What this calculator works out
A patio is more than a count of slabs. Underneath a proper full-bed patio sits a compacted sub-base to carry load and stop the ground moving, and a mortar bed the slabs are actually bedded into and levelled on, with a jointing compound swept into the gaps once they're down. Ordering the slabs alone and guessing the rest is how a job ends up with an extra van trip to the merchant. This calculator takes the patio's size, the slab size and the joint width, and works out the whole list in one pass: slabs to buy, MOT Type 1 sub-base in tonnes and bulk bags, sharp sand and cement for the mortar bed, and tubs of jointing compound.
It uses the same effective-footprint method as this site's Tile Calculator for the slab count: a slab plus its share of the joint around it is what actually tiles the ground. What sets a paving job apart from indoor tiling is everything underneath: the sub-base and mortar bed that carry outdoor loads and weather, covered further down this page.
How the slab count works
No slab sits edge to edge with the next one; there's a joint gap all the way round. That gap means every slab effectively "owns" a bit more ground than its own printed size. Add the joint width to the slab's width and its length, multiply those together, and that's the effective footprint one slab actually accounts for.
Divide the patio area by that effective footprint and round up, since a merchant can't sell part of a slab, and that's the slab count before any allowance for cutting and breakage.
Working the default example through: a 600 × 600mm slab with a 10mm joint has an effective footprint of 610 × 610mm, which is 0.3721 m². A 4m by 3m patio is 12 m². Divide 12 by 0.3721 and the result is 32.25, rounding up to 33 slabs before waste. Add a 10% allowance for cuts and breakages and 33 × 1.10 is 36.3, rounding up to 37 slabs to buy.
Full-bed construction: what's underneath the slabs
A full mortar bed patio is built in layers, each doing a different job. First comes a sub-base of MOT Type 1, a graded, compactable limestone or granite aggregate, compacted into a firm, level layer that spreads load and stops the ground beneath moving, settling or heaving with frost. 100mm compacted is the depth most patio laying guides quote, including Marshalls' own patio guide; other trade sources give a 75–100mm range for a pedestrian patio, with the fuller 100mm suiting softer ground or occasional vehicle weight. Compacted MOT Type 1 is taken here at 2.1 tonnes per cubic metre, the commonly used mid-point of the 2.0–2.2 t/m³ range quoted by aggregate suppliers.
On top of that goes the mortar bed the slabs are bedded into: a semi-dry mix of sharp (grit) sand and cement, mixed 4 parts sand to 1 part cement by volume, laid 30 to 40mm deep, thick enough to bed and level each slab by tapping it down but not so thick that it takes an age to cure or becomes prone to cracking. Sharp sand is used rather than soft building sand because its coarser, angular grains give the mix real bonding strength.
For the default 12 m² patio: 100mm of sub-base is 1.2 m³, which at 2.1 t/m³ comes to 2.52 tonnes, about 3 bulk bags. A 35mm mortar bed is 0.42 m³; split 4:1, that's 0.336 m³ of sharp sand (0.538 tonnes) and 0.084 m³ of cement, which at cement's typical 1,440 kg/m³ bulk density is about 121kg, rounding up to 5 bags of 25kg cement.
Spot bedding, full bed, and why this calculator assumes full bed
Spot bedding, sometimes called dab bedding, sets each slab on five mortar dabs, one under each corner and one in the middle, rather than a continuous bed. It's faster and uses less material, a common shortcut for a quick, low-traffic path. The trade-off is real: a slab on five dabs has unsupported spans between them, exactly where it's most likely to crack under a point load, a dropped object, or a car tyre if the patio ever doubles as extra parking. A full bed supports the entire underside of every slab continuously, which is why laying guides recommend it for anything meant to last, especially larger-format slabs that flex more across an unsupported span. This calculator works out quantities for a full bed throughout, the durable method most manufacturers assume in their own laying guides.
