Patio and paving slab calculator

How many paving slabs for a patio, with the Type 1 for the sub-base, the sharp sand and cement for the bed, and how deep to dig out.

Worked example

How the sum works

Slabs per m² come from the slab size plus the joint on two sides, so a 600 × 600 mm slab with a 10 mm joint covers 0.61 × 0.61 m. The calculator adds the extra you choose for cuts and breakages: 5 percent suits a simple rectangle, and 10 percent or more suits a patio with curves, steps or a lot of cutting round drains.

The sub-base is area × depth at about 2.1 tonnes per m³ for compacted Type 1, plus 10 percent for compaction, the same as the Type 1 calculator. The mortar bed is area × depth. Dry sand and cement shrink when they are mixed, so the calculator multiplies the bed by 1.33 for the dry materials, then splits it by the mix.

What makers advise

Marshalls' patio guide gives a compacted sub-base of 100 mm, a full mortar bed about 40 mm deep, and joints of 10 to 15 mm. Most makers say natural stone and porcelain need a full bed, not spot or dab bedding, and that porcelain needs a slurry primer on the back of each slab. Follow the maker's own guide for the slab you are laying: Marshalls: how to lay a patio.

Before you order

Fall the patio away from the house so it sheds water, and keep the finished level at least 150 mm below the damp-proof course, as building regulations guidance expects. Jointing compound or mortar for the joints isn't included: check the coverage on the tub or bag. Slab sizes are nominal, so check the actual size and thickness on the pallet, and order from one batch so the colour matches.