How much gravel do I need?

Multiply area by depth. A 30 m² path at 50 mm needs 1.5 m³, which is about 2.4 tonnes. Suppliers quote loose aggregate by the tonne and bulk bag, so both figures are worth having before you ring round.

For a domestic drive, 100–150 mm compacted is the usual specification, laid in layers and whacked between each one rather than in a single lift. Soft or clay ground takes more. Depth here matters far more than the surface above it.

A bulk bag is normally around 800 kg, so roughly half a cubic metre of aggregate. At 50 mm depth that covers about 10 m². Bag weights vary between suppliers, so check rather than assume when the order is close to a whole-bag boundary.

Not quite. Hardcore is a general term for crushed fill, which can be recycled material of variable quality. MOT Type 1 is a graded specification with a defined particle range that compacts to a stable surface, which is why it is what gets specified under drives and slabs.