Tarmac Calculator

How tarmac quantities are worked out

Multiply area by depth for volume, then convert to weight using about 2.4 tonnes per cubic metre for asphalt. A domestic driveway is typically laid 50mm thick over a compacted stone sub-base.

Worked example: a 40 m² driveway at 50 mm

Area × depth gives the volume: 40 m² × 0.05 m = 2 m³. Asphalt weighs roughly 2.4 tonnes per cubic metre, so that is about 4.8 tonnes. Because the layer compacts and some mix is lost around edges and on the wagon, order 5 tonnes — a full load is usually cheaper than a part load plus a second delivery.

Don't forget the sub-base

A tarmac surface is only as good as what sits under it. For a domestic driveway you want 100–150 mm of compacted Type 1 MOT stone. On the same 40 m² that is 4–6 m³, or roughly 8–10 tonnes of stone, laid in two passes and compacted with a plate or roller before the tarmac goes down. Paths carrying only foot traffic can drop to 75 mm.

How it usually arrives

Loose tarmac comes by tipper lorry in 6–20 tonne loads; you cannot order a single tonne loose in most areas. For patching jobs under about 5 m², 25 kg bags of cold-lay tarmac are the practical option — expect to use about 4–5 bags per square metre at 50 mm. Hot-lay tarmac must be laid the day it arrives, so book the surface course and the labour for the same day.

Five mistakes that shorten a driveway's life

1. Laying over soft ground with no sub-base, which settles and cracks within a season. 2. Going under 40 mm for a driveway — it ravels under tyres. 3. No fall: aim for about 1:60 so water runs off rather than pooling. 4. Laying in rain or when the temperature is near freezing, which stops the mix binding. 5. No edge restraint, so the edges creep outwards. A concrete kerb or timber edging holds the surface together.