Gravel Calculator

How to Calculate Gravel for Paths, Driveways and Sub-bases

Gravel is used for two very different purposes: as a structural sub-base under paving, and as a decorative surface for paths and driveways. The calculation method is the same for both, but the depth and gravel type differ. This guide covers both scenarios.

The Gravel Formula

Tonnes needed = Area (sqm) x Depth (m) x Density (tonnes/m3)

Most gravel types have a density of approximately 1.8 tonnes per cubic metre when compacted. For loose decorative gravel, use 1.5 tonnes/m3.

Example: 20 sqm driveway with 150mm sub-base:

20 x 0.15 x 1.8 = 5.4 tonnes

Gravel Depth Guide

These figures assume compacted density (1.8 t/m3). Order 10% extra to account for compaction loss and spreading variations.

Gravel Types and Costs

Buying by the tonne is always cheaper than buying bags. A tonne of pea gravel costs EUR 35-55 loose delivered, compared to EUR 80-120 if bought as 25 kg bags (40 bags = 1 tonne).

Bags vs Bulk Delivery

For projects needing more than 500 kg, bulk delivery is significantly cheaper and saves hours of bag-opening. Check access for the delivery lorry — most tip directly from the truck and need a hard surface within reach of the chute.

Compaction and Settlement

Loose gravel settles by 15-20% after compaction. If you need a finished depth of 100mm, order enough for 120mm depth. For MOT Type 1 sub-bases:

1. Spread in layers of no more than 50mm
2. Compact each layer with a plate compactor (hire for EUR 30-60/day)
3. Check levels with a spirit level and straight edge
4. The finished surface should be firm enough to walk on without sinking

Decorative gravel does not compact — it remains loose. Top up by 10-15% annually as stones migrate or sink into the ground. A weed membrane underneath prevents mixing with the soil. Use our paving calculator if you are laying stones on top of a gravel sub-base.