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Material notes
Calculation basis
What this page assumes before you type anything, and how the answer is rounded. Every value below is this calculator's own default: change the box and the answer changes with it.
- Slab size
- 600 × 600 mmthis page's own default
- Joint width
- 10 mm joint
- MOT Type 1 sub-base
- 100 mm MOT Type 1 sub-basecompacted, the default
- Sharp sand bedding
- 40 mm sharp sand bedding
- Waste allowance
- 10%
- Rounding
- Up to the next whole unitslabs round up; tonnages are shown to 2 dp
- Last reviewed
- 25 September 2026checked against this page's own defaults
Confirm quantities against your actual product and site conditions before ordering — standard sizes vary between suppliers, and the ground the material lands on is yours, not ours. Figures are planning estimates, not a quote.
Don't forget — check before you order
On this patio the calculator returns these as estimates or optional extras rather than a fixed quantity, so they are listed here instead of being counted into the shopping list. Each one is the calculator's own wording.
- Jointing sand or paving grout — Check the manufacturer's coverage for your joint width.
Delivery access and what to do with the surplus are the two things that cause a second trip. Neither has a quantity here, because neither is a material.
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.
Paving FAQs
How many slabs do I need for a 6 m × 4 m patio?
With 600 × 600 mm slabs and 10 mm joints: 77 slabs. That is 10 across by 7 down (70 slabs) plus 10% for cuts and breakages. Order the extra — a breakage mid-lay with the merchant shut is a miserable afternoon.
How much MOT Type 1 do I need?
Multiply the area by the compacted depth. A 24 m² patio at 100 mm is 2.4 m³, which at 1.8 t/m³ is 4.32 tonnes — about six 850 kg bulk bags. Order it a little generously; spreading sub-base is much easier than discovering you are 200 kg short.
How deep should the sub-base be for a driveway?
150 mm compacted is the minimum for a domestic car, laid in two layers of 75 mm and compacted between each. If a heavier vehicle will use it, or the ground beneath is soft clay, go deeper — 200 mm or more, with a geotextile membrane under the sub-base.
How much sharp sand do I need for bedding?
Same method: area × depth. A 24 m² patio at a 40 mm bed is 0.96 m³, which at 1.6 t/m³ is 1.54 tonnes — about 62 bags of 25 kg, or two bulk bags. Buying in bulk bags is far cheaper if you have somewhere to keep it dry.
Do I need a patio to slope?
Yes. A fall of roughly 1:60 to 1:80 away from the building keeps water moving and stops puddles. That is 12–20 mm of fall across a 1 m width. Set the fall in the sub-base and in the bedding layer, not by tilting individual slabs.
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.