How many paving slabs do I need?

How the paving calculator works

Slab counts are worked out on the module — the slab plus its joint. If you are laying 600 × 600 mm slabs with 10 mm joints, each slab effectively occupies 610 mm. The calculator divides the length by that module to get the number across, does the same for the width, then multiplies the two together. A 6 m × 4 m patio comes out as 10 slabs across by 7 down, which is 70 slabs, plus 10% for cuts and breakages — 77 slabs.

Joints matter more than people think. Widening the joint from 10 mm to 15 mm on a large patio can push you into an extra slab per row. And the calculator does include the joints, which is why the answer can be one slab more than a simple area calculation suggests.

Sub-base: the layer people forget

MOT Type 1 goes down first, and it is the reason a patio stays flat for twenty years rather than two. For a footpath or a patio that only sees people and garden furniture, 100 mm compacted is fine. Anything a car will drive or park on needs 150 mm, and a driveway carrying a van at the front edge of the drive is better at 150–200 mm in two compacted layers of 75–100 mm each.

The calculator converts your sub-base depth into tonnes at 1.8 tonnes per cubic metre — a typical compacted figure for MOT Type 1 — and shows both tonnes and 850 kg bulk bags. Bulk bags are the practical unit for most domestic jobs: a 6 m × 4 m patio at 100 mm is 4.32 tonnes, or six bulk bags. The sub-base guide takes the same job through a patio, a driveway and a shed base.

Bedding and pointing

Sharp sand, not building sand, is used for the bedding layer: it compacts evenly and does not shrink away as much. A 40 mm bed is standard, laid slightly proud and screeded level. Sharp sand also runs at about 1.6 tonnes per cubic metre — noticeably heavier per cubic metre than the sub-base, which catches people out.

For pointing, a brush-in polymeric jointing compound is the quickest option and is calculated by area rather than by joint length; traditional mortar pointing is cheaper in materials but slower and harder to get right. Either way, build a fall of about 1:60 to 1:80 away from the house so water never sits on the surface.

Worked example: a 5 m × 4 m patio

A 5 m × 4 m patio — 20 m² — in 600 × 600 mm slabs with 10 mm joints, laid on 100 mm of MOT Type 1 over a 40 mm bedding course, with 10% waste.

How we got there

1. 9 slabs across × 7 down = 63 full slabs — 600 × 600 mm slabs with 10 mm joints cover a 20 m² patio as 9 across and 7 down, which is 63 slabs.
2. 63 + 10% waste = 70 slabs — The waste allowance covers cuts at the edges and breakages, taking the order to 70 slabs.
3. MOT Type 1 sub-base: 3.60 tonnes — At 100 mm deep over 20 m², that is 3.60 tonnes — about 5 bulk bags.
4. Bedding sand: 1.28 tonnes — 40 mm of sharp sand for bedding comes to 1.28 tonnes, or 2 bulk bags.
5. Edge restraint: 18 m — The perimeter is 18 m, and it needs a restraint and a concrete haunch.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Paving slabs 600 × 600 mm — 70 slabs (9 × 7 = 63 plus 10% for cuts.)
• MOT Type 1 sub-base — 3.60 tonnes (≈ 5 × 850 kg bulk bags at 100 mm compacted.)
• Sharp sand (bedding) — 1.28 tonnes (≈ 52 × 25 kg bags, or 2 bulk bags, at 40 mm.)
• Jointing sand or paving grout — estimate only (Check the manufacturer's coverage for your joint width.)
• Edge restraint / kerb and concrete haunch — 18 m (Rule of thumb: the open perimeter, 18.0 m here. An edge that is not restrained lets the paving spread — a kerb on a concrete haunch, or a proprietary edge, is what holds it. Only the edges that meet soil or a drop need it.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

The calculator adds 10% to the slab count for cuts and breakages. Raise it if you are laying a diagonal or circular pattern, or working around many edges — those jobs cut far more slabs than a plain rectangle.

Regs & planning for this calculator

A patio is a hard surface, a drainage decision and a level decision all at once.

• Planning — hard surfaces and driveways The 5 m² permeable-surface condition where a hard surface sits between the front wall and a highway. Read the requirement.
• Approved Document H — surface water Surface water should discharge to a soakaway or other infiltration system where practicable, with the separation rules that go with it. Read the requirement.
• Approved Document C — ground level External levels against a wall, and the 150 mm damp-proof course it has to stay below. Read the requirement.
• BS EN 752 — drainage design When the design goes beyond the approved document and the standard takes over. Read the requirement.