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How many paving slabs do I need for a patio?

Measure the patio area, add 5-10% wastage for square, straight-laid surfaces and 10-15% for irregular edges or a diagonal pattern — and remember a fall away from the house of 1-2 cm per metre.

Step 1: The area — measure the finished patio size

The patio area is the surface that needs paving, measured to the finished size. If the patio is rectangular: length × width.

Example: The patio is to be 4.00 m × 6.00 m = 24.00 m².

If the patio isn't rectangular, break it down into rectangles and triangles and add them up. For a triangle where you know all three sides, use Heron's formula: s = (a + b + c) ÷ 2, and area = the square root of (s × (s − a) × (s − b) × (s − c)). If the patio has a curve, measure the curve's chord and its rise, and estimate the area of the curved segment as roughly (2 ÷ 3) × chord × rise.

Be sure to measure the patio you want, not the existing one — most patio projects extend the surface. And measure from the house's foundation wall, not from the roof overhang: the overhang can easily stick out 40-60 cm further than the wall, and the patio needs to go right up to the wall.

Step 2: Wastage outdoors

Outdoors, wastage is often lower than in a bathroom, because there are fewer pipe penetrations and corners. On the other hand, the edges are more often irregular, and the pavers are thicker and harder to cut cleanly.

• Square patio, straight lay: 5-8%.
• Patio with irregular edges, steps or flower beds: 10-12%.
• Diagonal lay or bonded pattern: 12-15%.
• Large format over 60 × 60 cm: 15% — a single badly cut 60 × 120 cm paver costs you 0.72 m² in one go.
• Curved edges: add 5 percentage points on top. Every slab along the curve is cut at its own angle.

Example: A square patio of 24.00 m² with 8% wastage: 24.00 × 1.08 = 25.92 m². If a pack covers 1.44 m², that's 25.92 ÷ 1.44 = exactly 18 packs → buy 19 packs, so you have a reserve. Outdoor pavers are frequently discontinued from the range, and a cracked slab three winters on can otherwise not be replaced without a colour mismatch.

The joint width is wider outdoors

Outdoors, the joints need to absorb movement from temperature swings, and they're therefore wider than indoors. Patio slabs laid in mortar or on paving adhesive over a concrete base typically have a 5-10 mm joint; pavers laid on gravel or crushed stone dust typically 3-8 mm; and natural stone with an uneven edge can require 10-15 mm.

The wider joint has two effects on the order. First, fewer slabs fit into a square metre: a 60 × 60 cm paver with an 8 mm joint occupies 0.608 × 0.608 = 0.3697 m², i.e. 2.71 slabs per m², against 2.75 at a 3 mm joint. The difference is small, but it goes the way people don't expect. Second, grout consumption rises noticeably — allow for several kilograms of jointing material per square metre with a wide outdoor joint, where indoors a few hundred grams is enough for a narrow one.

The supplier's instructions determine the final joint width. Some outdoor pavers have a minimum joint stated, precisely because they have calibrated dimensions with a tolerance that needs to be absorbed in the joint.

The fall: 1-2 cm per metre away from the house

A patio needs a fall so rainwater runs away from the house and not in towards the foundation wall. The main rule is 1-2 cm per metre, i.e. 1-2%.

Working it out: A patio that's 4.00 m deep measured away from the house needs, at 1.5 cm per metre, to fall 4.00 × 1.5 = 6 cm from the edge at the house to the outer edge. You need to know this figure before deciding the patio's finished level at the door — and the door typically needs a certain height above the finished patio, so water and snow don't sit against the frame. How much depends on the construction and on whether there's drainage; ask your contractor if you're unsure.

The fall doesn't meaningfully change the number of slabs. A surface with a 1.5% slope is only about a hundredth of a percent larger than the flat surface — it disappears in the rounding. But the fall changes the build-up beneath the slabs, and that's where the work lies.

What's under the slabs — what else to work out

The same measurements that give you the slab count also give you the quantities in the layers underneath. Taking the 4.00 × 6.00 m = 24 m² patio as the example:

• Sub-base of compacted aggregate: typically 10-20 cm compacted. 24 m² × 0.15 m = 3.6 m³ → roughly 6-7 tonnes (aggregate typically weighs 1.7-1.9 tonnes per m³).
• Levelling layer of crushed stone dust: typically 3-5 cm. 24 m² × 0.04 m = 0.96 m³ → roughly 1.6-1.8 tonnes.
• Concrete base (if the slabs are to be adhesive-fixed): typically 8-12 cm reinforced concrete — a job for a professional.
• Edge restraint: the patio's free perimeter in running metres. If one 6 m side runs against the house, there's 4 + 6 + 4 = 14 m of free edge to restrain.
• Drainage: channel, soakaway or connection — it depends on the site and on what's permitted there.

Note that the aggregate is the heaviest item in more than one sense: six or seven tonnes has to be brought into the garden, and access to the back garden often decides the price of a patio project more than the slabs themselves do. A patio a mini digger can reach is a different job from one where everything has to be carried through a narrow garden gate.

What it comes to

A patio project can cost wildly different amounts, depending on what's underneath the slabs. Concrete slabs laid on gravel are the cheap end; ceramic pavers fixed with adhesive on a reinforced concrete slab are the expensive end, and the gap between the two is large — concrete under the slabs is a completely different job from gravel.

The slabs themselves are rarely the biggest item. The material price per square metre also spans a wide range, from ordinary concrete slabs at the low end to natural stone at the high end, where a square metre can easily cost many times as much. But on a finished patio, it's the build-up and the labour hours that carry the bill.

So don't use a table to budget. Use your own measurements — area, edge metres and cubic metres — and ask for a written quote based on exactly those. Then you can compare two quotes, because they're pricing the same thing.

Measure the patio before you order

Slab count, aggregate volume and edge metres all come from the same measurements: the patio's length, width and shape, plus the depth away from the house for the fall calculation. If the patio is irregular — and most are, once they follow a corner of the house and a boundary line — triangulate it and measure the diagonals rather than assuming the corners are 90°.

If you have a measured floor plan of the house, you know the house's facade lengths and corners, and the patio's outline can be set out relative to them. HouseSense produces that plan by scanning the home with the iPhone's LiDAR, or by you drawing it on graph paper with your finger and typing in the measured centimetres. The patio surface itself and the fall, though, you need to measure outside with a tape measure and a string level: outdoor terrain isn't part of an indoor takeoff.