Paving Slab Calculator

Calculate how many patio paving slabs you need, plus sub-base and bedding, for standard UK slab sizes.

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How to Use This Calculator

What This Calculator Does

This paving slab calculator works out how many patio slabs you need for a given area, plus the MOT Type 1 sub-base and bedding underneath. It's for anyone laying a patio, path or terrace who wants an accurate slab count — including standard UK slab sizes — before ordering.

How to Use

  1. Enter the length and width of the area you're paving.
  2. Pick a standard slab size, or choose “Custom” to enter your own.
  3. Set the joint (pointing) width and a waste percentage — 5-10% covers cuts and breakages.
  4. Read off the number of slabs plus the sub-base and bedding you'll need.

Worked Example

For a patio 5 m × 4 m using 600 × 600 mm slabs with a 10 mm joint:

  1. Area = 5 × 4 = 20 m².
  2. Each slab plus joint covers about 0.372 m², so you need about 54 slabs.
  3. Adding 5% waste gives roughly 57 slabs to order.

Tips

  • Common UK patio sizes are 600×600, 450×450 and 600×300 mm.
  • Lay slabs on a full mortar bed over a compacted MOT Type 1 sub-base — never on dabs alone for a patio.
  • Lay a slight fall (about 1:80) away from the house so water drains off.
  • Always order a few spare slabs from the same batch in case of breakages or future repairs.

Common Questions

How many paving slabs do I need per square metre?

For 600 × 600 mm slabs you need about 2.8 per m², for 450 × 450 mm about 4.9 per m², and for 600 × 300 mm about 5.6 per m². This calculator works it out exactly from your area, chosen slab size and joint width.

How much waste should I allow for a patio?

Allow around 5% for a simple square or rectangular patio and up to 10% for patios with lots of cuts, curves or diagonal laying patterns.

What goes under paving slabs?

A compacted MOT Type 1 sub-base (typically 100 mm) topped with a bedding layer of mortar or sharp sand (around 30-50 mm). This calculator estimates both for you.

Should paving slabs be laid on sand or mortar?

For a patio, a full wet mortar bed is best as it gives firm, even support. Sand or a screeded bed is usually only suitable for lighter-duty paths and pedestrian areas.

How much fall should a patio have for drainage?

Aim for a gentle fall of about 1:80 (roughly 12 mm per metre) directed away from the house so rainwater runs off rather than pooling.

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