Paving calculator

What it works out

This works out the materials for a patio or a garden path: the slabs, the sub-base under them, the sand and cement for the bed, and the jointing between slabs. It also tells you how much soil you'll dig out.

It's for paving you walk on. A drive that takes cars needs its sub-base designed for the load, by a groundworker or an engineer.

How it works

The slabs come from the area divided by one slab plus its joint, with the waste allowance on top. The sub-base, compacted crushed stone, is the area times its depth, turned into tonnes from its density once compacted.

A mortar bed is mostly sharp sand, with cement making up about a quarter of the sand's weight for a 4:1 mix. A sand bed is just sand. The joints are filled with jointing compound or mortar, worked out from the joint's width and depth for your slab size.

Digging out is the area times the total depth of sub-base, bed and slab. Soil bulks up once it's dug, so it fills more of a skip than the hole it came from.

A worked example

A patio 4 m by 3 m, with slabs 60 cm by 60 cm and 10 mm joints, on 10 cm of sub-base and a 4 cm mortar bed.

1. Area 1: 4 m × 3 m = 12 m²
2. Slabs: 12 m² ÷ 0.37 m² (one 60 cm × 60 cm slab with its 10 mm joint) = 32.2
3. 32.2 + 10% for waste → 36 slabs
4. Sub-base: 12 m² × 10 cm = 1.2 m³
5. Sub-base: 1.2 m³ × 2,100 kg/m³ + 10% for waste = 2,772 kg → 4 bulk bags of 850 kg
6. Bed: 12 m² × 4 cm = 0.48 m³
7. Sharp sand, about the bed's volume: 0.48 m³ × 1,600 kg/m³ + 10% for waste = 844.8 kg → 1 bulk bag of 850 kg
8. Cement, a quarter of that for a 4:1 mortar: 0.12 m³ × 1,440 kg/m³ + 10% for waste = 190.08 kg → 8 bags of 25 kg
9. Joints: 12 m² × (60 cm + 60 cm) ÷ (60 cm × 60 cm) × 10 mm × 3.5 cm = 0.01 m³
10. Jointing: 0.01 m³ × 1,600 kg/m³ + 10% for waste = 24.64 kg → 2 packs of 20 kg
11. Digging out: 12 m² × 17.5 cm (sub-base, bed and slab) = 2.1 m³, before it bulks up in a skip

Common mistakes

• Skimping on the sub-base. It's what stops slabs sinking and rocking. Lay it in layers and compact each one.
• Forgetting the fall. A patio needs a slight slope away from the house so rain runs off. Set it out before you dig.
• Underestimating the digging. Sub-base, bed and slab together can be deeper than you expect, and the spoil takes up more room once it's out.
• Using this for a drive. Cars and vans need a sub-base designed for the load and the ground. Ask a groundworker or an engineer.

Questions people ask