Raised Bed Calculator

Working out how much soil you need for a raised bed is straightforward: multiply the length by the width by the height to get the volume. But most people underestimate the quantity, which means a frustrating trip back to the garden centre mid-project. Our raised bed calculator does the maths for you and adds the recommended 60/30/10 soil mix breakdown so you know exactly what to buy.

The single biggest factor is the depth of your bed. A metric bed measured as 1.2m × 2.4m needs 864 litres at 30cm deep, but only 432 litres at 15cm deep. A literal 4ft × 8ft bed at 12 inches deep needs about 906 litres. Enter the dimensions you actually measured rather than treating the rounded metric and imperial labels as identical.

Here are quantities for common metric raised-bed sizes in the UK, calculated at a 30cm fill depth:

Buying check: Check the product’s stated volume against your calculated requirement and include the delivery charge when comparing the delivered price per litre.

Need to calculate soil for non-raised-bed projects? Our topsoil calculator handles borders, lawns and general garden beds. For mulching around your raised beds, use the mulch calculator.

These imperial examples use the exact feet-and-inches measurements. The metric table above uses metric inputs, which is why a nominal 1.2m × 2.4m bed and an exact 4ft × 8ft bed give different results.

Doubling the depth from 1ft (30cm) to 2ft (60cm) doubles the volume and bag count. Choose the growing depth for the crop, then compare the supplier’s stated volume with the result because bulk products are not a standard size.

If your bed dimensions are not listed here, enter your exact measurements into the raised bed calculator above for a precise figure including the 60/30/10 soil mix breakdown.

Let us walk through a real example so you can see exactly how the numbers work. Suppose you have a standard raised bed measuring 1.8m long × 1.2m wide × 0.3m deep (roughly 6ft × 4ft × 1ft).

Step 1: Calculate the total volume

Multiply length × width × depth:

1.8m × 1.2m × 0.3m = 0.648 m³

Convert to litres (multiply by 1,000):

0.648 × 1,000 = 648 litres

Step 2: Apply the 60/30/10 mix

Using the recommended soil mix ratio, split the 648 litres into three components:

• Topsoil (60%): 648 × 0.6 = 389 litres, which is 10 bags of 40-litre topsoil. Use screened, BS 3882 certified topsoil for the best results. Our topsoil calculator can help if you are buying topsoil for other projects at the same time.
• Compost (30%): 648 × 0.3 = 194 litres, or 5 bags of 40-litre compost. Well-rotted garden compost, mushroom compost or peat-free multi-purpose all work well. Use our compost calculator if you need compost for multiple beds.
• Perlite or grit (10%): 648 × 0.1 = 65 litres: 7 bags of 10-litre horticultural perlite, or roughly two 25kg bags of horticultural grit.

Step 3: Compare suitable products

Use the calculated litres and mix quantities to compare products from suppliers. Before ordering, check:

• the product specification is suitable for the planned crop;
• the supplier’s stated volume, not the product name alone;
• the delivery charge and access requirements; and
• the delivered price per litre.

Confirm the final quantity with the supplier, using the product’s stated volume rather than its name alone.

The soil mix you use in a raised bed makes a bigger difference to plant health and yield than almost any other factor. After years of testing different blends in UK conditions, the 60/30/10 ratio consistently produces the best results for vegetables, herbs and flowers.

The 60/30/10 ratio explained

• 60% quality topsoil provides the bulk, structure and weight that keeps your bed stable. Topsoil also contains the mineral content and beneficial organisms that plants need. Choose screened topsoil (ideally BS 3882 certified) to avoid stones and weed seeds.
• 30% garden compost adds the nutrient density and water-holding capacity that topsoil alone lacks. Well-rotted garden compost, mushroom compost or peat-free multi-purpose compost all work well. The compost is what feeds your plants through the growing season.
• 10% perlite or horticultural grit is the ingredient most people skip, and it makes a real difference. Perlite (or coarse horticultural grit) creates permanent drainage channels throughout the mix. Without it, the soil can become waterlogged after heavy rain, leading to root rot, especially over a British winter.

Why not 100% compost?

Compost can be used on its own when the product is labelled for raised beds and the intended crops. The RHS advises allowing the filled bed to settle and topping it up if necessary. Treat the 60/30/10 mix on this page as a planning example rather than a universal rule.

Alternative mixes for specific plants

Use the calculator above to work out your total volume, then split the order according to the mix ratio you need. Our compost calculator can help you work out the compost portion separately if you are buying from different suppliers.

How deep your raised bed needs to be depends entirely on what you plan to grow. Building too shallow wastes your effort, as roots hit the base and plants suffer. Building too deep wastes money on soil you do not need. Here is the definitive guide for UK growing conditions.

Our recommendation: Choose the depth for the crops you plan to grow and the surface beneath the bed. If the supplier says the chosen material may settle, follow its filling and topping-up guidance rather than applying a fixed allowance.

What Depth Raised Bed for Different Plants?

Choosing the right depth saves you money on soil and gives your plants exactly what they need. Here is a detailed breakdown by plant type, based on UK growing conditions where the bed sits on open ground (roots can extend below the bed):

• Salad leaves, herbs and spring onions (15–20cm minimum): Shallow-rooted crops like lettuce, rocket, radishes, chives and basil thrive in just 15cm of soil. A 20cm bed gives them a little extra room and reduces how often you need to water in summer. These are perfect for windowsill-height beds and balcony planters.
• Carrots, parsnips and beetroot (30cm minimum): Root vegetables need depth for straight, unforked growth. Carrots in particular will bend, fork or stop growing when they hit compacted soil or the base of a shallow bed. For long varieties like Autumn King, aim for 40cm. Short-root varieties such as Chantenay or Paris Market can manage in 25cm.
• Potatoes, courgettes and runner beans (45cm minimum): Heavy feeders with extensive root systems need deep, rich soil. Potatoes especially benefit from the extra depth because you can earth them up within the bed as they grow. Courgettes and squash send roots outward as well as downward, so width matters as much as depth for these crops.
• Fruit bushes and perennial herbs (60cm+): Blueberries, gooseberries, currants, rosemary, sage and other perennials stay in the same spot for years. Their root systems expand annually and need generous depth to establish properly. A 60cm bed also provides better frost insulation for roots over winter, which is important for borderline-hardy plants in northern UK gardens.

Plan drainage without a blanket base layer. The RHS raised-bed guide calls for free-draining ground where possible, drainage outlets on hard surfaces and a free-draining mix. On that basis, GardenCalc recommends not adding a blanket layer of gravel or crocks at the base.

Raised beds on hard surfaces

If your raised bed sits on concrete, paving or a patio, roots cannot grow into the ground beneath. Choose a depth that provides the crop’s full growing depth, provide plenty of drainage outlets and use a free-draining growing mix.

Start with the calculator’s litre figure and the growing depth needed for the crop. Check each product’s specification and stated volume rather than relying on the product name alone.

Buying checklist

• Check that each product is suitable for the crop and intended use.
• Compare the supplier’s stated litres or cubic metres with the calculated requirement.
• Include delivery charges when comparing the delivered price per litre.
• Confirm the final order quantity with the supplier.

For beds deeper than 50cm built on ground, the RHS raised-bed guide says rubble, subsoil or inverted turf can be used at the base before the growing mix is added. That guidance does not apply to every bed: keep the crop’s full growing depth and drainage needs in view.