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 × 1.8m raised bed at 30cm deep holds 0.65 m³ (648 litres) of soil. That is 18 bags of 40-litre compost with the default 10% settlement allowance, or 17 bags without it — and it fits in a single bulk bag (~0.85 m³). Each cubic metre is 1,000 litres, so a 40L bag fills about 0.04 m³.

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.

Raised Bed Soil Calculator — Free Soil & Compost Tool

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

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

Bed ShapeRectangular and circular use exact area; L-shaped estimates 75% of the bounding rectangle

Length

Longest side of the bed footprint, inside the frame

Width

Shorter side of the bed footprint, inside the frame — keep under 1.2 m so you can reach the centre

Depth

Fill depth: 20 cm for salad and herbs, 30 cm for most veg, 45 cm+ for carrots, parsnips and fruit

Price per 40L Bag (optional)

Shelf price of one 40-litre bag — leave at 0 to skip the cost estimate (bulk bags are usually far cheaper per litre)

Include 10% wastage/settlement

Adds 10% to allow for soil settling 10-20% in the first year — turn off to order the exact bed volume

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

For L-shaped beds we estimate the footprint as 75% of the enclosing Length × Width rectangle (a typical L removes about a quarter of the bounding box). If your L is very narrow or very wide, measure the two rectangular legs separately and add their volumes for an exact figure.

The result is converted into practical quantities: cubic metres, litres (1 m³ = 1,000 L), 40L bags, and bulk bags (0.85 m³ each). Soil weight is estimated at 1.2 tonnes per cubic metre for typical loose garden topsoil (screened or wet topsoil is heavier — adjust the soil density in advanced options).

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

Frequently Asked Questions

Last updated: February 2026

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

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

How We Calculate This

Frequently Asked Questions

Related Calculators

Compost Bin Size Calculator

Topsoil Calculator

Mulch Calculator

Soil Amendment Calculator

Potting Mix Calculator