Slabs are counted, not measured
Paving is bought as a number of slabs, so the area only matters as a route to that number. Put the area of one slab in and this counts them.
The common sizes, as area: 600 x 600 is 0.36, 450 x 450 is 0.20, 600 x 300 is 0.18, and 900 x 600 is 0.54. If yours is not on that list, multiply the two sides in metres.
Some ranges are sold as packs covering a set area in mixed sizes, usually natural stone in a random pattern. For those, put the pack coverage in instead and the answer is packs.
The two things that decide whether a patio lasts
The sub-base, which is most of the work
Slabs do not hold themselves up. Under them wants around 100mm of compacted MOT Type 1 for a patio, and 150mm or more for anything a vehicle drives on, on top of properly dug-out and firm ground.
On a 20 square metre patio that is 2 cubic metres of Type 1, which is roughly 4 tonnes. It is usually the single biggest cost and the largest part of the labour, and it is the bit people leave out when they price a job in their head.
Compact it in layers of no more than 75mm with a plate compactor, not all at once. A 150mm layer whacked from the top is loose underneath and settles later.
A full mortar bed, not dabs
Every slab goes on a solid, continuous bed of mortar. Not five dabs, one in each corner and one in the middle, however often you see it done.
Dabs leave voids under the slab. Water gets into the voids, freezes, expands, and the slab cracks or starts rocking. It is the most common reason a patio fails within a few years, and it cannot be repaired without lifting and relaying.
Fall and the damp proof course
A patio has to shed water, and it has to shed it away from the house. The standard fall is 1 in 80, which is about 12mm for every metre. Across a 4 metre patio that is 50mm of drop from the wall to the far edge.
Less than that and water sits in puddles. Much more and furniture rocks and it feels wrong underfoot.
The other rule matters more: the finished paving must sit at least 150mm below the damp proof course, which is the visible line of engineering brick or plastic strip two courses up from the ground. Pave above it and rain splashes and rising damp bridge straight into the wall. It is one of the most expensive mistakes on this page to undo.
Cutting and setting out
Set out from the house wall and work outwards, so the full slabs are against the building and the cuts finish at the open edge where they are least visible.
Dry lay a run in both directions first. If you finish with a 40mm sliver against a wall, moving the whole grid half a slab fixes it, and that decision is free before any mortar is mixed.
Cut with a disc cutter or a splitter rather than a bolster where you can. Natural stone in particular breaks unpredictably along its own bedding planes.
Jointing
Joints get filled after the slabs have set, either brushed in with a dry mix or with a resin-based jointing compound. Resin costs considerably more and lasts far longer, and it is the difference between weeding a patio every spring and not.
Whichever you use, the joint width has to be consistent. Butting slabs tight leaves nothing for movement and they spall at the edges.
Common questions
How many paving slabs do I need?
Measure the area, add wastage for cuts, then divide by the area of one slab and round up. A 20 square metre patio of 600 by 600 slabs, which are 0.36 each, needs 61 slabs at 10 per cent wastage.
How much wastage should I allow for paving?
Ten per cent for a square or rectangular patio. Fifteen if it has curves, steps or anything to cut around. Natural stone in random sizes wants a bit more again, because you are fitting shapes rather than laying a grid.
What fall does a patio need for drainage?
One in eighty, which is about 12mm per metre, running away from the building. On a 4 metre patio that is 50mm from the house to the far edge. It also has to finish at least 150mm below the damp proof course.
How much sub-base do I need under a patio?
Around 100mm of compacted MOT Type 1 for a patio, and 150mm or more for a driveway that takes vehicles. On a 20 square metre patio that is 2 cubic metres, roughly 4 tonnes, and it is usually the biggest single cost.
Should slabs be laid on a full mortar bed or on dabs?
A full bed, always. Laying on five dabs of mortar leaves voids underneath, water gets in and freezes, and the slabs crack or rock. More patios fail this way within a few years than for any other reason.