Joints and jointing compound
Once the slabs are down and the mortar bed has firmed up, the joints between them get filled. A swept-in resin jointing compound, such as Azpects' EASYJoint, is a common modern choice: a pre-mixed sand and resin compound brushed into the joint dry, then activated with water, setting into a firm, weed-resistant, all-weather fill rather than a wet grout worked in by hand. Azpects' own coverage calculator asks for the paver size, joint width and joint depth before estimating a tub count, precisely because coverage depends on all three rather than one fixed "m² per tub" figure. This calculator works from the same first principles: total joint length across a paved area is roughly the area multiplied by (1 divided by slab width, plus 1 divided by slab length), a standard grid-joint approximation. Multiply that length by the joint width and an assumed 20mm fill depth (close to EASYJoint's own quoted 19mm minimum depth for porcelain), convert to weight at a typical jointing-compound density, and round up into whole tubs.
Tub size is left as an editable field, defaulting to 15kg, since packaging varies between suppliers and some ranges use a 12.5kg tub instead, the same approach this site's Turf Calculator takes with pallet size.
Falls: why a patio needs a slight slope
A patio laid dead flat causes problems the first time it rains hard, since water has nowhere to go and starts pooling, or worse, running back towards the house. Every patio needs a gentle fall, built into the sub-base and mortar bed as they're laid, directing water away from any building. A gradient of about 1 in 60, roughly 16 to 17mm of drop per metre, is the figure most UK laying guides and drainage references quote, including pavingexpert.com's own gradients and falls guide. On the 4m run of the default example, that's a fall of around 65 to 70mm from the house edge to the far edge, small enough to be invisible to the eye but enough to keep water moving the right way. This calculator works out quantities for a flat area; the fall is built in during laying by tapering the sub-base and mortar bed slightly.
Sealing a new patio
Sealing isn't compulsory, but it's a common finishing step for concrete, natural stone and porous slabs, since it reduces staining from spills, algae and weathering and can bring out the stone's colour. It shouldn't be rushed: new mortar and jointing releases moisture and soluble salts as it cures, which show up on the surface as a white, powdery bloom called efflorescence. Sealing over that bloom traps it underneath rather than letting it weather away naturally, which is difficult to put right afterwards. Most suppliers and sealant manufacturers recommend waiting roughly 8 to 12 weeks after laying, long enough for efflorescence to largely clear and the mortar bed to cure, though the exact figure varies by product, weather and how porous the slab is.
Questions people ask
Do I need MOT Type 1 under a patio?
For a proper full-bed patio, yes; a compacted sub-base spreads load and stops the ground beneath moving or settling. 100mm compacted is the commonly quoted depth for a pedestrian patio, deeper on soft ground or where a vehicle might occasionally cross it. Skipping it is a common cause of slabs cracking or sinking unevenly within a year or two.
Spot bedding or full bed, and why does this calculator assume full bed?
Spot bedding sets each slab on five mortar dabs and is quicker with less material, but leaves unsupported spans between the dabs where a slab is more likely to crack under a point load or vehicle weight. A full bed supports the whole underside continuously, the method most manufacturers' laying guides recommend for a durable patio, which is why this calculator uses it throughout.
What fall should a patio have for drainage?
About 1 in 60, roughly 16 to 17mm of drop per metre, directing water away from any building, is the gradient most UK laying guides quote. It's built into the sub-base and mortar bed as they're laid rather than being a separate material to order.
Should I seal a new patio, and when?
Sealing is optional but common for concrete, natural stone and porous slabs, mainly to resist staining and weathering. Most guidance suggests waiting roughly 8 to 12 weeks after laying, so efflorescence, the white bloom new mortar releases as it cures, has time to clear before it gets sealed underneath.
Why isn't slab count just patio area divided by slab area?
Because slabs don't sit edge to edge; there's a joint gap around every one. This calculator adds that joint to the slab's width and length first to get an effective footprint, and divides the patio area by that instead, the same method used to order tiles.
How much extra should I buy for wastage?
5% suits a simple rectangular patio with a clean run of full slabs. 10% is the safer default once edges need cutting against a wall, a curved bed or a step, since cut slabs break more often than whole ones and off-cuts rarely reuse elsewhere on the same job.
This calculator uses standard UK and US slab sizes and published typical figures for sub-base depth, mortar mix ratio and bulk densities, checked against at least two independent trade sources named above. Real depths, densities and product coverage vary by supplier and ground conditions, so treat the figures here as a solid ordering estimate and confirm exact quantities with your supplier before a large order.