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Composite Decking

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Composite Decking Calculator

Composite Decking Calculator

A composite decking calculator works out exactly how many boards, joists, clips and screws a deck needs from its length and width, then converts that into a total material cost. Enter your dimensions below, or use the manual method in the next section to calculate it yourself.

Get expert decking advice for your project, call us on: 020 8088 4888.

How to calculate how many decking boards you need

Step 1 — Measure your deck in metres

Measure the length and width of the area you're decking, in metres, to two decimal places.

Step 2 — Work out boards needed across the width

Divide the width by the board width plus its side gap, then round up.

  • Explorer (145mm wide): width (m) ÷ 0.150m

  • Frontier (143mm wide): width (m) ÷ 0.148m

Example: a 3m-wide deck in Explorer needs 3 ÷ 0.150 = 20 boards across.

Step 3 — Work out board runs needed across the length

Both ranges come in 4m lengths.

Divide your deck's length by 4m and round up; for runs longer than 4m, boards are butt-jointed with a minimum 8mm gap (or 2% of the board's length, whichever is larger) between ends, staggered row to row.

Step 4 — Add wastage

Add 5% to your total board count for a standard square/rectangular layout, or 15% if laying the boards diagonally, to cover cutting waste and mistakes.

Step 5 — Work out joist spacing

  • Explorer: joists at a maximum of 300mm centres (250mm if laying diagonally)

  • Frontier: joists at a maximum of 400mm centres (300mm if laying diagonally)

Number of joists needed = (span in mm ÷ centre spacing) + 1, rounded up.

Step 6 — Size the subframe (bearers)

If using plastic lumber or aluminium bearers, match the profile to the support span required:

Bearer Profile

Material Type

Maximum Support Span

50 x 50mm

Plastic Lumber

500mm

50 x 100mm

Plastic Lumber

750mm

50 x 150mm

Plastic Lumber

1500mm

20 x 50mm

Aluminium

500mm

40 x 50mm

Aluminium

1000mm

80 x 50mm

Aluminium

1400mm

Each bearer must be supported in at least 3 places, and shouldn't overhang its last support by more than 50mm.

Step 7 — Count fixings

Allow for a starter clip roughly every 300mm along each joist, plus one hidden fastener per joist at every board (with extra fasteners at board ends).

Step 8 — Get an instant total

Enter the same dimensions into the calculator above to get an itemised list of boards, joists, clips and fixings in seconds.

Reference Table: Board Specifications

Range

Length

Width

Depth

Core

Joists (standard)

Joists (diagonal)

Weight / board

Explorer

4m

145mm

21mm

Hollow, uncapped

300mm

250mm

11.20kg

Frontier

4m

143mm

22mm

Solid, capped

400mm

300mm

13.44kg

Reference Table: Gaps & Clearances

Location

Minimum gap

Between board sides (set by hidden clips)

3-6mm

Board end butt joints

8mm, or 2% of board length if greater

Boards to walls or fixed objects

20mm

Decking overhang past joist

Max. 20mm

Ground clearance over soft ground

Min. 300mm

Drainage fall

0.5% (5mm per metre)

Decking Calculator FAQs

Divide your deck's width by the board width plus its side gap (0.150m for Explorer, 0.148m for Frontier) to get boards per row, then divide the length by the 4m board length for the number of rows. Add 5% for wastage (15% if laying diagonally). Our calculator above does this instantly for any size deck.

Hidden fastener clips automatically set a 3–6mm gap between board sides. Butt joints at board ends need at least 8mm (or 2% of the board's length). Leave a minimum 20mm gap between the decking and any wall or fixed object.

You can use timber, plastic lumber or aluminium joists. If using plastic lumber or aluminium bearers, match the profile to the span: 50×50mm bearers span up to 500mm, 50×100mm up to 750mm, and 50×150mm up to 1,500mm. Each bearer needs support in at least 3 places.

Allow for a starter clip roughly every 300mm along each joist, plus one hidden fastener per joist at every board crossing, with extra fasteners at board ends. So for a single 4m board this equates to ~14 fasteners.

Reduce your joist centres to 250mm (Explorer) or 300mm (Frontier), and increase your wastage allowance from 5% to 15% to account for the extra angled cuts.

In most cases, no, but this depends on the height and total garden coverage of the structure, and whether the property is listed or in a conservation area. Check with your local planning authority before starting work if you're unsure.

Allowing for the expansion gap, one 4m Explorer board covers roughly 0.6m² and one 4m Frontier board covers roughly 0.59m², measured along its length including the gap to the next board.

Explorer decking needs joists at a maximum of 300mm centres (250mm laid diagonally). Frontier decking needs a maximum of 400mm centres (300mm laid diagonally), because its solid core supports a wider span.

Add 5% to your calculated board count for a standard layout, or 15% if you're laying the boards diagonally, to cover cutting waste, mistakes and pattern-matching.

Explorer is a hollow-core, uncapped board (145mm wide, 300mm joist centres, 15-year warranty). Frontier is a solid-core, capped board (143mm wide, 400mm joist centres, 25-year fade and stain warranty). Frontier's solid core allows wider joist spacing, so it typically needs fewer joists for the same deck.

Leave a minimum of 300mm clearance between the underside of the decking and the ground where it sits over soft ground or 50mm when over hard ground, and build in a drainage fall of 1:80 (12.5mm per metre) to stop water pooling under the deck.

This calculator is built specifically around EnviroBuild's Explorer and Frontier composite ranges and their exact board and joist specifications, so results won't be accurate for timber decking, which has different board sizes and span tables.

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