How Much Soil for a Raised Garden Bed: Calculator and Soil Mix

The short answer

Multiply length × width × depth. A 2.4 × 1.2 m bed filled 30 cm deep needs 0.86 m³, or about 864 liters; an 8 × 4 ft bed filled 12 inches deep needs 32 cubic feet. Buy about 10% extra because new soil settles. Fill it with roughly 60% good topsoil or garden soil, 30% compost and 10% coarse material such as perlite or fine bark.

Most vegetables need 30 cm (12 in) of good soil; leafy greens and herbs manage with less.

Expect the soil to sink by around 10–20% in the first season — top it up with compost.

Bagged soil is easy for one small bed; bulk delivery is far cheaper for several.

Filling the bottom of a deep bed with logs and branches reduces how much soil you need to buy.

Filling a raised bed is usually the most expensive part of building one, and it’s easy to get the amount wrong. Measure once, use the calculator below, and you’ll know exactly what to order.

Raised bed soil calculator

Enter your bed’s inside measurements and how deep you’ll fill it. The calculator adds 10% for settling.

How to calculate it yourself

The volume of a rectangular bed is length × width × depth.

Metric: measure in meters. 2.4 m × 1.2 m × 0.3 m = 0.864 m³. Multiply by 1,000 to get liters: 864 liters.
Imperial: measure in feet. 8 ft × 4 ft × 1 ft = 32 cubic feet. Divide by 27 for cubic yards: about 1.2 cu yd.

Then add about 10% for settling, and divide by your bag size to get the number of bags.

Common bed size	Fill depth	Soil needed	With 10% extra
1.2 × 1.2 m (4 × 4 ft)	30 cm (12 in)	432 L (16 cu ft)	about 475 L (17.6 cu ft)
2.4 × 1.2 m (8 × 4 ft)	30 cm (12 in)	864 L (32 cu ft)	about 950 L (35 cu ft)
2.4 × 0.9 m (8 × 3 ft)	30 cm (12 in)	648 L (24 cu ft)	about 715 L (26.4 cu ft)
1.8 × 0.9 m (6 × 3 ft)	40 cm (16 in)	648 L (24 cu ft)	about 715 L (26.4 cu ft)

Metric and imperial sizes are close equivalents, not exact conversions.

What soil mix to use

A good raised bed mix drains freely but still holds moisture and feeds plants. A reliable starting point:

About 60% topsoil or quality garden soil — the body of the mix. It holds water and nutrients and doesn’t shrink much.
About 30% compost — feeds the soil life and plants. Use a blend of sources if you can, such as mushroom compost plus green-waste compost.
About 10% coarse material — perlite, pumice, coarse sand or fine composted bark keeps the mix open.

Many garden centers sell a ready-made “raised bed mix” or “vegetable mix” in bulk that’s close to this. Check that it’s mostly soil and compost, not mostly bark.

Tip: Test the pH before you plant. Most vegetables prefer slightly acidic to neutral soil, around pH 6.0–7.0.

How to fill a raised bed for less

Lay cardboard first. If the bed sits on lawn, flattened cardboard with the tape removed smothers grass and breaks down within a season.
Use the bottom third for bulk. In beds deeper than about 45 cm (18 in), fill the base with old logs, branches, then leaves and grass clippings. This is the idea behind hügelkultur. It rots slowly and feeds the bed, but it will sink, so plan to top up.
Buy in bulk for more than one bed. Bagged soil is convenient for a single small bed. For two or more beds, a delivered load from a landscape supplier usually costs much less per liter.
Water as you fill. Wetting each layer settles air pockets so the bed doesn’t sink as much later.

How deep each crop needs

Crop group	Minimum soil depth
Lettuce, salad greens, herbs, strawberries	15–20 cm (6–8 in)
Beans, peas, onions, garlic, most brassicas	25–30 cm (10–12 in)
Tomatoes, peppers, zucchini, cucumbers	30–45 cm (12–18 in)
Carrots, parsnips, potatoes	40–45 cm (16–18 in) or loose soil below

If your bed sits directly on loosened ground, roots can grow down into the soil beneath, so the bed itself can be shallower.

Choosing the bed itself

If you haven’t bought the bed yet, its height decides how much soil you’ll need. Galvanized steel beds don’t rot and are quick to assemble, while untreated timber is cheaper but typically lasts only a few years.

The bottom line

Measure length, width and fill depth, multiply, and add 10%. Fill with mostly soil and compost rather than straight potting mix, and top up with compost each season. A bed filled well in the first year needs very little soil ever again.

Common questions

How deep should a raised garden bed be?

Around 30 cm (12 in) of soil suits most vegetables. Lettuce, herbs and strawberries manage with 15–20 cm (6–8 in). Carrots, parsnips and potatoes do best with 40–45 cm (16–18 in) or loose soil beneath the bed.

Can I use only potting mix in a raised bed?

You can, but it's expensive at raised-bed volumes and tends to dry out and shrink quickly. A blend of topsoil and compost holds water and nutrients better and costs less.

How many bags of soil do I need for an 8 x 4 raised bed?

At 12 inches deep, an 8 × 4 ft bed holds 32 cubic feet. With 10% extra for settling that's about 35 cubic feet — roughly 24 bags of 1.5 cu ft, or 18 bags of 2 cu ft.

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

If the bed sits on soil, lay cardboard to smother grass and weeds, then fill. In deep beds you can add logs, branches and leaves at the bottom to save soil. Avoid gravel layers — they don't improve drainage and can make the soil above stay wetter.

How often should I replace raised bed soil?

You rarely need to. Top up with 3–5 cm (1–2 in) of compost each season to replace what settles and what plants use up.