Paving Slab Calculator — Slabs, Sand & Cement

Our paving slab calculator works out how many full slabs fit across the length and down the width of your area (allowing for the joint gap), rounds each direction up, then multiplies, so partial edge rows are counted as whole slabs to cut from. If the area is an L or T shape, add each rectangle as its own section: every section is counted as its own grid of whole slabs and the totals are added together (a cut cannot span the join), while the mortar bed uses the combined area.

The formula

Slabs across = round up [ Section length ÷ (Slab width + Joint width) ]

Slabs down = round up [ Section width ÷ (Slab height + Joint width) ]

Total slabs = round up [ (Slabs across × Slabs down, added over sections) × Wastage factor ]

Sand and cement for the mortar bed

Larger flags (600×600mm and up), wet-cast concrete and natural stone should be laid on a full sand:cement mortar bed per BS 7533-102:2025 — not loose sand. We split the bed volume by mix ratio (default 4:1 sharp sand to cement by volume) and convert each part to weight:

• Bed volume: Area × bed depth (50mm standard)
• Sharp sand: Bed volume × 4⁄5 × 1.6 t/m³ (sand fraction of a 4:1 mix)
• Cement: Bed volume × 1⁄5 × 1440 kg/m³ ÷ 25kg per bag (cement fraction)
• Joint width: 10mm standard (filled separately by pointing or kiln-dried sand — see FAQs)

For a 50mm 4:1 bed that is ≈ 64 kg sand + 14.4 kg cement per m². Use a 3:1 mix for vehicular loading. Loose sharp sand only suits small (≤450×450mm) calibrated pressed concrete flags.

Wastage

We recommend 10% wastage for straightforward layouts. An L or T shape entered as sections is already counted per section, so 10% still suits it. For areas with lots of cuts (curves, angled edges), increase to 15%. Paving slabs can break during cutting and handling, so always order extra.