How the paving slab calculation works
For the standard coverage method, the tool divides the total paved area by the face area of one slab. A 600 × 600 mm slab has a nominal face area of 0.36 m². A 20 m² patio therefore has a theoretical requirement of 55.56 slabs, which becomes a whole-unit minimum of 56 before any extra allowance.
What joint width can — and cannot — tell you
On a simple aligned rectangular grid, joints occupy part of the finished surface. Grid mode therefore counts how many slab-plus-joint pitches fit across the measured length and width. It is deliberately unavailable for circles, triangles, additional zones and mixed patterns because that apparent precision would be misleading.
For general ordering, especially where edge cutting is unavoidable, the face-area method plus an explicit wastage allowance is the clearer estimate.
Mixed-size project packs need the supplier's laying pattern
Mixed-size paving packs combine several slab dimensions into a planned pattern. Current UK suppliers publish pack coverage and pattern diagrams for these products. Treating every piece as an average slab can produce an unrealistic count, so use the pack's stated square-metre coverage and laying plan rather than this single-size tool.
Common paving quantity mistakes
- Using nominal pack area as if every piece were the same size. Mixed packs need their actual laying pattern.
- Hiding a fixed 10% inside the result. This tool leaves allowance visible and editable; 0% remains available.
- Ignoring awkward cuts. Curves, diagonal layouts, drains, steps and obstacles can change the useful yield from a pack.
- Measuring the excavation rather than the finished paved area. Use the surface you intend the slabs to cover.
Wickes currently adds 10% automatically in its paving calculator, while Bradstone recommends purchasing up to 10% extra and suppliers may recommend more for certain products or complicated cutting. Gardeners Toolkit keeps this field editable instead of treating one percentage as universal.