How this calculator works
Ordering the right amount of tarmac matters because hot mix cannot be stored: order too little and you stop mid-job, leaving a cold joint that weakens the surface; order too much and the surplus is wasted once it arrives and cools. This tool takes your area and compacted depth, works out the volume, and converts it to tonnes using the mix density. Once you have the tonnage, price it with the tarmac driveway cost calculator, and remember the surface sits on a separate MOT Type 1 sub-base you order on top of this.
The formula
Volume (m³) = Area (m²) × Depth (m)
Tonnes = Volume (m³) × Density (t/m³)
Order total = Tonnes × (1 + wastage %)
Keep the units consistent: convert a depth in millimetres to metres (÷ 1000) before multiplying. The default density of 2.4 t/m³ is for compacted dense tarmac; open-graded mixes are a little lighter and dense heavy-duty mixes a little heavier.
Worked example
Example — a 50 m² driveway surfaced at 50 mm
- Area:
- 50 m²
- Compacted depth:
- 50 mm = 0.05 m
- Density:
- 2.4 t/m³ (dense tarmac)
- Wastage:
- 8%
- Volume: 50 m² × 0.05 m = 2.5 m³
- Tonnage: 2.5 m³ × 2.4 t/m³ = 6.0 t
- With wastage: 6.0 t × 1.08 = 6.48 t
Tarmac to order: ≈ 6.5 tonnes
Round up to the supplier's delivery increment. A single 50mm layer here would usually be split into a 30mm binder and a 20mm surface course on a real drive.
Reference data
| Compacted depth | Tarmac per m² | Coverage per tonne |
|---|---|---|
| 20 mm | 0.048 t | ≈ 20.8 m² |
| 25 mm | 0.060 t | ≈ 16.7 m² |
| 30 mm | 0.072 t | ≈ 13.9 m² |
| 40 mm | 0.096 t | ≈ 10.4 m² |
| 50 mm | 0.120 t | ≈ 8.3 m² |
| 75 mm | 0.180 t | ≈ 5.6 m² |
Key facts
- Compacted dense tarmac is commonly estimated at 2.4 tonnes per cubic metre (2,400 kg/m³) for material ordering.
- At a 30mm surface course, one tonne of tarmac covers about 14 square metres; at 50mm it covers about 8 square metres.
- Tarmac tonnage is area (m²) × compacted depth (m) × density (t/m³); add 5–10% for wastage and unrecoverable material.
- Hot tarmac is laid roughly 20–25% thicker than the target and rolled down, so always enter the final compacted depth.
Who should use this and when
Anyone placing a tarmac order needs this: contractors converting a takeoff into a delivery, and homeowners checking that a quoted tonnage matches the drive they are paying for. It works equally for driveways, car parks and patch jobs — only the density changes between mixes. Because tarmac is priced and delivered by the tonne, an accurate quantity is what stops both a costly shortfall and a wasted surplus. Pair it with the material tools on the tarmac calculators hub.
Tips and common pitfalls
- Convert millimetres to metres first. Multiplying square metres by a depth in millimetres overstates the volume a thousandfold.
- Enter compacted, not loose, depth. The 2.4 t/m³ density already assumes the material is rolled down.
- Add wastage before ordering. 5–10% covers spillage and the material left in the lorry.
- Don't confuse tarmac with the sub-base. The MOT Type 1 layer underneath is ordered separately and at a different density.
Frequently asked questions
How do you calculate tarmac tonnage?+
Multiply the surface area in square metres by the compacted depth in metres to get the volume in cubic metres, then multiply that volume by the density — about 2.4 tonnes per cubic metre for dense tarmac. So 50 m² at 50mm is 50 × 0.05 × 2.4 = 6 tonnes before wastage.
How much does one tonne of tarmac cover?+
It depends entirely on depth. At a 30mm surface course, one tonne of tarmac covers roughly 14 square metres; at a 50mm layer it covers about 8 square metres; and at a 75mm base course only about 5.5 square metres. Thicker layers use far more material per square metre.
What is the density of tarmac?+
Compacted dense tarmac (hot-mix asphalt) is commonly estimated at about 2.4 tonnes per cubic metre, or 2,400 kg/m³. Open-graded mixes are a little lighter and heavy-duty dense mixes a little heavier, so the calculator lets you adjust the density between roughly 2.3 and 2.5.
Why do I need to add wastage to a tarmac order?+
Always add roughly 5% to 10% extra. This covers variation in the compacted depth, material spilled over edgings while spreading, and the unrecoverable tarmac left cooling in the delivery lorry. Ordering slightly over is cheaper than a second delivery for a small shortfall.
Should I enter loose or compacted depth?+
Enter the final compacted depth you want. Hot tarmac is delivered loose and rolled down, so a contractor lays it around 20–25% thicker than the target to allow for compaction. Using the 2.4 t/m³ compacted density already accounts for that in the tonnage.
Is tarmac tonnage the same as asphalt tonnage?+
For estimating purposes, yes — tarmac is a common UK name for hot-mix asphalt, and both use the same area × depth × density method. The only variable is the density of the specific mix, which the calculator lets you set.
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Estimates & assumptions
- Volume is exact (area × depth); tonnage depends on the mix density, which the calculator defaults to 2.4 t/m³ for compacted dense tarmac.
- Open-graded and specialist mixes differ in density — confirm the figure with your supplier for a precise order.
- The tool sizes the tarmac surface only. The MOT Type 1 sub-base beneath it is a separate calculation.
- Add a wastage allowance; the tool does not automatically account for spillage or delivery minimums.
Estimates are for material planning. Confirm mix density and delivery increments with your supplier before ordering.