Worked examples: four patios, same slab
Every patio below is 600 × 600 mm slabs with 10 mm joints, on 100 mm of MOT Type 1 over a 40 mm bedding course, with the calculator's 10% waste allowance already applied.
| Patio | Area | Full slabs | Slabs to buy | MOT Type 1 | Bedding sand | Edge |
|---|---|---|---|---|---|---|
| 5 m × 2 m | 10 m² | 36 | 40 | 1.80 t | 0.64 t | 14 m |
| 5 m × 4 m | 20 m² | 63 | 70 | 3.60 t | 1.28 t | 18 m |
| 6 m × 5 m | 30 m² | 90 | 99 | 5.40 t | 1.92 t | 22 m |
| 8 m × 5 m | 40 m² | 126 | 139 | 7.20 t | 2.56 t | 26 m |
The full slab count is the grid of whole slabs; the figure to buy adds the 10%. The sub-base and sand are the volumes converted at 1.8 tonnes per cubic metre.
Why the joints change the count
The slab and the joint together make a module, and the calculator counts how many modules fit across and down the patio. A 5 m run in 600 mm slabs with 10 mm joints takes 9 slabs across, and a 2 m depth takes 4 down — 36 slabs for a 10 m² patio. That is why the count never lands on a tidy number: the last slab in each direction is a cut, and the grid is rounded up.
Why the waste allowance matters most on small jobs
The 10 m² patio needs 40 slabs for 36 slab positions; the 40 m² patio needs 139 for 126. Small patios, patios with many cut edges and patios laid on a diagonal all waste proportionally more, because the offcut from a cut slab is rarely usable. If your layout cuts heavily, raise the waste allowance rather than relying on the default.
The layers under the slabs
Two depths drive the materials under the paving. The sub-base is 100 mm compacted MOT Type 1, and the bedding is a 40 mm course of sharp sand. On the 20 m² patio that is 3.60 tonnes of sub-base and 1.28 tonnes of sand; on the 40 m² patio it is 7.20 tonnes and 2.56 tonnes. The edge restraint runs with the perimeter — 18 m on the 20 m² patio and 26 m on the 40 m².
How the calculator works
- Enter the patio length and width; the area follows.
- Choose the slab size and the joint width — the two numbers that set the module.
- The calculator counts the grid across and down, then rounds up to whole slabs.
- The waste allowance is added to that count, giving the number to order.
- The sub-base, bedding sand and edge restraint come from the same area and depths.
Where to go next
Paving slab FAQs
How many paving slabs do I need for a 20 m² patio?
70, in 600 × 600 mm slabs with 10 mm joints. That is 63 slabs in the grid — 9 across and 7 down — plus the 10% allowance for the cuts.
Why is my slab count higher than the area divided by the slab size?
Because the slabs are counted as a grid of whole slabs, not as a fraction of the area. A 10 m² patio takes 36 slab positions, and the 10% waste allowance takes the order to 40.
How much sub-base does a patio need?
At 100 mm compacted, 1.80 tonnes for a 10 m² patio and 3.60 tonnes for 20 m². The sub-base guide covers the depth each job wants and how the tonnage becomes bulk bags.
What waste allowance should I use for paving?
10% is the calculator's default and suits a plain rectangular patio. A diagonal, circular or heavily cut layout wastes far more slabs, so allow a higher percentage.
Does the joint width really change the slab count?
Yes. The joint is added to the slab to make the module the calculator counts, so wider joints put fewer slabs in the same run — and the difference grows with the size of the patio.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and your supplier's current product data before ordering.