United Kingdom · Materials & DIY
Patio Calculator
Enter the patio area and your slab size. The calculator returns the number of slabs, and the guidance below covers the bedding and sub-base that make up most of the actual work.
How many patio slabs do I need?
Divide the patio area by the area of one slab, then add 5 to 10% for cuts. A 600 by 600 mm slab covers 0.36 square metres, so you need 2.78 slabs per square metre. A 20 square metre patio therefore needs about 56 slabs, or 60 once waste is allowed for.
How to use this calculator
- Measure the finished patioLength by width. For an irregular shape, break it into rectangles and add them, or measure the area directly and enter it.
- Set slabs per square metreOne divided by the slab area in square metres. The default is a 600 by 600 mm slab; the hint lists the common alternatives.
- Match the waste to the shapeA simple rectangle sized to whole slabs wastes very little. A circular or curved patio, or one that has to fit around a bay window, can waste 15% or more.
- Remember what is underneathThe slabs are the visible part and rarely the biggest quantity. Sub-base and bedding mortar are calculated separately.
How the calculation works
A 600 by 600 mm slab has a face area of 0.36 square metres, so one square metre needs 1 ÷ 0.36, which is 2.78 slabs. Multiply by the patio area and round up.
Joint width is usually ignored in slab calculations and that is reasonable. A 10 mm joint on a 600 mm slab adds under two per cent to the effective coverage — well inside the waste allowance. Where the joints are much wider, as with some riven or setts-style ranges, it is worth adding the joint to the slab dimension before working out the coverage.
What genuinely moves the number is the shape. Designing the patio to a whole number of slabs in each direction — even by adjusting the size by a few centimetres — can take the waste from 15% to almost nothing. It is the single cheapest decision available on a patio, and it happens at the drawing stage rather than the ordering stage.
slabs per m² = 1 ÷ (slab length × slab width)Slabs per square metre by size
| Slab size (mm) | Coverage (m²) | Slabs per m² |
|---|---|---|
| 450 × 450 | 0.2025 | 4.94 |
| 600 × 600 | 0.36 | 2.78 |
| 600 × 450 | 0.27 | 3.70 |
| 600 × 750 | 0.45 | 2.22 |
| 600 × 900 | 0.54 | 1.85 |
| 900 × 600 mixed packs | Varies | Use the pack coverage |
Patio ranges are increasingly sold as mixed-size project packs designed to lay in a repeating random pattern. For those, ignore the per-slab arithmetic and use the square metre coverage printed on the pack — the individual sizes are not the useful figure and the pattern relies on the mix.
Worked example: what the patio actually costs in material
A patio 5 m by 4 m is 20 m². In 600 by 600 mm slabs at 2.78 per square metre that is 56 slabs, and with an 8% allowance, 61.
Underneath, at 100 mm of compacted Type 1 over an area extended 100 mm each side, that is about 2.6 cubic metres loose — roughly 5.2 tonnes. The bedding mortar at a full 40 mm bed is 0.8 cubic metres, which at a 1:4 mix is around 1.3 tonnes of sharp sand and fourteen bags of cement.
So 61 slabs, five tonnes of sub-base, well over a tonne of sand and fourteen bags of cement. The slabs are the part everyone budgets for and frequently not the largest line on the order.
Bedding: full mortar bed, not five spots
The most damaging piece of received wisdom in domestic paving is bedding slabs on five dabs of mortar. It is quick, it uses less material, and it produces a patio that rocks, cracks and lets water sit under the middle of every slab.
A slab should be laid on a full mortar bed — a continuous layer, typically 30 to 50 mm, that supports the entire underside. Anything less leaves unsupported areas that crack under a point load, and voids where water collects and freezes.
Porcelain and some natural stone need a slurry primer on the back of the slab as well, because the mortar will not bond to a dense, low-absorption backing on its own. Skipping the primer is why porcelain patios lift, and it cannot be corrected afterwards.
Falls matter too. A patio should shed water away from the building at roughly 1 in 80, and the finished surface must sit at least 150 mm below any damp proof course. Getting either wrong causes problems that are considerably more expensive than the paving.
Natural stone often arrives with variable thickness — a 22 mm nominal sandstone can range from 18 to 30 mm across a pack. Bed depth has to absorb that variation, which is another reason a full mortar bed rather than a fixed screeded layer is the correct approach.
Assumptions and limitations
- Calculates slabs only — sub-base, bedding mortar, primer and jointing compound are separate
- Assumes a single slab size; mixed project packs should use the manufacturer's coverage
- Joint width is not deducted, which is immaterial for standard joints and matters for wide-jointed ranges
- Waste allowance assumes reasonably efficient cutting on a rectangular layout
- Does not account for falls, steps or level changes
- Does not check the finished level against the damp proof course, which is a Building Regulations matter
Questions people ask about this in United Kingdom
How many 600 x 600 slabs per square metre?
2.78. Each slab covers 0.36 square metres, so one divided by 0.36 gives 2.78 slabs per square metre. A 20 square metre patio therefore needs about 56 slabs before waste, and around 60 with a normal cutting allowance.
How thick should the mortar bed be under patio slabs?
Typically 30 to 50 mm of full-bed mortar, continuous under the whole slab rather than on dabs. Natural stone with variable thickness needs the bed depth to absorb that variation, so a workable full bed matters more than a precisely screeded one.
Can I lay patio slabs on sand?
Not for a patio. A sand bed works for block paving because the blocks interlock and share load, but a slab on sand is unsupported at the edges and will rock and crack. Slabs need a full mortar bed on a compacted sub-base.
What fall should a patio have?
About 1 in 80 away from the building — roughly 12 mm of fall for every metre. Any less and water sits on the surface; any more and furniture becomes noticeably unlevel. The finished surface must also be at least 150 mm below the damp proof course.
Do porcelain slabs need a primer?
Yes. Porcelain is dense and absorbs almost no water, so ordinary mortar will not form a bond with the back of the slab. A slurry primer applied to the underside immediately before bedding is essential, and porcelain patios that lift are almost always ones where it was skipped.
How much waste should I allow on patio slabs?
5% for a simple rectangle sized to whole slabs, 8 to 10% for a normal patio with some cuts, and 15% or more for curves, circles or a lot of edge detail. Designing the patio to whole slab dimensions is the cheapest way to cut waste and happens before you order.
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Method, source and review date
- How it is worked out
- Area divided by slab face area, with a cutting allowance
- Formula used
slabs = area (m²) ÷ (slab length × slab width in metres) × (1 + waste)- Basis
- Standard coverage rates and spacings, editable on the page
- Last reviewed
- Confidence
- High
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
Quantities are an estimate for planning and ordering. Site conditions, wastage, cuts and delivery minimums all change what you actually need — always confirm with your supplier or the person laying the material before ordering.