Patio slab calculator
A square metre of patio takes about 2.7 slabs of 600 × 600 mm with 10 mm joints, on 100 mm of sub-base and a 40 mm mortar bed.
- Sub-base
- 2.40 tonnes
- Sub-base bulk bags (850kg)
- 3
- Bedding mortar
- 0.48 m³
- Area
- 12.00 m²
4 × 3 m in 600 × 600 mm slabs with 10 mm joints, plus 5% extra.
Sub-base is compacted MOT Type 1 or similar. To turn the bedding volume into cement and ballast, use the concrete calculator.
Slabs per square metre
With 10 mm joints, before extra for cuts.
| Slab size | Slabs per m² | Slabs for 10 m² |
|---|---|---|
| 450 × 450 mm | 4.7 | 48 |
| 600 × 300 mm | 5.3 | 53 |
| 600 × 600 mm | 2.7 | 27 |
| 900 × 600 mm | 1.8 | 19 |
| 900 × 900 mm | 1.2 | 13 |
How we calculate this
We divide the area, plus extra for cuts, by the space one slab takes including its joint. Slabs are rounded up.
For the base, Marshalls recommends a 100 mm compacted sub-base and a 40 mm bed of mortar. We weigh compacted sub-base at about 2 tonnes per m³, which matches a supplier’s guide of 4–5 m² per bulk bag at 100 mm.
Questions people ask
How deep should a patio sub-base be?
Marshalls recommends 75 to 100 mm of compacted sub-base for patios and garden paths, and 100 to 150 mm for driveways. Their patio guide digs out 150 mm to fit the sub-base, mortar and slab.
How many paving slabs do I need for 10 square metres?
About 29 slabs of 600 × 600 mm, including 5% extra for cuts. The table shows other sizes.
How wide should the joints be?
Marshalls suggests 10 to 15 mm between slabs. Some porcelain paving uses narrower joints, so check the product guide.
How much does a bulk bag of sub-base cover?
About 4 to 5 m² at 100 mm compacted. With our figure, an 850kg bag covers about 4.3 m².
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