How the patio calculator works
The calculator works out the area, then each layer of a properly built patio: the sub-base, the mortar bed, the slabs and the joints.
Patio build-up (from the bottom)
- Excavation: to about 160–180mm below the finished level, with a fall away from the house.
- Membrane: woven geotextile on soft ground.
- Sub-base: 100mm of MOT Type 1 compacted in two layers (150mm for heavy use or clay).
- Bed: a full 30–40mm bed of 4:1 to 6:1 sharp sand and cement, damp but not wet.
- Slabs: primed (porcelain always; natural stone recommended), then tapped down to level.
- Joints: brush-in jointing compound or a 3:1 sand and cement pointing mortar after 24 hours.
Slabs per m²
| Slab size | Slabs per m² (10mm joint) |
|---|---|
| 450 × 450mm | 4.73 |
| 600 × 300mm | 5.29 |
| 600 × 600mm | 2.69 |
| 900 × 600mm | 1.80 |
| 800 × 800mm | 1.52 |
Worked example: a 4m × 3m patio in 600 × 600mm slabs
12m² plus 10% for cuts is 13.2m². With 10mm joints, each slab takes up 0.61m × 0.61m = 0.372m², so you need 36 slabs. Underneath, 100mm of MOT Type 1 is about 2.6 tonnes (4 bulk bags). A 40mm bed in a 6:1 mix needs about 950kg of sharp sand (2 bulk bags, with plenty to spare) and 7 bags of cement. Jointing 10mm gaps takes roughly 18kg of compound, or 2 tubs. The dig is about 165mm below the finished level, nearly 2m³ of soil once it's loose.
To plan the whole job, including the dig-out and skip, use the new patio planner, which opens each step with these measurements.
Setting out before you dig
- Dry-lay a row of slabs on the lawn first and adjust the patio size to whole slabs where you can. It saves cutting and looks better.
- Mixed-size (project) packs of sandstone come with a laying plan. Follow it, or the last few metres won't fit together.
- Set string lines for the finished level and fall: about 1:60–1:80 away from the house, with the top of the patio at least 150mm below the damp-proof course.
Porcelain or natural stone?
Porcelain is uniform, stain-resistant and low-maintenance, but it's harder to cut (you need a wet saw or diamond blade), needs a priming slurry on every slab, and suits narrower 3–5mm joints. Sandstone and other natural stone varies in thickness, so it's laid on a full bed and tapped down slab by slab. It weathers and needs occasional cleaning, but is more forgiving to lay. Concrete utility slabs are the cheapest option for paths and shed bases.
Common patio mistakes
- Spot (dot-and-dab) bedding. Five blobs of mortar under each slab leave hollows that crack and rock, and let water collect under the slab. Use a full, solid bed.
- A thin or uncompacted sub-base. This is the most common cause of sunken patios. See the sub-base depth guide for your ground.
- No fall, or a fall towards the house. Puddles stain the paving and can push water against the walls.
- Jointing in the wrong weather. Most jointing compounds need dry or damp conditions within a set temperature range. Read the tub before you open it.
For the sub-base alone, use the MOT Type 1 calculator. For a driveway, block paving is usually better: see the block paving calculator.