How to Fill a Raised Garden Bed
Filling a raised bed with bagged compost gets expensive fast. Here is how much soil you actually need, the mix that grows the best veg, and how to fill deep beds without spending a fortune.
By the Calculate My Reno Team / Published
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Open the Raised Bed Soil Calculator
A raised bed is the easiest way to grow vegetables - good drainage, fewer weeds, soil you control. The catch is that filling one with bagged compost can cost more than the bed itself. A bit of planning fixes that.
Step 1: Work out the volume
The soil you need is simply the bed area times the fill depth:
volume = length × width × fill depth
Filling several beds? Add each one in the raised bed soil calculator - it totals the volume and gives a per-bed breakdown, plus the number of bags for the bag size you choose.
A 4 ft × 8 ft bed filled 12 inches deep is 32 cubic feet (about 1.2 cubic yards, or ~900 litres). That single bed is roughly thirty 1-cubic-foot bags - which is exactly why buying in bulk and mixing your own pays off.
Step 2: Choose the soil mix
Plants do not grow well in pure topsoil (too dense) or pure compost (too rich and it slumps). Mix them:
| Mix | Ratio | Good for |
|---|---|---|
| Standard veg mix | 60% topsoil · 30% compost · 10% aeration | General beds |
| Mel’s Mix | ⅓ compost · ⅓ coir or peat · ⅓ vermiculite | Square-foot gardening |
Once the calculator gives you the total volume, split it by your chosen ratio to know how much of each ingredient to buy.
Step 3: Fill deep beds cheaply
For a tall bed, do not pay for premium soil all the way down - roots only use the top 250–300 mm. Fill the bottom third with bulky, free or cheap material:
- logs, branches and woody prunings (the “hugelkultur” method - they break down and feed the bed over years),
- coarse compost, leaves, or untreated cardboard.
Then top up with your good soil mix. When you use the calculator, set the depth to just the soil layer you are actually buying, not the full height of the bed.
Step 4: Allow for settling
Fresh soil and compost settle after the first few waterings. Fill slightly proud of the top, water it in, and top up after a week or two.
How deep, by crop
- Salad leaves, herbs: 150–200 mm is fine.
- Most vegetables and flowers: 250–300 mm.
- Carrots, parsnips and other roots: 300 mm+.
Spreading soil as a thin layer over a lawn instead of filling a bed? Use the topsoil calculator. And to mulch the bed once it is planted, see how much mulch do I need.
Frequently asked questions
How much soil do I need to fill a 4x8 raised bed?
A 4 ft × 8 ft bed filled 12 inches deep needs 32 cubic feet of soil - about 1.2 cubic yards, or roughly 900 litres. At 6 inches deep it is half that. Enter your exact bed size and fill depth in the raised bed soil calculator, and add a bed for each one you are filling.
What is the best soil mix for a raised bed?
A reliable mix is about 60% topsoil, 30% compost and 10% aeration material such as perlite or composted bark. The popular "Mel's Mix" uses equal thirds of compost, coir (or peat) and vermiculite. Work out your total volume first, then split it by the ratio to know how much of each to buy.
How deep should a raised bed be?
Most vegetables and flowers are happy in 250–300 mm (10–12 inches) of soil. Root crops like carrots prefer 300 mm or more. If the bed sits on open ground rather than a hard surface, roots can grow down further, so you only need to buy good soil for the upper portion.