Paving is priced per slab and bought by the pack, but it is sold against an area you have to supply, and the two only line up if the joint width and the cutting waste are in the calculation. This draws the patio, works out how many units the pattern actually consumes rather than how many would tile it perfectly, and gives you the two quantities that always get forgotten: the edge restraint that stops it spreading, and the sub-base that stops it sinking.
The patio
Set the shape, then the unit size, the joint and the laying pattern’s waste.
Fine tuning
10% straight, 15% at 45 degrees, 20% for circles and curves.
Result
The joint is part of the unit
This is the thing that catches everyone. A 400 mm slab laid with a 5 mm joint does not cover 0.16 m²; it occupies 0.405 × 0.405 = 0.164 m², because the joint belongs to the unit once it is laid. On a small patio that is a slab or two. On a 40 m² terrace with a wide joint it is thirty units, which is a pack and a half, and it is the difference between finishing on Saturday and waiting until Tuesday.
It works in the other direction too. People who measure a slab, find it is 400 mm, and divide the area by 0.16, are systematically under-ordering, and then discover that the shortfall arrives as a second delivery from a different batch — and batch colour variation on natural stone is visible from across the garden.
Where the waste actually comes from
Cutting waste is mostly perimeter. A rectangle made of whole units has almost none; a circle, where every edge unit is a one-off cut, has a great deal; and a diagonal pattern wastes at both ends of every row regardless of shape. The percentage is a proxy for how many of your units end up as offcuts you cannot use elsewhere.
| Job | Waste | Why |
|---|---|---|
| Rectangle, straight lay | 8 to 10% | Only the boundary row is cut, and offcuts often fit |
| Rectangle, 45 degree lay | 15% | Both ends of every row are cut |
| L shape, straight lay | 12 to 15% | The notch generates offcuts that rarely fit elsewhere |
| Circle or curve | 20% | Every edge unit is a one-off, cut to a radius |
| Mixed sizes, random course | 12% | Pattern flexibility uses more offcuts, but not all |
| Permeable, wide joint | 10% | Joint width is in the unit size, not the waste |
The waste percentage exists so the job finishes. Treating it as a margin you can shave means cutting the last row from whatever is left, which is how patios end up with a row of visibly narrower units against the house.
There is also the damaged-unit allowance. Stone and concrete arrive chipped, and one unit in twenty will have a corner you cannot lay face up. That is what rounding up to whole packs is for, and it is not the same as the cutting waste.
The two quantities nobody orders
Ask anyone who has laid a patio what went wrong and it is rarely the slabs. It is the edge restraint and the base, both of which are separate purchases in different units and both of which are invisible until they fail.
Per metre of perimeter
- Concrete haunching, or a proprietary restraint rail.
- Without it, the perimeter units creep outward within two winters.
- Order off the perimeter, which an L shape does not reduce.
Per cubic metre
- 100 mm compacted for foot traffic, 150 mm for vehicle.
- Type 1 MOT, laid and compacted in layers, not in one.
- Order by volume, allow 30% for compaction.
Per square metre
- Sharp sand or grit sand bedding at about 50 mm.
- Brush-in jointing compound, bought by area covered.
- The most commonly under-ordered item on the list.
The mortar calculator and the grout calculator cover the bedding and jointing side properly, and the tile adhesive calculator is the right one where units are being bonded to a slab rather than bedded on sand. If the patio has a path leading to it, the walkway width calculator settles how wide that needs to be before you pave it.
Measuring the ground, not the drawing
Patio dimensions should come off strings and canes on the actual ground, because the house wall is rarely where the plan says and the boundary rarely runs straight. What matters is the paved area, not the plot it sits in.
- Set out with stringsFour canes and a string line give you the rectangle you intend to build, and let you check it against the house before anything is ordered.
- Check it is squareMeasure both diagonals. If they differ, the paving will not finish square against the wall, and you will cut every unit on one side to hide it.
- Measure the perimeter separatelyEdge restraint is per metre and comes off the outline, not the net area.
- Decide the fallPaving needs 1 in 60 away from the building. It does not change the area, but it changes the setting-out and the edge detail.
- Allow for the notchDrain covers, meter boxes, beds and the corner of the house are all not paving. Measure them and use the L shape rather than hoping they are small.
Do
- Include the joint width in the unit size before dividing.
- Order the edge restraint off the perimeter, not the area.
- Set the waste from the shape and the pattern, not from habit.
- Keep a few units back for repairs; ranges get discontinued.
Avoid
- Dividing the area by the bare slab size and ignoring the joint.
- Assuming a circle wastes the same as a rectangle.
- Ordering sub-base by area rather than volume.
- Laying without restraint because the units feel heavy enough to stay put.
A worked example
A plain rectangular terrace, 600 mm slabs, standard joint, which is the commonest job there is and the one most often under-ordered.
- Area6 m by 4 m is 24.00 m².
- Unit including joint0.40 + 0.005 = 0.405 m, so each unit occupies 0.164 m², not 0.16.
