Paving Slab Calculator UK - How Many Slabs Do I Need?

How Many Slabs Per m², By Size

The number of slabs per m² depends on the slab's coordinating size - its actual face size plus one joint width, since that's the space each slab genuinely occupies once laid. At a 10mm joint: 600 × 600mm needs 2.69/m², 450 × 450mm needs 4.73/m², 900 × 600mm needs 1.80/m², 600 × 300mm needs 5.29/m², and 400 × 400mm needs 5.95/m².

Joint width matters more than people expect over a large area. Widen the joint and the coordinating cell gets bigger, so you need fewer slabs for the same area - narrow it and you need more. The effect compounds: on a 20m² patio of 600 × 600mm slabs, moving from a 10mm to a 15mm joint drops you from around 54 slabs to about 52, a small but real difference once you're pricing a job precisely.

Laying Patterns and What Each Suits

• Stack bond - a plain grid, joints running straight through in both directions. Quickest to lay and easiest to plan cuts for, but shows up any inconsistency in slab size most obviously, since every joint lines up.
• Stretcher / running bond - each row offset by half a slab from the one below, exactly like brickwork. Hides minor size variation better than stack bond and is the standard choice for rectangular precision-cut slabs like porcelain or calibrated sandstone.
• Random / mixed sizes - several slab sizes combined in a deliberately non-repeating layout, the standard look for riven Indian sandstone patio packs. Needs more waste allowance since the layout is planned by eye on site, not to a strict grid.
• Basketweave - pairs of slabs alternating orientation in a woven grid, usually with sizes chosen so two short slabs equal one long one (e.g. 300 × 600mm pairs). Distinctive look, but only works cleanly with the right size ratio.

Full Mortar Bed vs Dabs, and Why Dabs Fail

A full mortar bed means the slab sits on a continuous, uniform layer of bedding mortar across its whole underside. Mortar dabs - blobs of mortar at the corners and centre, leaving gaps between - are quicker but leave voids under the slab. Water gets into those voids, and freeze-thaw cycling or the flex of foot traffic on an unsupported area eventually cracks the slab or rocks it loose.

BS 7533-102:2025, the current UK code of practice for installing modular paving (it replaced the older, now-withdrawn BS 7533-3 for blocks and BS 7533-4 for flags, covering both under one part since March 2025), describes full bedding as the specification-compliant method for laying flags. Dabs are a widely-used shortcut on smaller DIY jobs, not something the standard documents as an approved alternative.

Falls and Drainage: 1:80 Away From the House

Every patio needs a fall so water runs away rather than sitting on the surface or, worse, running back towards the house. The standard UK guidance is a minimum fall of 1:80 away from any building - that's 12.5mm of drop for every metre of distance. For a 4m-deep patio, that's 50mm of total fall from the house side to the far edge.

The finished patio surface should also sit at least 150mm below the damp-proof course (DPC) once complete, factoring in the sub-base, bedding and slab thickness together - not just the slab on its own.

Sub-Base Depth: Patios vs Driveways

A domestic patio only carries foot traffic, so 100mm of compacted MOT Type 1 is the standard UK default - the same figure our sub-base calculator uses for its own patio preset. A driveway carries vehicle weight and needs significantly more: 150mm as a baseline, more again for regular heavy vehicles or soft ground. Getting this wrong in either direction either wastes material (over-specifying a patio) or risks the surface failing under load (under-specifying a driveway).

Jointing Compound vs Pointing Mortar

Pointing mortar - a 4:1 building sand to cement mix, often with a plasticiser - is the traditional method: cheap, and once cured it's rock hard and structurally supports the slab edges for 20+ years. It's slow work, though, and wet mortar can permanently stain porous slabs like sandstone if it's not cleaned off straight away.

Brush-in jointing compound is a resin-coated sand sold ready-mixed in tubs. Swept into dry joints and compacted with a little water, it's dramatically faster to apply (a 20m² patio might take an afternoon with compound against a full day or two hand-pointing) and far less prone to staining, at a higher material cost per m². Coverage varies by product, joint width and depth, so this calculator's jointing compound figure is an estimate - always check the specific product's technical data sheet before ordering.

Paving Slabs By Area: Quick Reference

Stack bond, 10mm joints, 10% waste, figures regenerated directly from the calculator above.

Figures include 10% waste, rounded up to whole slabs. Travis Perkins and Jewson both stock paving slabs individually and by the pack for trade quantities, while Wickes and B&Q suit smaller DIY-scale orders. Use the calculator above for cuts, bedding and jointing quantities at your own size and pattern.

Frequently Asked Questions

How many paving slabs do I need per m2?

It depends on slab size and joint width. At a 10mm joint: 600 × 600mm slabs need 2.69 per m², 450 × 450mm need 4.73, 900 × 600mm need 1.80, 600 × 300mm need 5.29, and 400 × 400mm need 5.95. A wider joint uses slightly fewer slabs per m² since each coordinating "cell" is bigger; a narrower joint needs slightly more. Use the calculator above for your exact size and joint width.

What's the difference between stack bond, stretcher bond and basketweave?

Stack bond lines slabs up in a plain grid, joints running straight through in both directions - simplest to lay, shows any size variation between slabs most obviously. Stretcher (running) bond offsets each row by half a slab, like brickwork, hiding minor size variation better and looking less like a chessboard. Basketweave alternates pairs of slabs in a woven grid pattern, usually with slabs sized so two short ones equal one long one. Random or mixed-size layouts use several slab sizes in a deliberately non-repeating pattern, common with riven Indian sandstone patio packs.

Should I use a full mortar bed or mortar dabs for paving slabs?

Full mortar bed, not dabs. Dabs (blobs of mortar at the corners and centre of each slab) leave voids underneath the slab that trap water and let the slab rock and crack under load, load, especially with modern thinner, more consistent slabs. BS 7533-102:2025, the current UK code of practice for installing modular paving, describes full bedding as the specification-compliant method. Dabs are a widely-used shortcut, not a documented standard.

What fall (slope) does a patio need for drainage?

A fall of 1:80 away from the house is the standard minimum UK guidance - that's 12.5mm of drop for every metre you move away from the building. Get this wrong and water pools against the wall or, worse, tracks back towards the damp-proof course. A patio should also sit at least 150mm below the damp-proof course once finished.

What's the difference between jointing compound and pointing mortar?

Pointing mortar is a traditional sand and cement mix (typically 4:1 building sand to cement, with a plasticiser) brushed or pointed into the joints and left to cure - cheap, and sets rock hard, but slow to apply and can stain porous slabs if mortar is left on the surface. Brush-in jointing compound is a resin-coated sand sold ready-mixed in tubs, swept into dry joints and compacted with water - much faster to apply and less prone to staining, at a higher material cost. Both need dry, clean joints of a consistent depth to work properly.

How deep should the sub-base be under a patio?

100mm of compacted MOT Type 1 is the standard UK default for a domestic patio, the same figure our sub-base calculator uses for its patio preset. A driveway needs more (150mm or more) since it carries vehicle loads. Soft or clay ground may need a deeper sub-base regardless of what's going on top - see our sub-base calculator for ground-condition adjustments.