Gravel Calculator: How Much Do I Need?

How to Work Out Gravel Quantities

Whether you're laying a driveway, filling a sub-base, or covering a garden path, getting the gravel quantity right means you don't end up short or with half a tonne of leftover stone blocking your drive. Gravel is sold by the tonne, so you need to convert your area and depth into weight. Here's how.

Step 1: Measure Your Area

Measure the length and width of the area you need to cover:

Area (m²) = Length (m) × Width (m)

For irregular shapes, break the area into rectangles and add them up. For curved paths or circular areas, use:

Circle area (m²) = π × radius² (3.14 × r × r)

Common Project Areas

| Project | Typical Area | |---|---| | Single driveway (3m × 8m) | 24 m² | | Double driveway (5m × 8m) | 40 m² | | Garden path (1m × 10m) | 10 m² | | Patio sub-base (4m × 3m) | 12 m² | | Shed base sub-base (3m × 2.5m) | 7.5 m² |

Step 2: Decide on Depth

The depth of gravel you need depends entirely on the purpose:

| Application | Recommended Depth | Notes | |---|---|---| | Decorative garden gravel | 30–50mm | 50mm minimum to hide the membrane | | Garden path | 40–50mm | Compacts down, so lay slightly deeper | | Driveway (over existing hard surface) | 50–75mm | Loose gravel on a solid base | | Driveway sub-base (MOT Type 1) | 100–150mm | Compacted, under block paving or tarmac | | Patio sub-base | 100–150mm | Compacted MOT Type 1 | | Drainage/soakaway fill | 300–500mm+ | 20–40mm clean stone (no fines) | | French drain trench fill | 200–300mm | Around the perforated pipe |

Key point: Loose gravel compacts by roughly 10–15% when trafficked or vibrated. If you need 100mm finished depth, lay 110–115mm.

Step 3: Calculate the Volume

Volume (m³) = Area (m²) × Depth (m)

For a single driveway sub-base (24 m² at 150mm deep):

24 × 0.15 = 3.6 m³

Step 4: Convert Volume to Tonnes

This is where people get caught out. Gravel and aggregate are sold by weight (tonnes), not volume. You need to know the bulk density of the material:

| Material | Bulk Density (tonnes per m³) | Typical Use | |---|---|---| | MOT Type 1 | 2.1 | Sub-base, driveways, hard standings | | 20mm gravel (pea shingle) | 1.7 | Paths, decorative, drainage | | 10mm gravel | 1.6 | Decorative, fine paths | | Slate chippings | 1.5 | Decorative | | Cotswold/golden gravel | 1.6 | Decorative | | Sharp sand | 1.6 | Bedding, mortar | | Building sand (soft) | 1.5 | Mortar, rendering | | Topsoil | 1.3 | Gardens, turf prep | | Ballast (all-in aggregate) | 1.8 | Concrete mixing |

Tonnes needed = Volume (m³) × Bulk density (tonnes/m³)

For our driveway sub-base:

3.6 × 2.1 = 7.56 tonnes of MOT Type 1

Add 10% for Waste

Always add 10% for uneven ground, spillage, and compaction losses:

7.56 × 1.10 = 8.3 tonnes — order 8.5 tonnes to be safe.

Rather than working through all this by hand, the Gravel Calculator does the conversion instantly. Enter your dimensions and material type, and it gives you tonnage, bulk bag counts, and loose tipper loads.

How Gravel Is Sold

| Format | Typical Weight | Best For | Approximate Cost | |---|---|---|---| | 25kg bag (from merchant) | 25kg | Tiny areas, decorative top-ups | £3–£6 per bag | | Bulk bag (dumpy bag) | 850kg–1,000kg | Medium projects, driveways | £50–£90 per bag | | Loose load (tipper delivery) | 10–20 tonnes | Large driveways, sub-bases | £25–£45 per tonne |

Bulk Bags vs Loose Loads

• Bulk bags are easier to place precisely and the driver drops them on your drive with a crane lorry. No mess.
• Loose loads are much cheaper per tonne but the tipper empties onto your driveway or road. You'll need to barrow it into position yourself.

