Patio calculator

Enter the size of the patio and the slabs you plan to buy. You get the slab count with joints, the sub-base in tonnes, the bedding materials, how deep to dig and how much fall to allow.

Plan view. Darker slabs are cut to fit at the edges.

How the numbers are worked out

Slabs: along each side, the number of slabs is (length + joint) ÷ (slab size + joint), rounded up. Multiply the two counts, then add your allowance for cuts and breakages. For a 4 m side with 600 mm slabs and 10 mm joints: (4000 + 10) ÷ 610 = 6.6, so 7 slabs, the last one cut.

Sub-base = area × compacted depth × density. A 12 m² patio at 100 mm is 1.2 m³, which is about 2.5 tonnes at 2.1 t/m³ before the allowance.

Bedding = area × bed depth, then 33% more to turn wet mortar into dry sand and cement, then your allowance. The mix is 4 parts sand to 1 part cement by volume, and cement is taken as 1,440 kg per cubic metre.

Dig depth = slab thickness + bedding + sub-base. Fall = the distance the water runs over, divided by 80 or 60.

Planning the build-up

From the bottom: firm, level ground; a membrane if the soil is soft; compacted MOT Type 1 in layers; the bedding; then the slab. Supplier guides disagree on depths. Wickes shows 100 mm of Type 1 laid in two 50 mm layers, and other suppliers give 100 mm as a minimum for foot traffic with 150 mm common. Driveways and soft or clay ground need more. The calculator lets you set the depth, so use the guide for your slabs and your ground.

Fall and drainage

A patio needs a slight slope so water runs off. Guides quote 1 in 80 and about 1 in 60, which is why both are offered above. Set it with a spirit level and a spacer block under the low end, and make the fall run away from the house.

Next to the house

Keep the finished surface at least 150 mm below the damp proof course, so splashing rain and rising damp do not cross it. If you cannot, ask building control or a builder before laying.

Front gardens

Hard surfacing of more than 5 m² in a front garden is permitted development only if it is permeable, or if the rainwater drains to a lawn or border within the property. Otherwise it needs planning permission. This is a GOV.UK rule for England. Use the sources below to check the wording.

Common mistakes

Ignoring the joints. Leaving joints out under-counts slabs on every side.
Too little sub-base. It is the part you cannot fix later. Thin or poorly compacted sub-base is the usual reason patios sink.
No fall. A flat patio holds water against the house.
Forgetting the dig-out. Soil to remove is often more than the materials you bring in. Check how you will get rid of it.

Questions

How many slabs do I need for a patio?

Divide each side by the slab size plus the joint, round up, and multiply the two numbers. Then add an allowance for cuts and breakages. The calculator does this and shows the layout.

How deep should a patio sub-base be?

Supplier guides commonly give 100 mm of compacted MOT Type 1 as a minimum for foot traffic, with 150 mm common, and more for vehicles or soft ground. Follow the guide for your slabs and ground.

How much fall does a patio need?

Guides quote 1 in 80, and some about 1 in 60. Make it run away from the house.

How much MOT Type 1 do I need?

Area times compacted depth gives the volume. Multiply by about 2.1 tonnes per cubic metre and add a little extra. Your supplier can confirm the density of their product.

Where the defaults come from

Show the 3 sources

Wickes, how to lay a patio. A UK retailer's guide. It gives 100 mm of MOT Type 1 in two 50 mm layers, 50 mm slabs, mortar of four parts sand to one of cement, a fall of 16 mm per metre, joint sizes, and a surface 150 mm below the damp proof course.
GOV.UK, permeable surfacing of front gardens. Hard surfacing of more than 5 square metres in a front garden is permitted development only if permeable.
UK paving suppliers' guides. Prime Thorpe Paving, Universal Paving and the Royal Gray natural stone laying guide give 100 mm minimum sub-base, 150 mm often used, bedding of about 30 to 50 mm, and a fall of 1 in 80 as the stricter figure.

Defaults we could not confirm from a primary source

The 2.1 tonnes per cubic metre density is typical for compacted Type 1. The figures we found ranged from about 2.0 to 2.2.
The 40 mm bedding depth is a middle value. Guides range from 25 mm (a bedding product) to 60 mm.
The 33% dry-volume factor, the 1,440 kg per cubic metre for cement, and the 10% allowances are our own working values.
The 50 mm slab thickness is typical for concrete slabs, not a standard. Natural stone and porcelain are usually thinner.
Joints are not counted in the mortar or sand figures for pointing. Buy pointing compound separately.