How to Use the Paving Slab Calculator
- Choose your units. UK mode uses metres and millimetres and gives tonnes and 25 kg bags. USA mode uses feet and inches and gives tons, cubic yards and bags.
- Enter the area as length × width, or as a known area. Add more areas for L-shaped patios.
- Pick the slab or paver size from common sizes, or enter your own. Enter its thickness.
- Set the joint width: about 3–5mm for porcelain, 10–15mm for natural stone and concrete slabs, and 1/16–3/16 inch for US pavers.
- Choose the laying method: full mortar bed (UK slabs) or sand bed (block paving and US pavers).
- Set the sub-base depth (the calculator suggests one for patios and driveways) and the waste allowance.
How Many Paving Slabs Do I Need?
The number of slabs per square metre depends on the slab size plus the joint:
Slabs per m² = 1 ÷ ((slab length + joint) × (slab width + joint)), all in metres.
Then multiply by the area and add waste for cuts and breakages.
| Slab size | Joint | Slabs per m² | Slabs for 20 m² (+5%) |
|---|---|---|---|
| 600 × 600mm | 10mm | 2.69 | 57 |
| 600 × 300mm | 10mm | 5.29 | 112 |
| 450 × 450mm | 10mm | 4.73 | 100 |
| 900 × 600mm (porcelain) | 3mm | 1.84 | 39 |
| 900 × 900mm | 10mm | 1.21 | 26 |
| 300 × 300mm | 10mm | 10.41 | 219 |
Waste allowance: add 5% for a simple rectangle with single-size slabs, 10% where there are curves, steps or lots of cuts, and 10–15% for mixed-size "project pack" natural stone laid in a random pattern. Indian sandstone project packs are usually sold to cover a set area (often around 15–20 m²) including joints, so check the pack's coverage figure.
How Much Sub-base Do I Need?
The sub-base is the compacted layer of crushed stone under the paving. It spreads the load and stops slabs sinking or rocking. In the UK, the standard material is MOT Type 1; in the USA, crushed stone paver base (often ¾-inch minus).
| Use | UK sub-base (MOT Type 1) | USA paver base |
|---|---|---|
| Garden path, patio | 100mm | 4–6 in |
| Patio on soft or clay soil | 150mm | 6–8 in |
| Driveway (cars) | 150–200mm | 8–12 in |
Lay it in layers of no more than about 75mm (3 inches) and compact each layer with a plate compactor.
Quantity: volume = area × depth. MOT Type 1 weighs about 2.1 tonnes per m³ once compacted. In the USA, crushed stone weighs about 1.4 tons per cubic yard loose; add about 15% to allow for compaction.
How Much Sand and Cement for a Patio?
Most UK patios with natural stone or porcelain slabs are laid on a full mortar bed (not dabs), typically 30–50mm thick, made from 4 parts sharp sand to 1 part cement. Some installers use 5:1 or 6:1 for natural stone; follow the slab supplier's guidance.
The calculator works out the bed volume, adds about 30% because dry materials shrink when mixed with water, and splits it into sand and cement:
- Sharp sand at about 1.6 tonnes per m³
- Cement in 25 kg bags (about 1,440 kg per m³)
For porcelain, paint a priming slurry or bonding primer onto the back of every slab before bedding. Porcelain absorbs almost no water, so without a primer it won't bond to the mortar.
Block paving and US pavers are laid on a sand bed instead: about 25–50mm (1 inch) of sharp sand or concrete sand, screeded level over the compacted base.
Worked example (UK): 20 m² sandstone patio
A 20 m² patio in 600 × 600mm calibrated Indian sandstone (22mm), 10mm joints, a 40mm mortar bed at 4:1, and 100mm of MOT Type 1:
- Slabs: 20 × 2.69 = 53.7, plus 5% = 57 slabs
- MOT Type 1: 20 × 0.1 × 2.1 = 4.2 tonnes
- Mortar bed: 20 × 0.04 = 0.8 m³ wet, about 1.04 m³ dry
- Sharp sand: about 1.33 tonnes (roughly 1.5 bulk bags of 850 kg)
- Cement: about 12 bags of 25 kg
- Jointing: about 66 m of joint, or roughly 26 kg of jointing compound or mortar
- Dig depth: 100 + 40 + 22 = 162mm below the finished level
Worked example (USA): 12 × 16 ft paver patio
A 192 sq ft patio in 6 × 9 inch pavers with 1/8-inch joints, 1 inch of bedding sand and a 6-inch gravel base:
- Pavers: 192 × 2.58 per sq ft = 495, plus 10% = 545 pavers
- Base: 192 × 0.5 ÷ 27 = 3.56 cubic yards; with 15% for compaction at 1.4 tons per yard = 5.7 tons
- Bedding sand: 16 cubic feet, or about 32 bags of 0.5 cubic feet (0.6 cubic yards bulk)
- Polymeric sand: about 3 bags of 50 lb
- Edge restraint: the perimeter not against the house, here 12 + 16 + 12 = 40 ft
How Deep Should I Dig for a Patio?
