What is the best soil mix for raised beds?

The recommended soil mix for raised garden beds is 70% topsoil and 30% compost (or well-rotted organic matter). This provides good structure from the topsoil with excellent nutrient content and drainage from the compost. For vegetable growing, some gardeners prefer 60% topsoil, 30% compost and 10% perlite for extra drainage. Need topsoil for a larger area? Use our Topsoil Calculator (/landscaping/topsoil-calculator).

How deep should a raised bed be?

A minimum depth of 200mm (8 inches) is needed for most plants. 300mm (12 inches) is ideal for vegetables, herbs and most flowers. For root vegetables like carrots and parsnips, aim for 400-450mm deep. If the raised bed sits on existing soil, roots can grow deeper into the ground below. This raised bed volume calculator supports any depth you enter.

How much soil do I need for a 4x8 raised bed?

A standard 4ft x 8ft (1.2m x 2.4m) raised bed at 300mm deep needs approximately 0.86 m³ of soil — roughly 1-2 bulk bags depending on bag size. Using the 70/30 mix, that is about 0.6 m³ topsoil and 0.26 m³ compost.

What material should I use for raised bed walls?

Popular options include timber sleepers, treated softwood planks, concrete blocks, or gabion baskets. For taller retaining walls (over 600mm), consider our Retaining Wall Calculator (/landscaping/retaining-wall-calculator) to estimate materials. For a lawn around the raised bed, see our Turf Calculator (/landscaping/turf-calculator).

Should I put anything at the bottom of a raised bed?

For beds over 300mm deep, you can line the bottom with cardboard or newspaper to suppress weeds, then add a layer of small branches, leaves or garden waste (hugelkultur method) before adding soil. This saves on soil and improves drainage. Do not use landscape fabric at the bottom as it restricts root growth.

TradeCalcs

Raised Bed Soil Calculator — Volume & Soil Mix

Calculate the soil volume needed to fill raised garden beds. Get quantities for the recommended 70/30 topsoil and compost mix.

Bed Shape

Depth

0.3m (300mm) ideal for most plants

Bed Length

Bed Width

Price Per Bulk Bag (optional)

Price per bulk bag for cost estimate

How We Calculate This

Our raised bed soil calculator computes the volume of your bed based on its shape and dimensions, then recommends the ideal topsoil and compost mix quantities.

Volume formulas

Rectangular: Length × Width × Depth
Circular: π × Radius² × Depth
L-Shaped: (Section 1 area + Section 2 area) × Depth

Recommended soil mix

We recommend the widely-used 70/30 mix:

70% screened topsoil — provides structure, minerals and moisture retention
30% compost or organic matter — adds nutrients, improves drainage and soil biology

Bulk bag sizing

Standard bulk bags contain 0.75 m³. We round up to whole bags as suppliers do not sell part-bags. Topsoil should meet BS 3882:2015 for landscaping quality.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.