How Gravel Calculator Works

Enter your length, width, and chosen depth, and the calculator finds the volume, multiplies it by the gravel's bulk density, and returns the weight in tonnes with bag equivalents. The core maths is straightforward: gravel weighs roughly 1.6 to 1.7 tonnes per cubic metre, so:

The reference figures the whole trade quotes: a bulk bag of around 850kg covers roughly 10 m² at a 50mm depth, and a 25kg poly bag covers about 0.3 m² at the same depth. A 5m × 4m gravel area at 50mm therefore needs 20 ÷ 10 = 2 bulk bags, or about 1.7 tonnes loose. If you are doing larger projects, you may need a solid aggregate foundation which you can calculate using our Aggregate Base Calculator.

Getting Depth Right

Depth is the decision that controls both cost and how gravel behaves underfoot. Different jobs want different depths:

• Decorative borders and beds: 30 to 40mm.
• Paths and general garden areas: 40 to 50mm.
• Driveways in 20mm gravel: 50mm over a proper sub-base.
• Pea gravel (10mm): 30 to 40mm, since it settles tighter.

Too shallow shows the membrane through thin patches within a season; too deep turns paths into energy-sapping shingle that is genuinely hard to walk on. The 50mm standard exists because it fully hides the ground on the first pour while staying firm underfoot. You can estimate other garden costs using our Garden Landscaping Cost Calculator or bark coverage using our Bark Mulch Calculator.

Choosing Gravel Size

The stone size changes how the surface performs. The workhorse is 20mm angular gravel, which locks together underfoot and under tyres, stays put, and suits driveways and paths. 10mm pea gravel looks softer and feels kinder underfoot but travels: it migrates into lawns, sticks in shoe treads, and finds its way indoors, which is why it belongs in borders rather than on main walking routes. Larger 40mm stone reads as a decorative aggregate for beds and drainage rather than a walking surface.

For driveways, angular 20mm over a compacted MOT Type 1 sub-base is the standard build. Gravel's natural permeability is a quiet planning advantage: a gravel front garden drains freely, keeping it on the right side of UK rules that require permeable surfacing or contained drainage on new front driveways over 5 m². You can contrast this with solid alternatives like block paving using our Block Paving Calculator or resin systems using our Resin Driveway Calculator.

Two Layers Under Good Gravel

Gravel that stays weed-free and firm has two things beneath it. A geotextile membrane over prepared ground stops weeds pushing up and, just as importantly, stops gravel slowly disappearing into the soil below, which is the silent reason gravel areas thin out and need topping up. On driveways, the second layer is structural: 100 to 150mm of compacted MOT Type 1 carries vehicle loads so gravel acts as a wearing surface rather than the foundation.

Edging completes the build, because loose stone spreads without a boundary. Timber, metal, or stone edging holds the sheet of gravel where it was laid, and on drives, a kerb or edge restraint stops tyre scatter onto lawns and paths.

Worked Example

A 12m × 3.5m gravel driveway in 20mm golden gravel at 50mm:

• Area: 42 m²
• Volume: 42 × 0.05 = 2.1 m³
• Weight at 1.7 t/m³: 3.6 tonnes
• As bulk bags at 850kg: 5 bags, or a loose tipper load (usually cheaper at this volume)
• Beneath it: membrane plus 100mm of MOT Type 1 (around 8.5 tonnes)

Loose loads beat bulk bags on price once an order passes roughly 3 to 4 bags, provided there is somewhere for a tipper to drop it.