Paving calculator

A patio is more than the slabs on top: there's the sub-base underneath and the bedding in between, and getting any of them short holds the whole job up. This calculator works out all three from the area and your slab size, with wastage built in. Here's how it's worked out.

The layers of a patio

A solid patio is built up in three layers. Get the depths right and it stays flat and drains; skimp on them and it sinks or heaves.

• Sub-base: compacted MOT Type 1 crushed stone, usually 100mm for a patio, spreading the load and draining water away.
• Bedding: a mortar layer, around 40mm of sharp sand mixed with cement, that the slabs are laid onto and levelled.
• Slabs: the paving on top, butted or with a joint gap, then pointed.

How many slabs?

Slabs come down to the patio area divided by the area of one slab, plus wastage.

Slabs = area × (1 + wastage) ÷ slab area.

A 600mm by 600mm slab covers 0.36 square metres, so a 5m by 3m patio (15 square metres) needs about 42 slabs bare, or 46 with 10% for cuts and breakages. The calculator works the area out from your patio size and the slab area from the slab size.

Sub-base: MOT Type 1

The sub-base is the load-spreading layer, and it's MOT Type 1 that does the job: graded crushed stone that compacts to a firm, free-draining base. Work out the volume as the area times the depth in metres, so 15 square metres at 100mm is 1.5 cubic metres. MOT Type 1 weighs roughly 2 tonnes per cubic metre, so that's about 3 tonnes, ordered loose or in bulk bags. A driveway or anything taking a car needs a deeper sub-base, often 150mm or more.

Bedding: sharp sand and cement

Slabs are laid on a mortar bed, typically around 40mm of sharp sand mixed with cement. The volume is the area times the bedding depth, so 40mm over 15 square metres is 0.6 cubic metres of sand. Buy the sharp sand by volume or weight, at roughly 1.6 tonnes per cubic metre, and add cement to your usual mix ratio. A full mortar bed under each slab is what stops them rocking later.

How much wastage to allow

Ten per cent covers the edge cuts and the occasional slab that cracks. Add more for a patio with curves, lots of edges, or a laying pattern, since those create more offcuts you can't reuse. Running short is the outcome to avoid, because paving batches sell through and a later batch may not match on shade.

Don't forget the finishing bits

Beyond the three main layers, a patio needs jointing compound or a mortar mix for pointing the gaps, and often an edge restraint or haunching to hold the perimeter. A sub-base needs compacting with a wacker plate, and the whole thing needs a slight fall for drainage, usually about 1 in 80 away from the house. Price those in so the quote is complete.

From materials to a priced job

Quantities are the start; a patio also has a lot of labour in the digging, the base and the laying. Price the whole job with how much to charge to lay a patio, drop the materials into a clean, itemised quote with the quote generator, and if you're setting posts or edgings in concrete, the concrete calculator handles that.

This tool is for general estimating and gives approximate quantities. Slab sizes, build-ups and aggregate weights vary, so check with your supplier and your own measurements, and order a sensible amount over rather than exactly to the calculation.