Construction
How to estimate materials for a patio, path or driveway
Most first-time patios fail on the layers you can't see. Here's how to work out what to order, from the sub-base up.
· 8 min read
Almost every guide to laying a patio starts with the pavers. That is the wrong end of the job. The pavers are the part you see, but the part that decides whether the patio is still flat in five years is the 6–10 inches of compacted material underneath it. Get the quantities wrong there and you either run short mid-job — which means a half-compacted base sitting overnight in the rain — or you over-order by a couple of tonnes and pay to have it taken away again.
This guide walks through the estimate the way you actually do it on site: from the bottom up, one layer at a time.
Start with the excavation, not the surface
Before any quantity makes sense you need to know how deep you are digging. The depth is the sum of every layer plus the paver itself:
- Sub-base (crushed stone / MOT Type 1): 4 inches for a foot-traffic patio or path, 6–8 inches for a driveway that will carry a car.
- Bedding layer (sharp sand or grit): 1 inch, screeded level. Resist the temptation to make this thicker to "fix" an uneven base — a deep sand layer will rut.
- The paver itself: typically 2 inches for patio slabs, up to 3 inches for driveway blocks.
So a straightforward patio is roughly 4 + 1 + 2 = 7 inches of excavation. A driveway is closer to 11 inches. That number matters twice: it tells you how much spoil you are removing, and it is the depth figure every material calculation below depends on.
One thing that catches people out: you are digging down from the finished level, and the finished level is not the same across the whole area. Any surface against a building needs a fall of about 1 inch per 6 feet running away from the wall so water drains off, and it must finish at least 6 inches below the damp-proof course. Work out your finished levels first, then dig to a consistent depth below them.
Working out volume: the one formula you need
Every loose material — sub-base, sand, gravel, concrete — is sold by volume, and the maths is always the same. The only trap is that your depth is in inches while your area is in feet, so the depth has to be converted before you multiply:
- Cubic feet = length (ft) × width (ft) × (depth in ÷ 12)
- Cubic yards = cubic feet ÷ 27
Take a 12 ft × 10 ft patio needing a 4-inch sub-base. That is 12 × 10 × (4 ÷ 12) = 40 cubic feet, or 40 ÷ 27 = 1.48 cubic yards of crushed stone. The same 120 ft² area needs about 0.37 cubic yards of sand for a 1-inch bedding layer.
If you are buying by weight rather than volume — which is normal for aggregate delivered loose — you need one more step, because suppliers quote tonnes. Crushed stone runs roughly 1.4 US tons per cubic yard, so that 1.48 yards is a little over 2 tons. Densities vary by material and moisture, so treat tonnage as an estimate and confirm with the supplier before ordering a full load.
Counting pavers
Pavers are a count, not a volume, and the arithmetic is a division: your area divided by the area of one unit. The units are the awkward part, since paver dimensions come in inches and patios are measured in feet.
- Paver area (ft²) = (paver length in × paver width in) ÷ 144
- Pavers needed = patio area ÷ paver area, rounded up
A 12 × 12 inch paver is exactly 1 ft², which makes it the easy case — a 120 ft² patio takes 120 of them. Switch to a 4 × 8 inch block paver and each covers 0.22 ft², so the same patio needs about 540. Smaller units mean more pieces, more joints and considerably more laying time.
The waste allowance is not optional
Every material list needs a margin on top, and how much depends on the shape of the job rather than the material:
| Layout | Add | Why |
|---|---|---|
| Straight runs, square edges | 5–10% | Minimal cutting; mostly breakage and a few spares |
| Diagonal or herringbone | 15% | Every edge course is a cut, and offcuts are rarely reusable |
| Curves, circles, borders | 15–20% | Radius cuts waste most of the unit they come from |
For loose aggregate the reasoning is different but the answer is similar. A dug-out area is never perfectly flat, so the real volume is always a little more than the geometry says, and compaction closes up the gaps — meaning the loose material you order settles into a smaller finished depth. Around 10% covers both.
There is a second, non-obvious reason to order spare pavers: colour. Concrete and clay units are made in batches, and batches differ. If you crack a paver in three years and go back for one replacement, the odds of matching the original run are poor. Keeping half a dozen spares in the shed is the cheapest insurance on the whole job.
Estimating cost, not just quantity
Once the quantities are settled, the cost estimate is straightforward multiplication — quantity × unit price — but be clear about which unit each supplier is quoting. Aggregate is priced per tonne or per cubic yard, sand often per bulk bag, pavers per unit or per square metre. Mixing the units up is the single most common way a materials budget goes wrong.
A few costs are easy to forget entirely when you are focused on the big-ticket items: edge restraints and the concrete to haunch them, jointing sand or a polymeric equivalent, weed membrane, plate-compactor hire, and skip hire for the spoil. On a small patio the spoil alone can be several tonnes — a 12 × 10 ft area dug 7 inches deep produces about 2.6 cubic yards of soil to get rid of, which is a skip in its own right.
A worked example, end to end
A 12 ft × 10 ft (120 ft²) patio, 12 × 12 inch slabs, straight layout, 10% waste:
- Excavation: 7 inches deep → 70 ft³ → about 2.6 cubic yards of spoil
- Sub-base: 4 inches → 40 ft³ → 1.48 cubic yards → order 1.6 cubic yards (~2.3 tons) with waste
- Bedding sand: 1 inch → 10 ft³ → 0.37 cubic yards → order 0.4 cubic yards
- Pavers: 120 units × 1.10 = 132 pavers
That is the whole estimate. Notice that the pavers — the part most people start with — are the last line, and the base material outweighs them several times over by volume and by delivery cost.
The mistakes that actually cost money
Skimping on the sub-base. It is invisible, it is the least interesting material to pay for, and it is the only layer that determines whether the patio stays flat. Two inches of stone under a driveway will fail, and it will fail after the pavers are laid.
Compacting in one go. A plate compactor only densifies the top few inches properly. Sub-base should go in and be compacted in two lifts of around 2–3 inches rather than one 5-inch dump. This does not change your material estimate, but it does change whether that material does its job.
Measuring the area but not the levels. An area calculation assumes a consistent depth. If one corner of the site sits 3 inches lower than the other, the real volume can be meaningfully higher than the arithmetic suggested. Take levels at several points and estimate from the average depth, not the shallowest one.
Ordering to the exact number. A second delivery for a quarter-tonne of stone often costs more in delivery charges than the material itself, and it stops the job for a day. The waste allowance is not padding — it is the cheapest scheduling insurance available.