Asphalt Calculator
Works out hot-mix asphalt by the tonne for a drive, path or car park from the area and compacted thickness, with the sub-base stone underneath.
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The formula
tonnes = area × compacted thickness × density × (1 + waste) ÷ 1,000 sub-base = area × depth × 1.25 compaction × 2,100 kg/m³
Asphalt is ordered by weight and specified by compacted thickness, so density is the bridge: about 2.4 tonnes per cubic metre compacted. At 50 mm that is 120 kg for every square metre, which is the figure most suppliers quote from.
What to watch out for
Asphalt is unforgiving about timing and about what is underneath it:
- The base does the work. 150 mm of compacted crushed stone under a drive. Asphalt on soft ground cracks in the first winter regardless of thickness.
- Thickness is compacted, not loose. The mix loses about 20% in rolling, so 50 mm finished is laid at around 60 mm.
- It is a race. Below about 85 °C the mix will not compact. Everything is set out, the base is ready and the roller is running before the lorry tips.
- Edges need restraint. A kerb or edging stops the edge crumbling under tyres.
Frequently Asked Questions
How many tonnes of asphalt do I need?
Area × thickness × 2.4 t/m³, plus 5% waste. A 12 × 3 m drive at 50 mm is 1.8 m³, about 4.5 tonnes.
How thick should a tarmac driveway be?
50 mm of surface course over 150 mm of compacted sub-base for a domestic drive. Where lorries turn, 75–100 mm in two layers. Paths can be 40 mm over 100 mm of base.
How much does asphalt weigh per cubic metre?
About 2,400 kg compacted — 2,350 to 2,450 depending on the mix. Loose in the lorry it is less dense, which is why thickness is always specified compacted.