Paving Slab Calculator
Calculate paving slabs, sharp sand, cement and jointing compound for any patio or path. Includes joint gaps, wastage, BS 7533 falls guidance and 2026 UK pricing.
Concrete flags: robust, cost-effective. 10mm joints, standard mortar bed. ~£15–£30/m².
Area = π × (diameter ÷ 2)²
Use if you already know your total area from a plan or survey.
Standard: 10mm. Porcelain: 3–5mm. Wide pointing: 15mm.
10% covers edge cuts and breakages for most projects. Use 15% for porcelain and complex layouts.
Standard 40mm full wet bed (BS 7533). Increase to 50–75mm on uneven ground.
MOT Type 1 at 2.1 t/m³ × 1.2 compaction factor
Every patio needs a minimum 1:60 fall (≈17mm per metre) to drain away from buildings. Enter the drainage run length:
The low side of your patio should be at least this much lower than the high side. Always drain away from any building structure.
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Installation Guide
How to Lay Paving Slabs
Full Wet Bed — BS 7533 Specification
BS 7533-101:2021 (structural design) and BS 7533-102:2025 (installation) together govern modular paving in the UK. Both require all paving to be laid on a full wet mortar bed — not "dot and dab" (five blobs of mortar). Dot and dab leaves voids beneath the slab that trap water, which then freezes and expands in winter, cracking or lifting slabs within a few seasons.
A standard 40mm wet bed uses a 4:1 sharp sand to cement mix. Lay the bed, tamp it flat, then use a straightedge to check the level across the mortar before placing each slab. Do not place slabs more than 30 minutes after laying the bed as the mortar will begin to skin.
Falls — Drainage Away from Structures
Every patio must slope away from any building at a minimum gradient of 1:60 — approximately 17mm per linear metre. For a 4m deep patio draining away from the house, the far edge must be at least 67mm lower than the house edge.
Use the falls calculator above to get the height drop for your drainage run. Set this on your string lines or laser level before laying the sub-base. Getting the fall right at the sub-base stage is far easier than trying to correct it in the mortar bed.
Mortar Joints & Pointing
Standard natural stone and concrete flags use a 10–15mm joint, pointed with a semi-dry mortar mix (brush-in pointing mix or fresh mortar pressed in and tooled). Point within 24 hours of laying while the bed mortar is still green — this creates a monolithic bond.
Porcelain requires a 3–5mm joint and must be pointed with a proprietary flexible grout or slurry-based pointing compound — standard sand-cement mortar shrinks too much for tight porcelain joints and will crack and fall out. Always check the slab manufacturer's specification.
DPC & Building Regulations
The finished paving surface must be at least 150mm (two brick courses) below the Damp Proof Course (DPC) of any adjoining building. Paving above the DPC level bridges it, creating a pathway for moisture to enter the wall cavity — one of the most common causes of damp in UK homes.
The Splash-Back Rule: The 150mm clearance isn't just about bridging the DPC — it's also about splash-back. When rain hits a hard paved surface adjacent to a wall, water bounces upward. On a properly recessed patio, this splash lands on the masonry below the DPC where it can dry harmlessly. On a patio laid too high, the splash hits the brickwork above the DPC, saturating the wall and eventually bypassing the membrane. Even a correctly positioned DPC can be overwhelmed by persistent splash-back from a patio that's only 50–100mm below it.
Never block or cover air bricks with paving or compacted material. Air bricks ventilate suspended timber floors and must remain clear.
Under SuDS rules (Building Regulations Part H), any new paved area over 5 m² must either use a permeable surface or direct surface water to a soakaway within the property. Permeable paving with a MOT Type 3 sub-base is the simplest solution.
Project Packs — Random Pattern Paving
If you are buying a "project pack" (a mixed pack containing 4 or 5 different slab sizes laid in a random pattern), you cannot calculate by slab count. Instead, calculate the total m² of your patio, add your wastage allowance, and buy enough packs to cover that area. The pack coverage in m² will be printed on the product listing or packaging.
FAQ
Common Questions
For 600×600mm slabs with 10mm joints: approximately 2.7 slabs per m². For 450×450mm: approximately 4.8 per m². Always add wastage — 10% for standard paving, 15% for porcelain. Our calculator works this out precisely including the joint gap in the slab footprint.
Standard concrete or natural stone flags use a 10–15mm joint. Porcelain paving uses 3–5mm for its precision edges. The gap matters for the slab count — a 10mm joint adds roughly 3% more slabs compared to a 5mm joint for 600mm slabs.
A standard 40mm wet mortar bed (4:1 mix) requires approximately 0.056 tonnes of sharp sand per m². For a 20m² patio you need around 1.1 tonnes of sharp sand and 9 bags of cement (25kg). Our calculator shows quantities for your specific patio size and bed depth.
Most garden patios are permitted development and don't need planning permission, as long as they use permeable materials (or drain to a soakaway) if over 5m², remain at least 150mm below the DPC of the house, don't raise the ground level by more than 30cm, and the total hardstanding doesn't cover more than 50% of your garden.
Yes, but porcelain is more demanding than standard concrete flags. You must use SBR bonding slurry on the back of each slab, a full wet mortar bed (never dot and dab), and a proprietary flexible pointing compound for the 3–5mm joints. Porcelain cannot be cut with a standard angle grinder — you need a wet tile saw or a dedicated angle grinder with a continuous rim diamond blade.
BS 7533 requires a minimum fall of 1:60 — approximately 17mm per linear metre. For a 4m deep patio draining away from the house, the far edge must be at least 67mm lower than the near edge. Use the falls calculator on this page to get the figure for your drainage run.
Dot and dab means placing five blobs of mortar under each slab (one at each corner and one in the middle) rather than a continuous bed. It is quicker but leaves voids beneath the slab. Water collects in these voids, freezes in winter and lifts or cracks the slabs within a few seasons. BS 7533-101:2021 requires a full wet bed for all modular paving installations.