Patio Calculator

Key Takeaways

• A slab patio on firm ground needs 100mm of compacted MOT Type 1, a 40mm bed of sand and cement, and about 10% more slabs than the area covers.
• A patio on firm ground is built on 100mm of compacted MOT Type 1, and on soft or clay ground on 150mm. A 5m by 4m patio at 100mm takes 2.60m³ of loose MOT Type 1, about 3.87 tonnes, which is five 850kg bulk bags.
• Loose MOT Type 1 loses roughly a third of its depth under the plate compactor, so it is spread at about 130mm to finish at 100mm.
• The number of paving slabs for a patio is the area divided by the face of one slab, plus 10% for cuts and breakages.
• The dig for a slab patio is the slab thickness plus the bed plus the sub-base. 35mm slabs on a 40mm bed and 100mm of MOT Type 1 means digging 175mm below the finished level, and 3.50m³ of soil out of a 5m by 4m patio.
• A gravel patio of 20mm gravel is laid 40mm deep, twice the stone size, over 100mm of compacted MOT Type 1 with weed membrane between the two.

Every patio I priced started with the same question: how much material do I actually need? It sounds easy to some, then they go look at three calculators and get three different answers for the sub-base, none of them mention the bed, and the slab count comes back with no allowance for the cuts that need to be made. Outdoor Aggregates' patio calculator does the whole build in one go. Sub-base, slabs, the sand or grano dust and cement bed, membrane and pegs, with the products we sell priced live and the rest given as a typical UK price range.

How Much MOT Type 1 Do I Need for a Patio?

A patio needs 100mm of compacted MOT Type 1 on firm ground, about 4.4m² per 850kg bulk bag. Soft ground or clay takes 150mm, about 2.9m² per bag. The sum the calculator runs is area x compacted depth x 1.3, which gives the loose volume, then x 1,490kg per m³ for the weight. That 1,490 is not a figure I lifted from somewhere else. We weighed and measured our own bulk bags: 850kg of MOT Type 1 fills about 0.57m³ loose. The 1.3 is the compaction allowance, and it checks out against the trade's own coverage figures to within about 3%.

Run a 5m by 4m patio at 100mm through the other patio calculators and the answers come back between 4 and 5.04 tonnes. The top of that range is about 30% more than the job needs, and on a calculator that rounds to whole bags it is two extra bags. Most of them work the stone at a compacted weight of 2.0 to 2.3 tonnes per m³, and one of them adds a 20% compaction allowance on top of that, which counts the squash twice.

The calculator also gives you the loose depth to spread to and the number of wheelbarrow loads, taken at 85L a barrow. Over 150mm compacted it splits the build into layers, because a hire-shop plate compactor does not properly compact much more than 150mm in one go. A 200mm build goes down as two 100mm layers, each one compacted before the next.

One thing the calculator cannot know is how far your sub-base runs past the slabs. If there is an edging going in, I dug 200 to 300mm wider than the paving all round, so the edging and its concrete haunch had something solid to sit on. Add that to your length and width before you type them in. The full step-by-step for laying a sub-base covers setting that out, and the MOT Type 1 sub-base calculator does the base on its own if that is all you need.

How Many Paving Slabs Do I Need for My Patio?

The number of paving slabs for a patio is the area divided by the face of one slab, rounded up, plus 5 to 15% for cuts and breakages. Ten per cent suits a plain rectangle.

Pick 15% for a circle, a curve, a diagonal layout or a patio with lots of cut-outs round drains and steps. Large slabs make curves genuinely awkward, because every slab on the edge is a cut, and that is where the breakages come from. Pick 5% only for a small, square patio laid in slabs that fit it exactly. The calculator does not take the joints off. On a 600mm slab a 10mm joint saves well under 4% of the slab count, the cutting allowance swallows that, and a slab short is a far bigger problem than a slab spare. If your slabs are a size the list does not have, choose "Another size" and type the length and width in mm.

How many 600x600 slabs are in a square metre is one of the most searched patio questions there is, and the answer is 2.78. Three per m² is the rounded figure some calculators give, and on a 20m² patio it orders 56 slabs with nothing for the cuts.

The slab thickness buttons are there for the dig depth only. Porcelain is often around 20mm, most concrete and natural stone patio slabs are about 35mm, and 50mm is a heavier slab. Porcelain does not grip a semi-dry bed, so it goes down on a wet mortar with a priming slurry painted on the back of each slab. The bed quantity works out the same.

How Much Sand and Cement Do I Need for a Patio?

A patio's sand and cement is worked out from the bed: area x bed depth gives the volume of mix, then the mix splits it into sand and cement. A 40mm bed is 0.04m³ of mix for every m². At 6 to 1, six of every seven buckets are sharp sand and one is cement. The calculator turns that into kilograms and 25kg bags, at 1,600kg per m³ for dry sharp sand. The mix is measured by the bucket, the way it is made on site.

The mix matters less than people think. I bed a screeded slab patio on a semi-dry mix at 8 to 10 parts sharp sand to 1 part cement, because the bed is there to carry the slab and hold its level. A stronger 6 to 1 suits slabs bedded one at a time, such as reclaimed stone that varies in thickness, where each slab sits on its own pad of mortar. The guide to laying paving slabs on a dry mix goes through the semi-dry method, and how to get the slab to grip it, in full.

Two things I would never do. Use building sand, which is too fine and holds water, so the bed settles. Or lay slabs on sand alone with no cement. A sand-only bed is for interlocking block paving, where the blocks hold each other in place. A slab sits on its own and needs the cement to keep it there.

Grano dust does the same job as sharp sand in a bed. It is crushed granite, 0 to 4mm, and mixed with cement it makes a firm bedding mortar. Pick "Grano dust and cement" in the calculator and it works out the grano dust in our bulk bag or mini bags, with the cement as its own line. Wear gloves for any of it. Wet cement burns skin.

