How this calculator works
Each slab covers its own size plus one joint, so the number of slabs is the area divided by the area of one slab with its joint. Under the slabs go a compacted sub-base of MOT Type 1 and a bedding layer of sharp sand, or a full mortar bed for heavier use.
Worked example: a 4 × 3 m patio (12 m²) in 600 × 600 mm slabs with 10 mm joints and 10% for cuts: each slab with its joint covers 0.61 × 0.61 = 0.372 m², so you need 12 × 1.1 ÷ 0.372 = 35.5, which is 36 slabs. A 100 mm sub-base needs 12 × 0.1 × 2.2 × 1.05 = 2.8 tonnes of MOT Type 1, and a 30 mm sand bed 0.6 tonnes of sharp sand.
Materials for common patio sizes (600 × 600 mm slabs, 10 mm joints)
100 mm sub-base, 30 mm sharp sand bed, 10% for cuts.
| Patio | Slabs | MOT Type 1 | Sharp sand |
|---|---|---|---|
| 3 × 3 m | 27 | 2.1 tonnes | 0.5 tonnes |
| 4 × 3 m | 36 | 2.8 tonnes | 0.6 tonnes |
| 4 × 4 m | 48 | 3.7 tonnes | 0.8 tonnes |
| 5 × 4 m | 60 | 4.6 tonnes | 1.0 tonnes |
| 6 × 4 m | 71 | 5.5 tonnes | 1.2 tonnes |
| 6 × 5 m | 89 | 6.9 tonnes | 1.5 tonnes |
Tips before you buy
- Keep the finished patio at least 150 mm below the damp-proof course of the house, and lay it to a fall of about 1 in 80 (about 12 mm per metre) away from the building, so that water drains away.
- Compact the MOT Type 1 in layers of no more than about 100 mm, with a plate compactor. A hand rammer will not give a firm base.
- Lay slabs on a full mortar bed, or on mortar spots with sand, for a drive or any area that will carry a vehicle. A sand bed is fine for a garden patio.
- Buy slabs from one batch: colours and sizes vary between runs, and a later pack may not match.
- Check for buried services before you dig, and for the front garden rule: paving more than 5 m² with an impermeable surface needs permeable materials or drainage to a lawn or border.
- Fill the joints with a dry mortar mix, a resin-bound jointing compound or a brush-in gravel jointing product, as the slab maker recommends.
Tools & materials for this project
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Frequently asked questions
How many paving slabs do I need for a 4m x 3m patio?
36 slabs of 600 × 600 mm with 10 mm joints and 10% for cuts. Without any allowance for cuts it is 33 or 34.
How much MOT Type 1 do I need for a patio?
About 0.22 tonnes per m² at 100 mm deep, so a 12 m² patio needs about 2.8 tonnes with 5% extra for compaction.
How deep should the sub-base be for paving?
100 mm of compacted MOT Type 1 for a patio or path, and 150 mm or more for a driveway. On soft clay ground, go deeper.
How many 600x600 slabs are in a square metre?
About 2.8 without joints. With 10 mm joints it is 2.69 slabs per m².
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.