Pea Gravel Calculator UK: How Much Do I Need?

This pea gravel calculator works out the volume and weight of pea gravel you need for a UK garden project. You enter the length and width of the area in feet or metres, plus the depth you want, and it returns the volume in cubic yards and cubic feet. It also lets you estimate the weight in tonnes, which is how pea gravel is sold in the UK.

Formula

The calculator uses the standard volume formula for a rectangular area:

Volume (yd³) = Length (ft) × Width (ft) × Depth (in) ÷ 324

Where:

• Length – the longer side of the area in feet (ft)
• Width – the shorter side of the area in feet (ft)
• Depth – how deep you want the gravel layer in inches (in)
• 324 – the conversion factor: 1 cubic yard = 27 cubic feet, and 1 foot = 12 inches, so 27 × 12 = 324

The second output, cubic feet, uses the simpler formula: Volume (ft³) = Length × Width × Depth ÷ 12. That is useful if you are buying in small bags measured in litres (1 ft³ ≈ 28.3 litres).

Worked Example

Using the default values from the calculator – a 40 ft by 25 ft area at 3 inches deep:

1. Multiply length by width: 40 × 25 = 1000 ft²
2. Multiply by depth in inches: 1000 × 3 = 3000
3. Divide by 324: 3000 ÷ 324 = 9.26 yd³

The calculator shows 9.26 cubic yards (rounded to two decimal places). That is the headline answer. It also shows 250.0 cubic feet (1000 × 3 ÷ 12).

To convert to tonnes, multiply the cubic yards by the density of pea gravel. A typical density is 1.4 tonnes per cubic yard, so 9.26 × 1.4 ≈ 13 tonnes. But density varies from about 1.2 to 1.6 t/yd³ depending on the stone, so always ask your supplier for their specific figure.

Why Depth Matters

The depth you choose is the single biggest factor in how much gravel you need. For a decorative path, 2 inches (50 mm) is usually enough. For an area that will be walked on regularly, 3 inches (75 mm) is a common recommendation. If you are laying gravel over a membrane on soil that gets muddy, you might want 4 inches (100 mm) to prevent the stones from being pushed into the ground.

Going from 2 inches to 4 inches doubles the amount of gravel, so it is worth being realistic about how much you actually need. A shallow layer looks sparse and weeds can push through; an overly deep layer is heavy, expensive, and makes walking feel unstable.

How to Measure Your Area

Most garden areas are not perfect rectangles. To get a good estimate, break the space into rectangles, calculate each one separately, then add the volumes together. For a circular bed, use the formula π × radius² for the area, then multiply by depth and convert. The calculator here works in feet and inches, but you can also enter metres and convert: 1 metre = 3.281 feet, and 1 inch = 2.54 cm.

If your area is an L-shape, split it into two rectangles. If it is a triangle, use half the base times the height for the area. The more accurately you measure, the less waste you will have.

Buying Pea Gravel in the UK

In the UK, pea gravel is sold in two main ways:

• Bulk bags (builders’ bags) – typically hold around 0.8 to 1.0 cubic metres (roughly 1.0 to 1.3 yd³) and weigh about 1.2 to 1.6 tonnes. These are the most economical for large areas.
• Small bags (20–25 kg) – sold at garden centres and DIY stores. A 25 kg bag of pea gravel covers about 0.5 m² at 50 mm depth, but check the label because coverage varies by stone size.

Bulk bags are usually delivered on a pallet and tipped where you want them. Small bags are easier to move but work out much more expensive per tonne. For a 40 ft × 25 ft area, you would need roughly 13 tonnes, which is about 10 bulk bags – clearly not a DIY-bag job.

Common Mistakes

The most common error is mixing units. If you measure length and width in feet but depth in inches, you must divide by 324, not 27. If you use metres for length and width but centimetres for depth, the conversion is different: volume (m³) = L(m) × W(m) × D(cm) ÷ 100. Another frequent mistake is forgetting that the depth is in inches, not feet – entering 3 instead of 0.25 will give you 12 times too much gravel.

Rounding errors also add up. The calculator rounds the cubic yards to two decimal places, but when you order, round up to the nearest half tonne or whole bag. It is always better to have a little spare than to run out mid-project.

Alternatives to Pea Gravel

Pea gravel is popular for paths, driveways, and play areas because it is smooth, cheap, and easy to lay. But it has downsides: it shifts underfoot, can be kicked onto lawns, and needs a weed membrane underneath. If you want a firmer surface, consider:

• Self-binding gravel – contains fines that compact into a hard surface, good for paths.
• Gravel grids – plastic honeycomb cells that hold the gravel in place, reducing movement.
• Decorative chippings – larger, angular stones that lock together better and stay put.
• Slate or rubber mulch – lighter, softer, and often used for play areas.

Each has a different density and coverage, so always check the supplier’s spec before calculating.

Frequently Asked Questions

How many tonnes of pea gravel do I need for a 10×10 ft area at 2 inches deep?

First calculate the volume: 10 × 10 × 2 ÷ 324 = 0.62 yd³. Then multiply by density (say 1.4 t/yd³) to get 0.87 tonnes. So you would need about 1 tonne, or one bulk bag if the supplier’s bag holds around 0.8 m³ (which is roughly 1.1 tonnes).

What depth of pea gravel do I need for a driveway?

For a driveway that will take cars, you need at least 4 inches (100 mm) of gravel, often over a geotextile membrane and a base of hardcore. 2–3 inches is fine for footpaths only. If you use less than 4 inches on a driveway, the gravel will rut and move under tyre weight.

Can I use pea gravel on top of soil directly?

Yes, but it is not recommended. Without a weed membrane, weeds will grow through and the gravel will gradually sink into the soil. Laying a weed-suppressing membrane first keeps the gravel clean and reduces maintenance. For heavy traffic, add a geotextile layer and a compacted sub-base.

How do I convert cubic yards to tonnes for pea gravel?

Multiply the volume in cubic yards by the density in tonnes per cubic yard. Typical pea gravel density is 1.4 t/yd³, but it can range from 1.2 to 1.6 depending on moisture and stone size. Always ask your supplier for the exact density of the product you are buying.

Is pea gravel safe for children’s play areas?

Yes, pea gravel is a common surface for play areas because it is soft and has no sharp edges. However, it must be kept at a depth of at least 6 inches (150 mm) to provide adequate cushioning under swings and slides. Check with safety guidelines for the specific equipment. Also, small stones can be a choking hazard for very young children, so supervision is advised.

Can I lay pea gravel over concrete?

Yes, you can lay pea gravel over a solid concrete base, but you need to contain it with edging, otherwise it will spread. A 2–3 inch layer is decorative and helps with drainage. Make sure the concrete has a slight slope to shed water, and consider using a membrane to prevent weeds from growing in the joints.

Related

• How to Calculate Pea Gravel Needed
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• Topsoil Calculator: How Much Do You Need?

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1