Road Base and Sub-Base Calculator
Works out MOT Type 1 or equivalent sub-base. Enter the finished compacted depth you want — the loose volume you have to order is larger, and that allowance is already included.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × compaction × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Sub-base runs about 2100 kg per cubic metre loose, so a cubic metre weighs roughly 2.10 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
Sub-base is the one material where ordering the obvious volume leaves you short:
- It compacts by about 20–25%. 150 mm of finished depth needs closer to 190 mm of loose material. That allowance is applied here.
- Compact in layers. Maximum 75 mm per pass with a plate compactor. A single 150 mm layer compacts at the top and stays loose underneath, which is exactly where you do not want it loose.
- Depth follows the load. 100 mm under a patio or footpath, 150 mm under a car drive, 200 mm or more where delivery lorries will turn.
Frequently Asked Questions
How much sub-base do I need for a driveway?
At 150 mm compacted, a 10 × 3 m drive needs about 4.5 m³ finished — roughly 5.6 m³ loose once compaction is allowed for, or 6.2 m³ with a 10% waste allowance on top. That is close to 13 tonnes.
How much does MOT Type 1 compact?
Around 20–25% by volume. Order about a quarter more loose material than the finished depth suggests, which this calculator does for you.
How thick should a sub-base be?
100 mm under a patio or path, 150 mm under a domestic driveway, 200 mm or more where heavy vehicles will turn. Soft or clay ground needs more, plus a geotextile membrane underneath.