Raised Bed Soil Calculator
Calculate how much soil and compost you need for a raised garden bed. Shows volume in litres, bags needed and cost with recommended soil mix ratios.
Total Soil Cost
£48.00
Soil Mix Breakdown
Pro Tip
Mix perlite in for better drainage. Add more compost for heavy feeders like tomatoes.
How Much Compost a Raised Bed Actually Needs
Volume in litres is the only number that matters when buying soil. The default here is a 2m by 1m bed at 30cm deep, which works out to 0.6 cubic metres or 600 litres. At 50 litres per bag, that is 12 bags. At £4 per bag, total spend is £48. People consistently underbuy because they think in bags rather than litres, then run out halfway through filling and end up driving back to the garden centre on a wet Saturday.
Standard raised beds in UK gardens are typically 1.2m by 2.4m at 30cm deep, giving around 864 litres. A pair of those beds, common in starter veg gardens, eats through 35 to 40 bags of compost. Bulk bags (sometimes called 'jumbo bags') deliver around 800 litres for £80 to £120 and usually work out cheaper than 16 individual bags once you factor in delivery and the back ache of carrying them.
Soil Mix Matters More Than Brand
Pure compost shrinks dramatically as the organic matter breaks down, dropping the bed level by 5 to 10cm in the first year. The 60/40 mix (compost to topsoil) holds structure better and is the default the calculator uses. For drainage-sensitive crops like Mediterranean herbs, alpines and root vegetables, the 50/30/20 mix adds 20 percent perlite or horticultural grit to keep the soil airy and prevent the dense, waterlogged conditions that rot tomato roots in wet summers.
Top up every spring with a 5cm layer of fresh compost; this replaces what has broken down and feeds the new season's growth without disturbing established roots. A bed that started at 30cm deep will likely settle to 25cm by the second spring; that is normal and means the soil is alive and active, not a sign you skimped on the original fill.
Frequently Asked Questions
How deep should a raised bed be?
30cm is the practical minimum for most vegetables and salads. Carrots, parsnips and other deep-rooting crops want 45cm. Strawberries, herbs and shallow-rooted brassicas thrive in 20 to 25cm. Deeper beds need more soil but require less watering in summer because the root zone stays cooler.
Can I fill the bottom of a raised bed with cheaper material?
Yes. A hugelkultur layer of logs, branches, cardboard and grass clippings at the base saves on compost and improves long-term drainage. Top with 25 to 30cm of proper soil mix for the active root zone. The bottom layer breaks down slowly over five to ten years, releasing nutrients.
What's the best soil mix for a raised vegetable bed?
A 60/40 split of multi-purpose compost to good quality topsoil works for most vegetables. For root crops add 20 percent horticultural grit or perlite. Avoid pure compost-only fills; they shrink fast and become hydrophobic in dry spells, so watering just runs off the surface.
How many bags of soil for a 4ft x 8ft raised bed?
A 4ft by 8ft bed (roughly 1.2m by 2.4m) at 30cm deep needs about 864 litres. That is around 17 standard 50L bags. A single 800L bulk bag delivered usually works out cheaper than buying 16 to 17 individual bags from a garden centre.
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