Dig depth = sub-base + bedding layer + slab thickness, measured down from where the finished surface will be.
For a typical UK patio: 100mm sub-base + 40mm mortar + 22mm slab = about 160mm. For a 20mm porcelain patio on a 30mm bed: about 150mm. For a US paver patio with a 6-inch base, 1-inch sand and 2⅜-inch pavers: about 9½ inches.
Two rules matter when the patio meets the house:
- Keep the finished surface at least 150mm below the damp-proof course (DPC) in the house wall, so rain can't splash above it and cause damp.
- Build in a fall away from the house so water drains off. A fall of about 1:60 to 1:80 (roughly 13–17mm per metre) is common in the UK; in the USA, about 1/8–1/4 inch per foot.
Paving Costs
UK (installed, per m², including groundwork)
| Material | Typical cost |
|---|---|
| Concrete slabs | £120–£160 |
| Natural stone (sandstone, limestone) | £150–£220 |
| Porcelain | £180–£250 |
| Premium stone (granite, slate) | £200–£300 |
A 20 m² patio typically costs about £3,000–£6,500 installed, depending on the material, drainage and access.
USA: paver patios are usually priced per square foot installed, with concrete pavers the cheapest option, followed by brick and natural stone. Costs vary widely by region, so get local quotes.
For a concrete patio instead of slabs, use the Concrete Slab Calculator. For gravel or a sand base on its own, the Cubic Yard Calculator gives volumes and tonnes.
Porcelain vs Natural Stone vs Concrete Slabs
- Porcelain: usually 20mm thick, very consistent in size, low maintenance and stain resistant. Needs a priming slurry, narrow joints (3–5mm), and a wet saw with a porcelain blade for cuts.
- Natural stone (Indian sandstone, limestone, slate): varied colour and texture. Calibrated stone is a consistent thickness (about 20–25mm); hand-cut stone varies more and needs a deeper, adjustable bed. Usually laid with 10–15mm joints.
- Concrete slabs: the most affordable, easy to cut, and available in many finishes. Typically 35–50mm thick.
- Block paving and US pavers: small units laid on sand, which flex with the ground and suit driveways. Use a compacted base and solid edge restraints so the blocks can't spread.
Frequently Asked Questions
How many 600x600 paving slabs are in a square metre?
2.78 without joints, or about 2.69 with 10mm joints. For 20 m², order about 57 slabs including 5% waste.
How much MOT Type 1 do I need for a patio?
Multiply the area by the depth (usually 0.1 m) and by 2.1. A 20 m² patio at 100mm needs about 4.2 tonnes.
How much sand and cement do I need to lay slabs?
For a 40mm full bed at 4:1, allow about 67 kg of sharp sand and 0.6 bags (15 kg) of cement per m². A 20 m² patio needs about 1.33 tonnes of sand and 12 bags of cement.
How deep should a patio base be?
100mm of compacted MOT Type 1 for most garden patios, or 150mm on soft ground. In the USA, 4–6 inches of crushed stone base for patios and 8–12 inches for driveways.
How many pavers do I need per square foot?
About 4.5 for 4 × 8 inch pavers, 2.6 for 6 × 9 inch pavers and 1 for 12 × 12 inch pavers, allowing for narrow joints. Add 5–10% for cuts.
How much bedding sand do I need for pavers?
1 inch deep: 1 cubic yard covers about 324 sq ft. A 192 sq ft patio needs 16 cubic feet, or about 32 bags of 0.5 cubic feet.
What gap should I leave between paving slabs?
3–5mm for porcelain, about 10mm for calibrated sandstone and concrete slabs, and 10–15mm for hand-cut natural stone.
Do I need to prime porcelain paving slabs?
Yes. Porcelain is almost non-porous, so brush a priming slurry or bonding primer onto the back of each slab just before you lay it on the wet mortar bed. Without it, slabs can debond and rock within a season.
Can I lay paving slabs on sand only?
Small concrete slabs on a light-use path can be laid on a sand bed, but natural stone and porcelain need a full mortar bed to stop them rocking and cracking.
Get quotes from local landscapers and patio installers
Drainage, falls and the base decide how long a patio lasts. Compare at least three quotes.