How deep should a raised bed be?

Most vegetables need at least 30 cm of soil depth. Root vegetables like carrots and parsnips need 45 cm or more. For shallow-rooted crops like lettuce and herbs, 20 cm is sufficient.

What soil mix should I use for a raised bed?

A good raised bed mix is roughly 60% topsoil, 30% compost, and 10% perlite or sharp sand for drainage. Some gardeners use a 50/50 mix of topsoil and compost.

How many bags of soil do I need?

A standard 1.2m x 1.8m raised bed at 30cm deep needs approximately 0.65 m³ of soil, which is about 16-17 bags of 40-litre compost or 1 bulk bag.

Does soil settle in raised beds?

Yes, soil typically settles by 10-20% in the first year. We include a 10% wastage/settlement allowance by default. You may need to top up after the first season.

What is a bulk bag of soil?

A bulk bag (also called a dumpy bag or tonne bag) typically holds around 850 litres or 0.85 m³ of soil. They usually weigh about 1 tonne when filled with topsoil.

GardenCalcs

Raised Bed Soil Calculator — How Much Soil for a Raised Bed

Calculate the exact amount of soil needed to fill your raised bed.

Bed Shape

Length

Width

Depth

Price per 40L Bag (optional)

Include 10% wastage/settlement

How We Calculate This

This calculator works out the volume of soil needed by multiplying the area of your raised bed by the depth.

Formula: Volume = Length × Width × Depth

For circular beds, we use: Volume = π × (diameter/2)² × Depth

The result is converted into practical quantities: cubic metres, litres, 40L bags, and bulk bags (0.85 m³ each). Soil weight is estimated at 1.2 tonnes per cubic metre for typical garden topsoil.

The optional 10% wastage allowance covers settlement and minor spillage during filling.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.