What is the best soil mix for a raised bed?

The Royal Horticultural Society recommends a mix of around 60% topsoil and 40% compost for most raised beds. This blend provides good drainage while retaining moisture and nutrients. For vegetable growing, add extra organic matter such as well-rotted manure or garden compost each season to keep the soil fertile.

How deep should a raised bed be for vegetables?

Most vegetables need a minimum depth of 20 to 30 cm of quality soil to develop healthy root systems. Root vegetables like carrots and parsnips benefit from 45 cm or more. If your raised bed sits on top of existing soil, roots can grow deeper, but on paving or concrete, aim for at least 30 cm depth.

How many bags of compost do I need for a raised bed?

It depends on your bed dimensions. A standard 2.4m by 1.2m raised bed at 30 cm deep requires roughly 864 litres, which is about 18 bags of 50-litre compost. Use our calculator above to get a precise estimate for your specific bed size and depth.

Raised Bed Soil Calculator

Calculate how much soil or compost you need to fill a raised bed

Published by UltraCalculatorsLast reviewed: 28 Mar 2026v1.0Sources:RHS: Raised bedsReport an issue

Raised beds are one of the most popular ways to grow vegetables, herbs, and flowers in UK gardens. They offer better drainage, warmer soil in spring, and make gardening more accessible. The key challenge is working out exactly how much soil or compost you need to fill them, as getting this wrong means either running short mid-project or wasting money on excess bags. Whether you are building a new raised bed from timber sleepers, metal edging, or brick, the volume of fill material depends on the bed's length, width, and depth. Different soil types also have different densities, which affects both the weight you need to transport and the number of bags to buy. Our raised bed soil calculator takes your bed dimensions and chosen soil type, then gives you the volume in litres, the number of standard 50-litre bags required, and an estimated weight so you can plan delivery or collection.

How to Use This Calculator

To calculate soil for your raised bed: 1. Enter the bed length in metres. Measure the inside dimensions of the frame for the most accurate result. 2. Enter the bed width in metres. 3. Enter the bed depth in centimetres. This is how deep the bed frame is, typically 15 to 45 cm. 4. Choose your soil type. Topsoil is the cheapest option, compost mix (60% topsoil, 40% compost) is ideal for vegetables, and multi-purpose compost suits containers and shallow beds. 5. Review the results showing total volume in litres, the number of 50-litre bags needed, and the estimated weight for planning delivery.

How It Works

The raised bed soil formula converts dimensions to a volume and then to practical units: Volume (cubic metres) = Length (m) x Width (m) x Depth (cm) / 100 Volume (litres) = Volume (m3) x 1,000 Bags Needed = Ceiling of (Volume in litres / 50) Weight depends on the soil type density: - Topsoil: approximately 1,200 kg per m3 - Compost mix: approximately 800 kg per m3 - Multi-purpose compost: approximately 400 kg per m3 Weight (kg) = Volume (m3) x Density (kg/m3) For example, a 2.4m x 1.2m bed at 30 cm deep: Volume = 2.4 x 1.2 x 0.30 = 0.864 m3 = 864 litres. That requires 18 bags of 50L compost mix, weighing approximately 691 kg.

Worked Examples

Example 1: Standard vegetable bed with compost mix

Inputs: Length: 2.4 m. Width: 1.2 m. Depth: 30 cm. Soil type: Compost mix.

Volume = 2.4 x 1.2 x (30 / 100) = 2.4 x 1.2 x 0.3 = 0.864 m3.
Volume in litres = 0.864 x 1,000 = 864 litres.
Bags needed = ceiling(864 / 50) = 18 bags of 50 litres.
Weight = 0.864 x 800 = 691 kg.

Result: 864 litres of compost mix needed, which is 18 bags of 50L. The total weight is approximately 691 kg, so a bulk bag delivery may be more economical than individual bags.

Example 2: Deep bed for root vegetables with topsoil

Inputs: Length: 3.0 m. Width: 1.0 m. Depth: 45 cm. Soil type: Topsoil.

Volume = 3.0 x 1.0 x (45 / 100) = 3.0 x 1.0 x 0.45 = 1.35 m3.
Volume in litres = 1.35 x 1,000 = 1,350 litres.
Bags needed = ceiling(1,350 / 50) = 27 bags of 50 litres.
Weight = 1.35 x 1,200 = 1,620 kg.

Result: 1,350 litres of topsoil needed, which is 27 bags. At 1,620 kg total, a bulk delivery of loose topsoil would be far more practical and cost-effective than buying individual bags.

Example 3: Shallow herb bed with multi-purpose compost

Inputs: Length: 1.2 m. Width: 0.6 m. Depth: 20 cm. Soil type: Multi-purpose compost.

Volume = 1.2 x 0.6 x (20 / 100) = 1.2 x 0.6 x 0.2 = 0.144 m3.
Volume in litres = 0.144 x 1,000 = 144 litres.
Bags needed = ceiling(144 / 50) = 3 bags of 50 litres.
Weight = 0.144 x 400 = 58 kg.

Result: 144 litres of multi-purpose compost, requiring just 3 bags. At only 58 kg total, this is easily manageable by hand from a garden centre visit.

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How to Use This Calculator

How It Works

Worked Examples

Example 1: Standard vegetable bed with compost mix

Inputs: Length: 2.4 m. Width: 1.2 m. Depth: 30 cm. Soil type: Compost mix.

Volume = 2.4 x 1.2 x (30 / 100) = 2.4 x 1.2 x 0.3 = 0.864 m3.
Volume in litres = 0.864 x 1,000 = 864 litres.
Bags needed = ceiling(864 / 50) = 18 bags of 50 litres.
Weight = 0.864 x 800 = 691 kg.

Result: 864 litres of compost mix needed, which is 18 bags of 50L. The total weight is approximately 691 kg, so a bulk bag delivery may be more economical than individual bags.

Example 2: Deep bed for root vegetables with topsoil

Inputs: Length: 3.0 m. Width: 1.0 m. Depth: 45 cm. Soil type: Topsoil.

Volume = 3.0 x 1.0 x (45 / 100) = 3.0 x 1.0 x 0.45 = 1.35 m3.
Volume in litres = 1.35 x 1,000 = 1,350 litres.
Bags needed = ceiling(1,350 / 50) = 27 bags of 50 litres.
Weight = 1.35 x 1,200 = 1,620 kg.

Result: 1,350 litres of topsoil needed, which is 27 bags. At 1,620 kg total, a bulk delivery of loose topsoil would be far more practical and cost-effective than buying individual bags.

Example 3: Shallow herb bed with multi-purpose compost

Inputs: Length: 1.2 m. Width: 0.6 m. Depth: 20 cm. Soil type: Multi-purpose compost.

Volume = 1.2 x 0.6 x (20 / 100) = 1.2 x 0.6 x 0.2 = 0.144 m3.
Volume in litres = 0.144 x 1,000 = 144 litres.
Bags needed = ceiling(144 / 50) = 3 bags of 50 litres.
Weight = 0.144 x 400 = 58 kg.

Result: 144 litres of multi-purpose compost, requiring just 3 bags. At only 58 kg total, this is easily manageable by hand from a garden centre visit.