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Patio Paving Calculator — Slabs, Sand & Cement
Calculate paving slabs, sand, cement and sub-base quantities for your patio project.
Longest dimension of the area
Shortest dimension of the area
Patio not a simple rectangle? An L-shaped patio is just two rectangles: measure each one edge to edge and add it as a section. Use three sections for a T or U shape. The calculator adds every section together.
Gap between slabs
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How We Calculate This
This calculator works out the number of paving slabs needed based on your patio dimensions, chosen slab size, joint width and laying pattern. It also estimates sand, cement and sub-base quantities. If your patio is an L or T shape, add each rectangle as its own section: slabs are rounded up within each section (a cut cannot span the join), while sand, cement and sub-base use the combined area.
The calculation
- Effective slab area = (slab width + joint) × (slab height + joint)
- Base slabs = section area ÷ effective slab area, rounded up per section and added together
- A typical trade cutting allowance is added by pattern (herringbone +10%, stretcher bond +5%, stack bond +3%), then your wastage percentage on top
- Bedding mortar is a 4:1 sharp sand : cement mix by volume, scaled to your chosen bed depth (default 40mm)
- Sharp sand = bed depth × 1,280 kg/m³ (the 4/5 sand fraction at 1,600 kg/m³) — 51 kg/m² at 40mm
- Cement = bed depth × 288 kg/m³ (the 1/5 cement fraction at 1,440 kg/m³) — about 0.5 of a 25kg bag per m² at 40mm
- Edge restraint = the full perimeter of every section. On L or T shapes that counts both sides of the line where sections join, so it is an upper bound: enter your measured run of open edge in the advanced options to trim it
Modular paving is laid on a full wet mortar bed per the installation code BS 7533-102:2025 (structural design is covered separately by BS 7533-101:2021). The per-square-metre sand and cement figures are mix-derived estimates, not values prescribed by the standard; wastage allowances are typical trade rules of thumb, not regulated figures.
Frequently Asked Questions
It depends on slab size. For 600x600mm slabs with 10mm joints you need approximately 2.7 slabs per m². For 450x450mm slabs you need about 4.7 per m². Always add 10% for cutting waste and breakages.
Patio slabs are laid on a full wet mortar bed, typically 25–50mm thick on a compacted sub-base (40mm is a common residential choice). Use a 4:1 sharp sand to cement mix. A thicker bed uses proportionally more sand and cement, so set the bedding depth in the advanced options to match the bed you are laying.
A minimum of 100mm compacted Type 1 MOT sub-base is recommended for patios. For driveways or areas with vehicle access, increase to 150-200mm. The sub-base should be compacted in layers using a plate compactor. Ensure the patio drains correctly with our Drainage Fall Calculator.
Yes. Edge restraint prevents slabs from shifting outward over time. Options include concrete haunching, aluminium or plastic edge restraints, or a brick/block border. Allow for every exposed edge of the paved area: edges that finish against the house or another solid wall are already restrained, and where two sections of an L-shaped patio join there is no edge at all. The calculator uses the full perimeter of every section by default, which over-orders on those shapes, so measure your actual open edge and enter it under advanced options for a tighter figure.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.