Gravel Calculator — How Much Gravel Do I Need?

How much gravel do I need?

Multiply your area (m²) × depth (m) × 1.6 to get tonnes of standard gravel. For example, a 5 m × 4 m gravel area at 50 mm deep needs 20 × 0.05 × 1.6 = 1.6 tonnes — about two bulk bags. The calculator above does this instantly, adjusts for different stone types, and adds a wastage allowance so you don't run short.

A simple rule of thumb: one bulk bag (around 0.85 tonnes) of 20 mm gravel covers about 10 m² at 50 mm depth.

Choosing the right gravel size

10 mm pea gravel is comfortable underfoot and suits paths, borders, and play areas — but it migrates easily, so contain it with edging. 20 mm gravel is the standard for driveways and larger paths: big enough to stay put under tyres, small enough to walk on comfortably. Anything smaller than 20 mm on a driveway tends to get stuck in tyre treads, while larger stones (40 mm+) are hard to walk on and are better for drainage beds.

Slate chippings are lighter (about 1.4 t/m³), so a tonne goes further, and their flat shape locks together nicely on paths. Limestone and granite chippings offer different colours and hardness — granite is the most durable for driveways.

Laying gravel properly

For paths and decorative areas on firm ground: excavate 50–80 mm, lay a weed membrane, and spread your gravel at 40–50 mm. For a gravel driveway, the gravel is only the surface layer — it needs a compacted MOT Type 1 sub-base of 100–150 mm underneath, or the stones will sink and rut within months. Work out that layer with our MOT Type 1 calculator. Gravel grids (plastic honeycomb panels) on top of the sub-base keep stones in place and reduce raking.

Frequently asked questions

How much does a bulk bag of gravel cover?
Around 10 m² at 50 mm depth for 20 mm gravel. A bulk bag holds roughly 0.85 tonnes, though sizes vary slightly by supplier.

How many tonnes of gravel do I need for a driveway?
For the gravel surface alone at 50 mm: area in m² × 0.08 tonnes. A 50 m² driveway needs about 4 tonnes of gravel — plus the MOT Type 1 sub-base underneath, which is calculated separately.

What depth should gravel be?
40 mm for paths with 10 mm gravel, 50 mm for paths and driveways with 20 mm gravel. Deeper than 60 mm becomes hard to walk on; shallower than 30 mm shows bald patches.

Do I need a membrane under gravel?
Yes, in almost all cases — a permeable geotextile membrane stops weeds growing through and prevents gravel sinking into the soil, while still letting rain drain away.

How much gravel fits in a tonne?
A tonne of 20 mm gravel is about 0.625 m³ — enough to cover 12.5 m² at 50 mm depth.

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