How Many Paving Slabs Do I Need? Worked UK Examples

How Many Paving Slabs Do I Need?

Quick Answer

Slabs = area ÷ slab area, rounded up, plus 10% for cuts

A 4 × 3 m patio is 12 m². With 450 × 450 mm slabs that is 12 ÷ 0.2025 = 59.3, rounded up to 60 slabs, or 66 with the 10% wastage allowance. With 600 × 600 mm slabs it is 34, or 38 with wastage.

Slabs Per Square Metre by Size

Slab size	Slab area	Slabs per m²
300 × 300 mm	0.09 m²	11.1
450 × 450 mm	0.2025 m²	4.9
600 × 600 mm	0.36 m²	2.8
900 × 600 mm	0.54 m²	1.9

Figures are slab area maths only (1 ÷ slab area in m²) and assume tight-fit laying; with 10 mm joints you need slightly fewer. Always round the final count up and add wastage before ordering. 450 × 450 mm and 600 × 600 mm are the most popular sizes in the UK, and 450 mm slabs are easier to handle.

Worked Example: a 4 × 3 m Patio

Area: 4 × 3 = 12 m²
450 × 450 mm slab area: 0.45 × 0.45 = 0.2025 m²
12 ÷ 0.2025 = 59.3, rounded up to 60 slabs
Add 10% for cuts and breakages: 60 × 1.1 = 66 slabs
In 600 × 600 mm (0.36 m² each): 12 ÷ 0.36 = 33.3 → 34, plus 10% = 38 slabs

The same method covers paths: a 3 × 2 m path is 6 m², which is 30 slabs at 450 × 450 mm, or 33 with the 10% allowance.

How Much Wastage to Allow

Allow 10% for cuts and breakages on a straight-laid patio. Diagonal and herringbone patterns waste more at the edges, so allow 12-15%, and complex curved shapes more still. The spare slabs are not money down the drain: keep one or two for future repairs, since colours weather and batches change.

What Goes Under the Slabs

The standard build-up is a compacted sub-base of MOT Type 1 at 75-100 mm, topped with a 30-50 mm layer of sharp sand or mortar. Mortar gives a stronger, more permanent fix ideal for patios; sand is easier for DIY and lets individual slabs be lifted later. Use polymeric sand for the joints, and size it with the jointing sand calculator.

For the 12 m² patio above at a 100 mm sub-base depth: 12 × 0.1 = 1.2 m³ of MOT Type 1, which at about 1,950 kg/m³ compacted is 2.34 tonnes, or about 2.6 tonnes with 10% waste. The 50 mm sand layer is 0.6 m³, about 1.1 tonnes including waste, roughly 5 bulk bags of material in total. The sand and sub-base calculator prices it and flags under-specified depths.

Odd-Shaped Patios

The slab count only works once you know the area. For L-shaped patios, measure up with the patio area calculator first; for anything more irregular, break the shape down with the odd shape area calculator. Then divide by your slab size and add wastage as above.