Decking calculator

How many decking boards do I need?

Rows first: the deck's width ÷ (board width + gap), rounded up. Then boards per row from the lengths your merchant stocks. For 4.8m × 3.6m with 145mm boards and a 5mm gap, that is 25 rows, each one 4.8m board: 25 boards, or 28 with 10% waste. In 3.6m boards the same deck takes 34, because each row needs a 3.6m board and a 1.2m piece, and one board makes three pieces.

Where a row is longer than your boards, join them end to end over a joist and stagger the joints from row to row.

How much decking do I need per m²?

About 6.7 linear metres of 145mm board per m² with a 5mm gap, which is 1.9 boards of 3.6m or 1.4 of 4.8m. Add 10% waste for a straight lay and 15% for diagonal.

How many decking boards for 30m², or a 3m × 3m deck?

A 30m² deck, 6m × 5m, takes 34 rows of 145mm boards: 68 × 3m, or 75 with 10% waste. A 3m × 3m deck takes 21 × 3m boards, or 24 with waste.

Decking frame calculator: joists, bearers and posts

The frame is joists under the boards, bearers under the joists, and posts under the bearers. Joists should not span more than 1.8m without a bearer under them, so a wider deck gets a bearer in the middle. Posts are 100 × 100mm, 600mm into the ground with Postcrete, at up to 1.5m along each bearer. Noggins, short pieces between the joists, go in a row across the middle of each bay.

For 4.8m × 3.6m at 400mm centres: 13 joists of 47 × 150mm at 3.6m, 3 bearer lines at 4.8m, 15 posts, 24 noggins at 353mm, and 650 screws.

Decking joist spacing and span

• 400mm centres for standard timber boards, 350mm for 28mm finished softwood, 300mm for a diagonal lay (tradecalculator).
• 300mm for most composite boards, 250 to 300mm laid diagonally (Envirobuild).
• 47 × 100mm joists for short spans; 47 × 150mm where the span passes 1.5m, up to 1.8m between bearers (materialcalculator).

600mm centres suit floor joists under chipboard, not decking: the boards bounce. Only thick, high-grade boards are rated to 600mm.

Composite decking calculator: what changes

Composite boards need closer joists (300mm), a 8mm gap at butt joints, 20mm where the deck meets a wall, and hidden clips instead of screws: one per board at every joist. The same 4.8m × 3.6m deck in 145mm composite is 25 rows on 17 joists, which is 425 clips. Board widths vary between makers, so enter yours.

What gap should I put between decking boards?

About 5mm between timber boards (materialcalculator gives 5 to 6mm), for drainage and so the boards can swell. Composite: 3 to 6mm at the sides, which most clip systems set for you, at least 8mm at butt joints and 20mm at walls (Envirobuild).

What is the 3/4/5 rule for decking?

A way to square the frame: mark 3 units along one side and 4 along the other from the same corner, and adjust until the diagonal between the marks is exactly 5. In a deck frame, 600mm, 800mm and 1,000mm is easy to measure.

What is the 30cm rule for decking?

In England, decking is permitted development only if it is no more than 30cm above the ground, covers no more than half the garden together with other extensions and outbuildings, and is not in front of the house (Planning Portal). Higher than that needs planning permission. Above 600mm it also needs a balustrade.

How much does decking cost?

BestBuilders' 2026 figures for supply and fit: softwood £80 to £140 per m², hardwood £140 to £220, composite £120 to £220. A 15m² deck comes to £1,500 to £2,200 in softwood or £2,200 to £3,300 in composite, frame and labour included. A balustrade adds £60 to £150 a metre, steps £80 to £200 each, uneven ground £150 to £600, and taking out an old deck £150 to £400. At those rates a 20m² softwood deck is about £1,600 to £2,800.

Common decking mistakes

• Joists too far apart, so the boards bounce.
• No air under the deck: Envirobuild wants 50mm over paving and 300mm over soil.
• No end gaps on composite boards, so they buckle in summer.
• Joints in the same place row after row, or not over a joist.
• Timber touching the soil: keep it at least 50mm clear (materialcalculator).