Tarmac Calculator UK
Work out how much tarmac you actually need before you order, in tonnes, so you're not left short halfway through a pour or paying for a part-load you didn't use. This UK tarmac calculator turns your driveway, path or car-park dimensions into a compacted tonnage, factors in the laying depth, and adds a wastage allowance for edges and cutting. It also has a cold-lay mode that converts the same job into 25 kg bags for small repairs and patch jobs. Everything is metric-first, with an imperial toggle if you're measuring in feet, and an optional cost estimate for the material supply.
Read the guide: How Much Tarmac Do I Need? →
Depth is always in mm regardless of the measurement toggle above.
Enter your area and depth to see how much tarmac you need.
| Laying depth | Volume per m² | Tonnes per m² | Coverage per tonne |
|---|---|---|---|
| 40 mm | 0.040 m³ | 0.094 t | ~10.6 m² |
| 50 mm | 0.050 m³ | 0.118 t | ~8.5 m² |
| 60 mm | 0.060 m³ | 0.142 t | ~7.1 m² |
| 70 mm | 0.070 m³ | 0.165 t | ~6.1 m² |
| 100 mm | 0.100 m³ | 0.236 t | ~4.2 m² |
Based on a compacted density of 2.36 t/m³. Different mixes and compaction vary this slightly.
How this calculator works
It takes your area (from length x width, or a direct area entry), multiplies by the laying depth to get the compacted volume in cubic metres, then converts that to tonnes using a compacted asphalt density of 2.36 tonnes per cubic metre. Your wastage allowance is added on top, because you always lose a little to edges, cutting-in and the bit that goes cold before it's rolled. In hot-lay mode you get a tonnage to order from your local asphalt plant; in cold-lay mode the same tonnage is converted into 25 kg bags of the sort you buy for pothole and path repairs.
Enter the area and the depth and you get an order tonnage with waste already built in — depth is the number people forget, and it changes everything.
Formulas used
- Area = length x width (or entered directly)
- Volume (m³) = area x (depth in mm / 1000)
- Tonnes = volume x 2.36 (compacted asphalt density, t/m³)
- With wastage = tonnes x (1 + waste %)
- Coverage per tonne = 1 / (depth in m x 2.36)
- Cold-lay bags = ceiling of (tonnes needed x 1000 / bag weight in kg)
Tonnage is driven by depth as much as area — doubling the depth doubles the tarmac, so get the depth right before you trust the number.
Worked example — driveway (hot-lay)
A driveway measuring 10 m x 4 m = 40 m², laid as a 60 mm total build-up (a 40 mm binder course with a 20 mm surface course), on a compacted Type 1 sub-base.
- Volume = 40 x 0.06 = 2.40 m³
- Tonnes = 2.40 x 2.36 = 5.66 t
- With 10% wastage: 5.66 x 1.10 = 6.23 t — order around 6.5 tonnes
- Coverage check: at 60 mm you get ~7.1 m² per tonne, and 40 / 7.1 = 5.6 t before waste — matches.
A two-car driveway at 60 mm is roughly six tonnes plus waste — order to the nearest half-tonne your supplier delivers in, and round up, never down.
Worked example — path repair (cold-lay bags)
A garden path repair measuring 3 m x 1 m = 3 m², filled to a 25 mm finished depth using 25 kg cold-lay bags.
- Volume = 3 x 0.025 = 0.075 m³
- Tonnes = 0.075 x 2.36 = 0.177 t (177 kg)
- With 10% wastage: 0.177 x 1.10 = 0.195 t — 8 x 25 kg bags
For small patch jobs, cold-lay bags from Wickes, B&Q or Travis Perkins are the sensible route — hot-lay only makes sense once you're into whole tonnes.
Depth — how thick should tarmac be?
For a domestic driveway the tarmac itself is usually 50 to 70 mm of compacted material, most often a 40 to 50 mm binder (base) course with a 20 to 25 mm surface course on top. A footpath or light patio can drop to 40 mm. What sits underneath matters just as much: a 100 to 150 mm compacted Type 1 (MOT) sub-base is what actually carries the load. This tool sizes the tarmac only — use the Gravel & Sub-Base Calculator for the Type 1 layer.
The tarmac is the thin bit on top — the sub-base is what stops it cracking, so budget for both.
UK standards and a note on planning
Asphalt surfacing in the UK is specified under BS 594987 (transport, laying and compaction of asphalt) and the Specification for Highway Works (SHW) Series 900 for bituminous bound layers; the sub-base beneath follows SHW Clause 803 for Type 1 unbound material. On planning: since the 2008 permitted-development changes, a new or replacement front driveway over 5 m² that drains to a hard impermeable surface needs planning permission unless it's permeable or drains to a permeable area within your boundary. Tarmac is impermeable, so plan the drainage.
Lay to BS 594987 over a proper Type 1 base, and on a front driveway route the water to a border or soakaway to stay inside permitted development.
Frequently asked questions
Related tools
- Driveway Cost Calculator UK — full supply-and-fit price for tarmac, block paving, resin and more
- Gravel & Sub-Base Calculator — Type 1 MOT sub-base under a tarmac driveway
- Gravel Calculator UK — loose gravel volume, tonnes and bulk bags
- Paving Calculator UK — slabs or block pavers as an alternative surface
Guides worth reading
- How Much Tarmac Do I Need? A UK Guide — depth, tonnage and cost worked through
Disclaimer: This calculator is for planning and estimating only. Actual tonnage varies with mix type, compaction, laying depth, sub-base and site conditions. Compacted asphalt density is taken as 2.36 t/m³; your supplier's mix may differ. Always confirm the final quantity with your asphalt plant or contractor before ordering.