooked rip against a house wall that turns out not to be square, or the two or three lengths that arrive with a bow you would not accept in a finished surface. On a diagonal deck every board takes two angled cuts and the triangular offcuts are useless. The percentage cannot fix the other problem, which is that decking comes in fixed lengths of 2.4, 3.0, 3.6, 4.2 and 4.8 metres, so a deck 3.9 metres across means binning 300 mm off every board. Setting the deck out to land near a board length is worth doing on paper before anything is ordered.

Board widths, cover widths and the lengths they come in

British decking is sold at its sawn size and delivered at its finished size, and the two are not the same. A board sold as 32 by 125 mm is planed down to roughly 26 by 120 mm, and a board sold as 32 by 150 mm finishes at about 27 by 145 mm, though some mills finish the wider board at 141 or 144 mm. Check the merchant's own specification rather than assuming the nominal figure.

Take the 145 mm board as the common case. Laid with a 5 mm gap it occupies 150 mm of deck, so a linear metre of board covers 0.15 square metres of finished surface. The gap works slightly in your favour and the effect is small enough that the waste allowance swamps it. Composite boards are usually 145 mm wide in the same 3.6 and 4.8 metre lengths, so they drop into the same arithmetic. What changes with composite is the frame underneath.

Joist centres in millimetres, and why 600 mm is usually too wide

Joists here are set out at 400 mm or 600 mm centres, the working equivalents of the American 16 and 24 inch spacings. Those are floor joist conventions, and 600 mm carried straight out into the garden is the commonest reason a deck feels springy underfoot.

For softwood decking the published trade guidance is 400 mm centres for a 28 mm board, rising to about 500 mm for thicker 34 and 45 mm boards. Hardwood goes tighter because the boards are thinner: 350 to 400 mm at 21 or 22 mm, 400 to 450 mm at 28 mm. Composite is tighter again, commonly 300 mm on a new frame and 400 mm on a sound existing frame stiffened with noggins.

The reason is stiffness rather than strength. Composite deflects further than timber of the same thickness over the same span, and it softens as it warms, so a deck in full sun is working harder than the same deck in March. Running boards at an angle lengthens the span between joists: at 45 degrees, multiply the centres by about 0.707, so 400 mm becomes roughly 280 mm.

C16, C24 and the Use Class the joists actually need

Structural softwood here is strength graded, and the two grades you will be offered are C16 and C24. The Timber Decking and Cladding Association treats C16 as the minimum for a deck more than 600 mm above the ground, and points to C18 or C24 where you want smaller sections or longer spans. C24 is stiffer at the same section, so it is the grade to reach for when joist depth is tight.

Treatment is specified by Use Class, and this is where a lot of garden decks are built wrong. Deck boards need Use Class 3, which covers timber used outside above ground. The substructure does not. Since January 2020 the TDCA has recommended Use Class 4 for joists, beams and posts whether or not they touch the ground, because a joist in a shaded, poorly ventilated void spends much of the year damp. Merchants often supply UC3 by default, so it is worth asking. Every cut end is an untreated end as well, so brush end grain preservative on as you cut.

Spans: what we can tell you, and what we cannot

Most decking pages at this point reprint an American span table, and we are not going to. The American Wood Council's DCA6 tables are written for Southern Yellow Pine, Douglas Fir-Larch, Hem-Fir and Western Cedar at imperial sections under American loading. Converting them to millimetres does not make them British.

The British equivalent exists but is not free. The TDCA publishes deck beam and joist spans in the Timber Decking Handbook, and TRADA publishes span tables for solid timber members in floors, ceilings and roofs. We have not been able to verify a public, authoritative UK span table written specifically for deck joists, and we would rather say so than print a figure we cannot stand behind.

What is public is domestic floor joist guidance calculated to Eurocode 5, and it gives a sense of scale. A 47 by 150 mm C16 joist spans about 2.6 metres at 400 mm centres and the same section in C24 about 2.9 metres, at a domestic floor loading of 1.5 kN per square metre. A deck also takes wind, weather and sometimes a hot tub. Treat those figures as a check that a design is in the right area, and for anything raised more than 600 mm take the spans from the TDCA handbook or have the frame checked.

Gaps and fixings, which the TDCA does publish

The gapping guidance is clear and worth following. The TDCA specifies 5 to 8 mm between adjacent boards so water drains and the timber has somewhere to go as it moves. Leave 5 mm where a board end meets a post and 10 mm where decking abuts a building. Board ends may be butted rather than tightly jointed, and every butt joint needs support under both ends.

Composite has its own numbers and they vary by brand, so read the pack rather than applying the timber figures. Millboard calls for 4 mm at the sides, 1 mm at the ends and 10 mm against walls. Trex asks for at least 5 mm at the sides and 3 mm at the ends above 5 degrees, opening to 5 mm below that, because boards fitted cold will grow when the weather turns.

Fixings are the detail most often fudged. The TDCA specifies stainless steel annular ring shank nails or screws at least two and a half times the board thickness in length, two per joist crossing, set at the outer quarter points. Plated screws cost less and will stain a treated deck within a couple of winters.

Joist centres by decking type

Maximum centres in millimetres for the joists under the boards. Framing tighter is always acceptable and costs a few more joists; framing wider is what produces bounce and sag.

Joist centres from the GripDeck timber decking joist spacing guide and Tough Decking's composite spacing guidance, with 300 to 400 mm recycled plastic joist centres from Millboard. Board gaps, strength grades, Use Class treatment and fixing specification from the Timber Decking and Cladding Association. Indicative floor joist spans calculated to BS EN 1995-1-1 with the UK National Annex. We could not find a free, authoritative UK span table written specifically for deck joists; the TDCA's own figures are in the Timber Decking Handbook, which is not published online.

What goes wrong on British decks

Framing at 600 mm centres because that is what floor joists do. A 28 mm softwood board wants 400 mm and capped composite usually wants 300 mm. The deck will not fall down, but it will flex underfoot and the sag between joists shows in low sun.

Buying Use Class 3 joists. UC3 is right for the boards and wrong for the frame. The TDCA has recommended Use Class 4 for joists, beams and posts since January 2020, ground contact or not, and merchants often supply UC3 unless asked.

Converting an American span table into millimetres. DCA6 is written for American species under American loading. The arithmetic works and the answer still does not apply here.

Laying boards tight, or tight against the house. The TDCA specifies 5 to 8 mm between boards and 10 mm where decking abuts a building. Without it, water sits in the joints and the boards have nowhere to move.

Setting out a deck that does not suit the board lengths. Boards come in 2.4, 3.0, 3.6, 4.2 and 4.8 metre lengths. A deck 3.9 metres across means cutting 300 mm off every board in the field.

Frequently asked questions