- Units fitted24.00 ÷ 0.164 = 146 units. Using the bare slab size would have said 150, which is four short before any cutting.
- With wasteAt 10% for a straight lay on a rectangle: 161 units to buy.
- The rest of the order20 m of edge restraint and 2.40 m³ of sub-base at 100 mm — about 3.1 m³ once compaction is allowed for.
Slab size changes more than the count
Unit size is usually chosen on appearance, and it quietly decides how the rest of the job goes. Large units lay fast and have fewer joints, which means fewer places for weeds and less jointing compound, but every cut is a big cut and an offcut from a 600 mm slab is either reused immediately or wasted outright. Small units are slower, need more edge restraint per metre because the perimeter units are shorter, and absorb mistakes far more gracefully.
| Unit size | What it does | Where it suits |
|---|---|---|
| 300 × 300 mm | Lays slowly, forgiving on curves, most joints to maintain | Small courtyards, curved edges, mixed-size patio packs |
| 400 × 400 mm | The middle ground; offcuts often reusable | Small to medium rectangular patios |
| 600 × 600 mm | Fast to lay, heavy, every cut is expensive | Large open terraces with straight edges |
| 900 × 600 mm | Two-person handling, dramatic joints, unforgiving | Large contemporary areas laid to a grid |
| Mixed-size pack | Sold by coverage, waste built into the pack | Informal areas where the pattern hides the cuts |
Handling is the hidden cost. A 900 × 600 mm slab at 45 mm thick weighs around 55 kg, which is a two-person lift and a different job from a 400 mm unit one person can carry. On a terrace of any size that is the difference between an afternoon and a weekend, and it is worth deciding before the order rather than on the day.
Laying pattern and what it costs you
The pattern is not decorative-only. It sets the waste percentage, the cutting time and whether the patio can absorb a slightly out-of-square set-out without it showing.
A 45 degree lay is the standard trick when the house wall and the set-out do not agree, because the eye reads the diagonal rather than the wall line. It works, and it costs 15% in waste instead of 8% plus half again in cutting time.
If you are choosing diagonal purely for looks on a square, straight-edged area, budget for both: the extra units and the extra hours.
Permeability, and where the water goes
A patio is an impermeable surface, and where the water goes after that is a design decision with a regulatory edge to it. In much of the UK, new or replaced paving over 5 m² at the front of a property needs to drain to a permeable area or be permeable itself, and the rules differ by location, so check before ordering rather than after laying.
The practical options are a fall to a lawn or border, a channel drain to a soakaway, or permeable paving with an open-graded base. None of them change the area calculation, but the permeable option changes two others: the base becomes deeper and open-graded, and the joint width becomes part of the drainage rather than part of the aesthetic.
Where the numbers stop being true
This assumes a single unit size laid in one pattern. Mixed-size patio packs are sold by area covered and the waste figure is baked into the pack, so the calculation changes: buy the number of packs the coverage says, plus one. Circles and curves are the weak spot — the 20% figure is a working average, and a circle laid with small units wastes less than one laid with large ones, because the cuts are shorter.
The sub-base figure assumes decent ground. On clay, or anywhere that holds water, the depth goes up and so does the volume, and no calculator can tell you what is under your garden. Dig a trial hole before ordering.
Before you order
- Set out on the ground with strings and check the diagonals.
- Add the joint width to the unit size before dividing.
- Choose the waste from the shape and the pattern.
- Order edge restraint by the metre, off the perimeter.
- Order sub-base by volume and allow for compaction.
- Keep four or five units back for future repairs.
How the arithmetic works
Area comes from the shape: width × length, or outline minus notch, or π × (diameter ÷ 2)². Each unit occupies (unit width + joint) × (unit length + joint), with the joint converted from millimetres. Units fitted is the area divided by that figure, and units to buy is that number × (1 + waste ÷ 100), rounded up because a part pack is still a pack.
Edge restraint is the perimeter: 2 × (width + length) for a rectangle and for an L shape alike, π × diameter for a circle. Sub-base is area × 0.1 for a 100 mm depth, and needs about 30% added for compaction when you convert it to tonnage.
Questions people ask
Why does the L shape not reduce the edging length?
Because the notch removes two outer edges and adds two inner ones of the same total length. The perimeter is that of the full outline, so the restraint is ordered off the outline even though the paving is ordered off the net area.
How much waste on a circle, really?
About 20%, and more if the units are large. Every perimeter unit is cut to a radius and no two are the same, so none of them can be reused at the opposite side.
Do I need a sub-base if the ground is already hard?
Yes. Hard clay is the worst case, because it holds water, freezes and heaves. The sub-base is there to drain and to spread load, not to stop the ground moving.
Can I lay directly on sand over soil?
It works for about a season. Soil moves, sand migrates into it, and the units settle unevenly. The base is the cheapest part of the job to get right and the most expensive to retrofit.
What about drainage?
Water has to go somewhere. A 1 in 60 fall to a permeable edge, a soakaway or a channel drain; and if the patio is over about 40 m² next to a house, check whether it needs planning consent as impermeable surfacing, because in many places it does.