Paving Calculator
Paving is bought by the slab or the pack, so the answer is a whole number of slabs, not an area. Enter the patio size, the slab size and the joint width, choose a laying pattern for the cutting waste, and this calculator rounds up to the slabs you need. It also works out the sub-base, the bedding layer and the jointing compound underneath and between them, which is where most of the tonnage on a patio job actually is.
Your measurements
Check your order
Your paving order
- Confirm slab size, joints, laying pattern and cutting allowance before rounding to packs.
- Check the specified bedding and sub-base materials, depths and compaction allowance; these vary with the installation system.
- Match the sand and sub-base quantities to the supplier’s density and bag size. List edging separately.
Tools for this job
Measure the paving footprint before ordering. A rake helps distribute loose base material before the specified compaction and final screeding.
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- Spread loose ground materials
Garden rake
Fiskars Pro garden rake
For spreading loose soil or gravel and preparing an even surface. Raking distributes material; it does not replace any compaction the installation requires.
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Long tape measure
Komelon 6611 100-foot open-reel tape
For measuring longer outdoor runs and marking out areas. Keep the tape straight and use the same units throughout; this listing is for the 100-foot model.
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How many paving slabs do I need?
Divide the patio area by the area of one slab plus its joint, add a cutting allowance, and round up. The joint is the detail people forget: a 600 x 600 mm slab with a 10 mm joint occupies 610 x 610 mm of patio, so 24 m² takes 2.69 slabs per square metre, not 2.78. On a large patio that difference is a couple of slabs, and on block paving with hundreds of units it is a whole pack.
The cutting allowance depends on the pattern. A simple grid of square slabs in a square patio cuts only along two edges and 5% covers it. Stretcher bond, where every second row is offset by half a slab, cuts at both ends of every other row. A random or mixed-size pattern, or any curved edge, wastes more because the cut pieces rarely fit anywhere else. Buy from one batch: the colour of pressed concrete and natural stone varies between batches, and a short order topped up later will show.
The formula
Common slab sizes
Natural stone and porcelain are often sold in project packs that mix three or four sizes to a set pattern, quoted by the square metre. For those, use the known-area mode and add 10 to 15% for cuts.
| Slab | Area with joint | Slabs per m² | Typical use |
|---|---|---|---|
| 600 x 600 mm | 0.372 m² | 2.69 | The standard UK patio slab |
| 450 x 450 mm | 0.212 m² | 4.73 | Paths and smaller patios |
| 600 x 300 mm | 0.189 m² | 5.29 | Stretcher bond, mixed-size layouts |
| 900 x 600 mm | 0.555 m² | 1.80 | Large-format porcelain and stone |
| 400 x 400 mm | 0.168 m² | 5.95 | Utility paving |
| 24 x 24 in (610 mm) | 0.384 m² | 2.60 | US patio pavers |
| 16 x 16 in (406 mm) | 0.173 m² | 5.77 | US patio pavers |
| 12 x 12 in (305 mm) | 0.099 m² | 10.1 | US patio pavers |
Worked example
A patio 6 m x 4 m = 24 m² in 600 x 600 mm slabs with 10 mm joints, laid in a typical pattern with edges to cut, so 10% waste. Each slab with its joint takes 0.61 × 0.61 = 0.3721 m². Area with allowance = 24 × 1.1 = 26.4 m². Slabs = 26.4 ÷ 0.3721 = 70.9, rounded up to 71 slabs.
Underneath: 10 cm of compacted MOT type 1 over 24 m² is 2.4 m³, and at 2.1 t/m³ compacted that is 5.04 tonnes. If the supplier sells 850 kg bulk bags, 5,040 ÷ 850 rounds up to six bags; use the actual bag weight if it differs, or order a loose load. The 40 mm mortar bed is 24 × 0.04 = 0.96 m³ of mortar, which the mortar calculator turns into cement and sharp sand. Add the jointing compound at the rate on the tub, typically 4 to 7 kg/m² for these joints, so roughly 100 to 170 kg.
The layers under a patio
Lay everything to a fall of at least 1 in 80 away from the house, and keep the finished surface 150 mm below the damp-proof course. The gravel calculator converts the sub-base to tonnes if you prefer to order by weight, and the concrete calculator covers a solid slab base where the ground is poor.
| Layer | Patio or path | Driveway | Notes |
|---|---|---|---|
| Dig out | 15 to 20 cm | 25 to 30 cm | Slab thickness plus bed plus sub-base |
| Sub-base, MOT type 1 | 10 cm compacted | 15 cm compacted | Compact in 5 cm layers with a plate compactor |
| Bedding | 30 to 50 mm mortar or sharp sand | 30 to 50 mm | Full mortar bed for slabs; sand for block paving |
| Slab or block | 20 to 40 mm | 50 to 80 mm | Driveway blocks are thicker |
| Joints | 3 to 15 mm | 3 to 5 mm | Kiln-dried sand, mortar or resin jointing compound |
Mistakes that cost slabs
- Measuring to the wall and forgetting the joint, so the last row does not fit and every slab in it is cut.
- Mixing batches. Order the whole job at once, plus a couple of spares for future repairs.
- Spot bedding on five dabs of mortar. The slab rocks, the corners crack and water collects underneath. Use a full bed.
- Skimping the sub-base under a car. 10 cm is a patio; a driveway needs 15 cm and thicker blocks.
- Ordering by the square metre for a random pattern without allowing 15%, then discovering the pack sizes do not fill the last strip.
Sources and assumptions
Slabs are counted from their size plus the selected joint and an area-based cutting allowance. The sub-base and bedding volumes use area × entered depth; sub-base weight uses the entered density.
- Marshalls: paving installation questions: The manufacturer distinguishes laying methods by paving type, including rigid mortar bedding for natural stone and wet-cast paving, and product suitability for driveways.
Before using this estimate
- The slab count is not a layout drawing. Mixed-size project packs, borders, curves and nominal sizes that already include joints need a separate laying plan.
- Base depth and density are estimates to replace with the project specification. The calculator does not design drainage, falls, retaining edges or a driveway foundation.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.