Patio calculator
Whole slabs from the slab size you enter, and the bed and sub-base in bulk bags or 25 kg bags, by the makers' guides.
How the number is worked out, a worked example and questions
How the number is worked out
- Measure the patio. Its length and width, or draw its shape if it is not a rectangle. The area is the area to be paved.
- Enter the slab and its joint. One slab covers its own size plus one joint each way, so a 600 × 600 mm slab with a 10 mm joint covers 0.3721 m². The area divided by that is the slabs, a visible percentage is added once for cuts, and the count is rounded up once to whole slabs; with the number a pack holds, to whole packs.
- The bed. The area times the bed's depth is the mortar, split into sharp sand and cement by volume at the mix the makers' guides give: 1 part cement to 3 or 4 parts sharp sand. The sand is then whole bulk bags or 25 kg bags, as you choose, and the cement whole 25 kg bags, from typical figures for what each holds that are named on the line, or from the figures on the bags you buy.
- The rest are checks with the area beside them. The primer for the backs of the slabs and the jointing compound are counted in tubs or pails when you enter what one covers. The sub-base is counted in tonnes when you enter its depth and your supplier's density, as the concrete calculator does.
This is a material quantity, not a design. It decides no fall, no level against a damp-proof course, no sub-base depth and no drainage: those are the makers' guides' and a designer's.
A worked example
A patio 4 m × 3 m is 12 m². Slabs 600 × 600 mm with a 10 mm joint, 10% for cuts, and a 30 mm bed at 1 part cement to 4 parts sharp sand. The slab and the 10% are generic example figures, not a product; the joint, the bed and the mix are the makers' guides' figures.
| Working | Figure | Result |
|---|---|---|
| Patio area | 4 m × 3 m | 12 m² |
| One slab with its joint | 600 × 600 mm, 10 mm joint | 0.3721 m² |
| Slabs before the allowance | 12 ÷ 0.3721 | 32.249395 |
| With 10% for cuts, rounded up | 35.474335 | 36 slabs |
| Mortar bed at 30 mm | 30 mm | 0.36 m³ |
| Sharp sand, 4 of 5 parts | 1 : 4 | 0.288 m³ |
| Sand in bulk bags of about 0.5 m³, or 25 kg bags of about 0.015 m³ | 1 bulk bag | 20 bags |
| Cement, 1 of 5 parts, in 25 kg bags of about 17 L | 0.072 m³ | 5 bags |
| Sub-base at the guides' 100 mm, in bulk bags of about 0.45 m³ | 1.2 m³ | 3 bulk bags |
| Primer at about 12 m² a tub; jointing at about 7 m² a pail | 1 tub | 2 pails |
What a bulk bag or a 25 kg bag holds, what a bag of cement fills and what a tub or pail covers are typical figures, each named for where it was read on the line it makes: a supplier's page for a bulk bag of sharp sand (about 0.5 m³ for 800 to 850 kg), a supplier's own words for its bulk bag of Type 1 (about 4 to 5 m² at 100 mm), the density figure trade calculators use for cement (a 25 kg bag about 17 L), and two products' stated coverages. Enter the figure from the bag, tub or pail you buy and the count follows it instead.
Stop and check first
- The fall and the damp-proof course. Both guides read lay a patio with a fall away from the house (one says around 1 in 60, the other 1 in 80 along and 1 in 40 across) and its surface at least 150 mm below the damp-proof course. The calculator decides neither.
- What is under it. A compacted Type 1 sub-base, which both guides give at 100 mm as the usual minimum, laid in layers with a plate compactor; the ground and the use decide the depth. Sub-base is sold by weight: enter the depth and your supplier's density and it is counted in tonnes.
- The bed and the primer. A full bed of mortar under the whole slab, not dabs, and a slurry primer on the back of every slab: both guides say so. Sharp sand, not building sand; ordinary Portland cement, not white.
- Cutting. Both guides cut with a water-fed diamond blade and the protective equipment for it: the dust from cutting slabs is a hazard. A slab cut by more than a quarter is taken from a corner.
- The weather. One guide stops laying below 3 °C on a falling thermometer or 1 °C on a rising one, and covers unfinished work and stockpiles.
Questions
How deep should the mortar bed be, and what mix?
The two makers' guides read give a full bed of 30 to 40 mm before tamping, at 1 part cement to 3 or 4 parts sharp sand. The calculator starts at 30 mm and 1 : 4, both editable, and counts the sand and cement by volume from the figures you leave in.
How many bags of cement is that?
The bed's cement share is a volume; a 25 kg bag fills about 17 litres by the density figure trade calculators use, as no maker's sheet read gives a bag's volume. The calculator counts whole bags from that typical figure, says so on the line, and uses the litres from your bag instead if you enter them.
Bulk bags or 25 kg bags?
Sharp sand and Type 1 sub-base come both ways. A bulk bag of about 800 kg is delivered on a pallet and holds about 0.5 m³ of sand (a supplier's own figure) or covers about 4 to 5 m² of sub-base at 100 mm (a supplier's own words); a 25 kg bag is about a bucket and a half. Choose on the patio step: the counts follow the choice.
How much jointing compound?
The pail says what it covers at its own joint: one 15 kg tub is described as covering 7 m², and another maker's 15 kg pail covers about 17 to 21 m² of 450 mm slabs with 5 × 20 mm joints, by its data sheet, and is not for porcelain. The calculator counts pails from the 7 m² figure, says it is typical, and uses your pail's coverage for your joint if you enter it.
Can I lay slabs on sand alone?
Not by the guides read: both lay flags on a full mortar bed with a primer on the back, and one says not to butt-joint the flags or brush dry sand into the joints. Block paving on a sand bed is a different job, and this calculator does not cover it.
The patio is not a rectangle. What do I do?
Draw its shape: the calculator takes a drawn patio and uses its area. Or split it into rectangles and plan each on its own.