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Project guide

How many paving slabs do I need, and how much sub-base?

To work out how many paving slabs you need, divide the patio area by the area of one slab and add 10% for cuts. The sub-base, mortar bed and jointing follow from the same area.

Updated Based on manufacturer guidance, sources below

The short answer

Slabs = patio area / slab area, plus 10%. A 600 x 600mm slab is 0.36 m². Under the slabs, Marshalls specifies at least 100mm of compacted MOT Type 1 and a full mortar bed of 4 parts sharp sand to 1 part cement. A 4 x 3m patio needs 37 slabs, about 2.8 tonnes of MOT Type 1 (4 bulk bags), 8 bags of cement and 1 bulk bag of sharp sand for a 40mm bed.

The formula

  • Slabs = area / slab area (0.36 m² for 600 x 600mm) x 1.1, rounded up
  • MOT Type 1 (tonnes) = area x 0.1 x 2.1 x 1.1
  • Mortar bed (m³) = area x 0.04 x 1.1
  • Cement (25kg bags) = mortar m³ x 14 for a 4:1 mix; sharp sand = mortar m³ x 1.5 tonnes

Worked example

A 4 x 3m patio (12 m²) in 600 x 600mm slabs, on 100mm of sub-base and a 40mm bed:

  • Slabs: 12 / 0.36 = 33.3, plus 10% = 36.7, so 37 slabs
  • Sub-base: 12 x 0.1 x 2.1 x 1.1 = 2.8 tonnes of MOT Type 1, or 4 bulk bags of 800kg
  • Mortar: 12 x 0.04 x 1.1 = 0.53 m³
  • Cement: 0.53 x 14 = 7.4, so 8 bags. Sharp sand: 0.79 tonnes, so 1 bulk bag

Work it out for your job

Change the measurements and the quantities update straight away.

Patio area
12 m²
MOT Type 1 (100mm)
about 2.8 tonnes
Bedding mortar (40mm)
0.53 m³
Jointing compound
26.2 kg

Your list

  • 37Paving slabs 600 x 600mm (+10%)
  • 1Weed-control fabric, 2 x 25m rolls
  • 4MOT Type 1 sub-base, 800kg bulk bags
  • 1Sharp sand, 800kg bulk bags
  • 8Cement 25kg bags (1:4 bed)
  • 3Paving jointing compound, 12.5kg tubs
Price this patio

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Materials checklist

  • Paving slabsSizes are nominal; check the pack for the actual size and the m² each pack covers.
  • MOT Type 1 sub-baseCrushed stone, 40mm to dust. At least 100mm compacted, laid in two 50mm layers.
  • Weed-control fabricUnder the sub-base to separate it from the soil.
  • Sharp sandFor the mortar bed, mixed 4 parts sand to 1 part cement.
  • Cement25kg bags of general purpose cement.
  • Jointing compoundBrush-in compound, or the same 4:1 mortar, to fill the joints.
  • Paving primerNeeded for porcelain and recommended for natural stone, painted on the back of each slab.

You'll also need

  • Plate compactor (hire)
  • Spirit level, string line and pegs
  • Rubber mallet
  • Cement mixer or mixing tub
  • Angle grinder with a stone disc for cuts, plus eye, ear and dust protection

Step by step

  1. Mark out and digMark the patio and dig out about 150mm plus the slab thickness, removing all topsoil and roots.
  2. Set the fallPlan a fall of about 1 in 60 away from the house so water runs off.
  3. Lay the sub-baseSpread MOT Type 1 in two 50mm layers, compacting each with a plate compactor.
  4. Bed the slabsLay a full mortar bed under each slab, never dabs, and tap the slab down with a mallet, leaving 8 to 15mm joints.
  5. Fill the jointsOnce set, fill the joints with jointing compound or a 4:1 mix.

Tips from the manufacturers

  • Keep the finished patio at least 150mm below the damp-proof course of the house (Marshalls).
  • Lay a full bed of mortar under every slab. Dabs leave hollows that crack under load.
  • Dry-lay a few rows first to check the joint widths and the blend of colours across packs.
  • Marshalls gives a bed of about 30mm for sandstone and 40mm in its general patio guide; this calculator uses 40mm so you do not run short.

Mistakes to avoid

  • Forgetting cuts round edges and corners. Always add 10%.
  • Skipping the sub-base or not compacting it, so slabs sink.
  • Spot-bedding slabs on five dabs of mortar.
  • Laying porcelain without a primer, so it does not bond.

Frequently asked questions

How many paving slabs for 10 m²?

In 600 x 600mm slabs, 10 / 0.36 = 27.8, plus 10% for cuts is 31 slabs.

How deep should a patio sub-base be?

At least 100mm of compacted MOT Type 1 for a patio used on foot, laid in two 50mm layers. Soft or clay ground may need more.

What mix for laying patio slabs?

A full mortar bed of 4 parts sharp sand to 1 part cement, about 30 to 40mm thick, is what Marshalls and Screwfix both give for a domestic patio.

How much MOT Type 1 do I need for a patio?

Area x depth x about 2.1 tonnes per m³. A 12 m² patio at 100mm needs about 2.5 tonnes, or 2.8 tonnes with 10% extra.

How many bags of cement for a patio?

For a 40mm 4:1 bed, about 0.6 bags per m². A 4 x 3m patio needs about 8 bags.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

How much sand and cement do I need for bricks, blocks or slabs?Work out the volume of mortar first, then split it into cement and sand by the mix. Here are the standard UK rates for brickwork, blockwork and laying paving slabs. How much gravel do I need? Tonnes and bagsTo work out how much gravel you need, multiply the area by the depth to get cubic metres, then by the weight of a cubic metre of your gravel to get tonnes. How much concrete for a shed base?Make the base a little bigger than the shed, work out the volume, and convert it to bags of cement and ballast for a 1:5 mix.

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