How Deep Do I Dig for a Patio?

A slab patio is dug to the slab thickness plus the bed plus the compacted sub-base. For 35mm slabs on 40mm of bed and 100mm of MOT Type 1, that is 175mm below the finished paving.

The calculator shows the dig depth and the volume of soil it takes out, so you know what to plan for in skips or bags before the first spade goes in. A 5m by 4m patio dug to 175mm is 3.50m³ of soil. Allow an extra 50mm or so where turf and plant roots come off first, and keep going wherever the ground is soft. Firm ground at the bottom of the dig counts for more than a dead flat one.

Check the finished level against the house before you decide on the dig. The top of a patio by the house has to finish at least 150mm below the damp-proof course, the line of slate, plastic or engineering brick in the wall a couple of courses above the ground. That is a Building Regulations requirement. If it pushes the patio down, the dig goes deeper.

How Much Fall Does a Patio Need?

A patio needs a fall of at least 1 in 60 away from the house, about 17mm for every metre. A patio running 4m out from the wall drops about 67mm from one side to the other. I set the fall on the sub-base, with pegs and a string line, before the bed goes down, so the bed is an even thickness all over. The calculator works the bed as one even depth, and an even bed on a falling base is what makes that figure right. The commercial minimums are 1 in 80 along a run and 1 in 40 across it, but a home patio wants the water gone faster than that.

Does a Patio Need Weed Membrane?

A patio on firm, dry ground does not strictly need a weed membrane, because weeds in a patio grow down into the joints from seed rather than up through 100mm of sub-base. Where it earns its place is soft clay, made-up ground or ground that moves.

On a slab patio the membrane goes on the dug-out ground, under the MOT Type 1, to keep the stone from being pushed down into soft soil. The calculator works it in 1m strips with a 300mm overlap and a peg every 500mm along the edges and the joins. On a gravel patio it goes on top of the compacted MOT Type 1, under the gravel, with a 100mm overlap, because there the job is keeping the gravel and the sub-base apart.

The membrane and pegs are on the materials list either way. If your ground is firm and dry and you would rather leave them out, just do not order those two lines. Step 4 of the sub-base laying guide goes through which ground needs a membrane.

How Much Gravel Do I Need for a Gravel Patio?

A gravel patio of 20mm gravel is laid 40mm deep, twice the size of the stone, which works out at about 60kg per m². Deeper and chair legs and feet sink in. Pick "Gravel patio" in the calculator, then the gravel size and which gravel, and the slab, bed and cement lines are replaced by that gravel, worked at 1,490kg per m³ from our measured bulk bag. A 5m by 4m gravel patio takes 0.80m³, about 1.19 tonnes, or 1.25 tonnes with the calculator's 5% allowance. That is two 850kg bulk bags. The dig is shallower too: 40mm of gravel on 100mm of MOT Type 1 is 140mm. The calculator takes 10mm, 14mm and 20mm gravel, laid at 20mm, 28mm and 40mm, and lists every gravel we sell in each size.

Worked Example: A 5m by 4m Slab Patio

A 5m by 4m slab patio on firm ground, in 600 x 600mm slabs on a 40mm bed at 6 to 1, with no extra allowance, needs five bulk bags of MOT Type 1, 62 slabs, 44 bags of sharp sand, six bags of cement, three rolls of membrane and one 100 pack of pegs.

The calculator starts on a 5% allowance, which takes it to 4.07 tonnes of MOT Type 1 (still five bags), 47 bags of sharp sand and seven bags of cement. Click 0% to see the figures in the table.

From us, that is £570 for the MOT Type 1, £46.50 for the membrane and £15.95 for the pegs. The calculator adds a typical UK price range for the slabs, sand and cement, which come from your local merchant.

Next Steps

Put your measurements into the patio calculator and tap "Add to basket" on the MOT Type 1, grano dust, gravel, membrane or pegs, or "Take me there" to see the product first with your measurements already filled in. "Click to share this answer" copies a link that opens the calculator with everything you typed, so you can send it to whoever is helping or keep it for the day the merchant rings back.

If you would like to go over your patio with one of our team, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever laid a patio yourself, and what did you bed the slabs on? Let us know in the comments how deep you went and how it has held up since, we love reading these.

FAQs

How do I work out m² for a patio?

A rectangular patio's area in m² is the length times the width in metres, so 5m by 4m is 20m². For an L shape, split it into two rectangles and add them together, and for a circle use 3.14 x the radius x the radius.

How many 900x600 slabs do I need?

Paving slabs of 900 x 600mm cover 0.54m² each, which is 1.85 slabs per m². A 20m² patio needs 38 slabs to cover the area exactly, or 41 with 10% added for cuts and breakages.

How much sub-base do I need for a patio?

A patio needs 100mm of compacted MOT Type 1 sub-base on firm ground and 150mm on soft or clay ground. At 100mm one 850kg bulk bag covers about 4.4m², so a 20m² patio takes five bags.

Is grano dust or sharp sand better for laying slabs?

Grano dust and sharp sand both make a good slab bed when mixed with cement, because both are coarse and angular enough to lock together and drain. The one to avoid is building sand, which is too fine and holds water.

Can I lay patio slabs on sand only?

Patio slabs on a sand-only bed move and settle over time, because each slab sits on its own with nothing locking it to its neighbours. I would put cement in the bed of any slab patio, even at a lean 8 to 1.

Do I need to add extra for wastage on the sand and cement?

Sand and cement for a patio bed is worked out on an even depth, and an uneven sub-base eats more. The calculator starts on 5% extra for the MOT Type 1, the bed and the gravel together, because hardly any base is dead level. Pick 10% or more for an awkward shape or uneven ground.