Paving Slab Calculator — Slabs, Sand & Cement

As an area-based estimate (with 10mm joints): 600x600mm slabs = approximately 2.8 per m², 450x450mm = approximately 4.7 per m², 400x400mm = approximately 5.9 per m², and 600x300mm = approximately 5.4 per m². These figures divide 1 m² by the slab-plus-joint footprint. The calculator counts physical slabs slightly differently — it works out how many full slabs fit across the length and down the width and rounds each up, because a partial edge row still needs a whole slab to cut from. So a 5m × 3m area in 600x600 returns 45 slabs (effectively 3.0/m²) rather than 42, before wastage. That edge rounding is deliberate and matches how you actually order. For indoor floor tiles, see our Floor Tile Calculator.

For 600x600mm and larger flags, wet-cast concrete slabs and all natural stone (Indian sandstone, limestone, granite), lay onto a full sand:cement mortar bed — not loose sand and not dot-and-dab blobs. This is the bedding method required by BS 7533-102:2025 and recommended by Pavingexpert: a solid, fully-supported bound bed resists frost heave, ground-water washout and rocking. A typical bed is 4:1 sharp (grit) sand to cement, 30-50mm thick on a compacted sub-base; use 3:1 for vehicular loading. Unbound loose sharp sand is only acceptable for small (450x450mm or smaller) calibrated pressed-concrete flags. Natural stone and porcelain should also be primed (SBR/cement slurry on the underside) for a full bond. Never use building (soft) sand — it goes fluid when wet and the slabs settle.

A 10mm joint (also called a gap or spacing) is standard for most paving slabs in the UK. This allows room for pointing with mortar (4:1 sand:cement mix) or kiln-dried sand. Wider joints of 15mm may be used with natural stone. Too narrow and pointing becomes difficult; too wide and it looks uneven.

Mortar pointing (4:1 sharp sand to cement) is the most durable option for patios and high-traffic areas. Brush-in kiln-dried sand is quicker but less durable and can wash out. Resin-based jointing compound is a modern alternative that sets hard but remains slightly flexible. Ensure your patio has the correct drainage fall — use our Drainage Fall Calculator.

Yes — always lay paving on a proper sub-base. Use 100mm of compacted Type 1 MOT sub-base for pedestrian areas, or 150mm for driveways — our Type 1 Sub-Base Calculator works out quantities. The sub-base goes down first, then a 30-50mm sand:cement mortar bed, then the slabs. For a gravel surface instead, see our Gravel Calculator.