Rule of thumb: For anything over 4 tonnes, a loose load is almost always cheaper. Below 2 tonnes, bags from the merchant work out about the same once you factor in the delivery charge.

Worked Examples

Example 1: Decorative Gravel for a Front Garden

• Area: 5m × 3m = 15 m²
• Depth: 50mm (0.05m)
• Material: 20mm golden gravel (density 1.6 t/m³)

Volume: 15 × 0.05 = 0.75 m³

Tonnes: 0.75 × 1.6 = 1.2 tonnes

Plus 10%: 1.32 tonnes — order 1.5 tonnes (2 bulk bags)

Example 2: Driveway Sub-Base

• Area: 5m × 10m = 50 m²
• Depth: 150mm (0.15m)
• Material: MOT Type 1 (density 2.1 t/m³)

Volume: 50 × 0.15 = 7.5 m³

Tonnes: 7.5 × 2.1 = 15.75 tonnes

Plus 10%: 17.3 tonnes — order a loose load of 18 tonnes

Example 3: Garden Path

• Area: 12m × 0.9m = 10.8 m²
• Depth: 40mm (0.04m)
• Material: 10mm pea shingle (density 1.6 t/m³)

Volume: 10.8 × 0.04 = 0.43 m³

Tonnes: 0.43 × 1.6 = 0.69 tonnes

Plus 10%: 0.76 tonnes — one bulk bag will cover it

Laying Tips

• Always lay membrane first. Heavy-duty weed membrane under decorative gravel stops weeds pushing through. The Landscape Institute provides useful guidance on landscape construction best practices. Pin it down with galvanised pegs at 300mm intervals along the edges.
• Compact your sub-base. MOT Type 1 (conforming to the Specification for Highway Works) should be compacted in layers of no more than 100mm with a vibrating plate compactor. Hire one for £25–£35 a day.
• Edge restraints matter. Without timber edging, metal edging, or kerbing, loose gravel migrates into flower beds and onto paths. Budget for edging on every gravel project.
• Don't mix materials. Laying decorative gravel over old gravel of a different type creates a two-tone mess within a year as it gets kicked around.
• Order the right size. 20mm is the most versatile. 10mm is too fine for driveways (sticks to shoes). 40mm is hard to walk on and doesn't compact well for paths.

If you're using gravel as a sub-base for a patio, the Patio Calculator can help with the full materials list including slabs, bedding, and jointing. For deck sub-base preparation, check the Decking Calculator.

Delivery Considerations

• Check access. A tipper lorry is typically 2.5m wide and 10m long. Measure your gate and driveway access before ordering a loose load.
• Specify a drop point. If the tipper can't reach your site, you'll need to barrow it — 1 tonne of gravel is roughly 15–20 barrow loads.
• Watch the weight on drives. A loaded tipper weighs 20+ tonnes. If you've got a new drive or one over a cellar, check it can take the weight.
• Book early. Good aggregates suppliers get booked up in spring and summer. Order a week ahead for loose loads.

Common Mistakes

• Underestimating depth. 30mm of loose gravel barely covers the membrane and looks thin within weeks. Go for 50mm minimum on decorative areas.
• Forgetting compaction on sub-bases. Uncompacted MOT settles unevenly and your paving will dip and rock. Compact in layers.
• Not accounting for falls. Driveways and hard standings need a drainage fall of at least 1:60 as set out in Approved Document H. This means the depth varies across the area — the low end is deeper.
• Ordering by volume instead of weight. Suppliers sell by the tonne. If you ask for "4 cubic metres," they'll convert it for you, but they might use a different bulk density than you expected.

Quick Summary

1. Measure the area (length × width)
2. Choose the right depth for the application (30mm decorative, 150mm sub-base)
3. Calculate volume: area × depth
4. Convert to tonnes: volume × bulk density
5. Add 10% for waste
6. Decide: bags, bulk bags, or loose load based on quantity

The Gravel Calculator handles all the conversions and gives you tonnage, bulk bag counts, and cost estimates. Enter your dimensions and material type and you've got your order sorted.