Sub-base Calculator
Estimate MOT Type 1 tonnage and compaction.
Sub-base Order
The Foundation Layer: UK Sub-base Guide
The Compaction Rule
Every solid surface—whether it's a block-paved driveway, a porcelain patio, or a garden shed base—is only as good as the sub-base beneath it. Without a compacted layer of Type 1 MOT, your surface will eventually sink and crack.
Our **sub-base calculator** automatically adds a **20% compaction allowance**. This accounts for the fact that loose stone takes up more room in a bag than it does once it has been hammered into place by a heavy wacker plate.
Depth Requirements
Geotextile Membranes
Before you pour your Type 1 MOT, you should lay a **non-woven geotextile membrane** (often called 'Terram') over the soil.
This membrane prevents the expensive sub-base stone from being pushed down into the soft mud over time. It acts as a separator, ensuring the stone stays clean and the interlocking structure remains intact.
Loose Density
One cubic metre (m³) of loose Type 1 MOT weighs approximately **1.8 to 1.9 tonnes**. Once fully compacted, that same cubic metre will weigh approximately **2.2 tonnes**. This is why the volume seems to 'disappear' during installation.
Permeable Paving
If you are building a driveway without a drainage channel (SuDS compliant), you cannot use standard Type 1 MOT. You must use **Type 3** or **4/20mm clean stone**, which allows water to pass through into the ground.
Pro Tip: The String Line
When digging out your area, don't just guess the depth. Use a string line across the area at the final finished height. Measure down from the string to ensure you have consistent depth everywhere. An extra 25mm of depth over a 50m² driveway can cost you an extra 3 bulk bags of stone!
Sub-base & MOT FAQ
Project Disclaimer
Sub-base requirements vary based on the underlying soil (CBR value). If you are building on soft clay or peat, you may need a deeper sub-base or a layer of 'capping' stone. Always consult with a groundwork professional for heavy-duty driveway installations.