How Deep Should a Raised Bed Be for Vegetables?

Quick answer: For most vegetables, a raised bed should be at least 30cm (12 inches) deep. Leafy greens and herbs manage in 15–20cm (6–8in), while root crops like carrots and parsnips want 45cm+ (18in+). If the bed sits on open soil, roots can grow deeper than the frame; on a hard surface like a patio or balcony, fill to the full working depth because roots have nowhere else to go.

Depth is the decision that quietly controls everything else about a raised bed — how well your crops grow, how much soil you buy, and what it costs to fill. Get it wrong in one direction and you starve your plants of root space; get it wrong in the other and you spend far more on compost than you needed to. This guide gives the right depth for each crop type, explains how the surface underneath changes the answer, and shows how depth drives the fill cost. Use the calculator above to see the cost difference for yourself.

The short version, by crop

Root depth needs vary more than most beginners realise, and matching the bed to what you’ll grow is the whole game:

• 15–20cm (6–8in): lettuce, spinach, rocket, most herbs, radishes, and other shallow-rooted crops.
• 30cm (12in): the reliable all-rounder — beans, courgettes, brassicas, tomatoes, peppers, garlic, onions.
• 45cm+ (18in+): carrots, parsnips, and other long roots that need room to grow straight and unforked.

If you only build one bed and want to grow a bit of everything, 30cm is the sweet spot. It handles the great majority of vegetables and keeps the fill cost reasonable. You can always grow the occasional deep-rooted crop in a taller container alongside.

Does the surface underneath matter?

Yes — this changes how deep you actually need to build, and it’s the reason two experienced gardeners can give you different “correct” depths and both be right.

On open soil or grass: roots can push past the bottom of the frame into the ground below. A 20cm frame on good soil can grow crops that “want” 30cm, because the extra depth is available underneath. You can build shallower and save on compost, as long as the ground below isn’t compacted or waterlogged. To help roots down, it’s worth loosening the soil beneath the bed before you fill it, and laying cardboard rather than a solid liner so roots can eventually pass through.

On concrete, paving, decking, or a balcony: the frame depth is all the root depth there is. Here you must build to the full working depth — 30cm minimum for general vegetables, more for roots — because there’s no ground to borrow from. You also need to make sure the bed can drain, since water can’t soak away into the ground.

Why deeper isn’t automatically better

It’s tempting to build tall beds for everything, but depth has real costs that rise directly with it:

• Compost. Volume rises directly with depth. A 4×4 bed at 18 inches needs 50% more compost than the same bed at 12 inches — that’s several extra bags, or a bigger share of a bulk delivery.
• Weight. Wet soil weighs roughly 0.5–0.6 kg per litre. A deep bed on a balcony or roof terrace can exceed what the structure should safely carry once it’s watered and planted.
• Diminishing returns. Once a crop has the depth it needs, extra depth does nothing for it — you’re paying to fill space that roots never reach.

Match depth to what you’ll actually grow. If that’s salad and herbs, a shallow bed is not a compromise — it’s the correct, cheaper choice.

Filling deep beds without overspending

For beds deeper than about 30cm, you don’t need premium compost all the way down. Roots concentrate in the top 20–30cm, so a common approach fills the bottom third with cheaper bulk material — garden soil, well-rotted manure, or woody debris — and reserves good compost for the top two-thirds where roots are busiest. This cuts the cost of a tall bed substantially without any effect on how your plants grow, because the root zone is identical either way.

If you do this, calculate the compost for the top layer only. Set the depth in the calculator to your compost layer (say 30cm), not the full height of a 45cm or 60cm bed, and the free base material makes up the rest.

How depth affects your fill cost

The clearest way to feel the impact of depth is to change one number and watch the volume move. A 1.2m × 0.8m bed needs about 288 litres at 30cm, but only 192 litres at 20cm and 432 litres at 45cm. That’s the difference between roughly 6, 4, and 9 bags of 50-litre compost for the same footprint. Before you commit to a tall bed, it’s worth deciding whether the crops you want actually need that depth, or whether a standard 30cm bed would grow them just as well for less.

Common mistakes to avoid

The most common depth mistakes are building a shallow bed on a hard surface (where roots can’t escape downward and quickly run out of room), building a needlessly deep bed for shallow crops and paying for compost that’s never used, and — on balconies and roofs — building deep beds without checking the structure can carry the loaded weight. One more: assuming the frame height equals the fill depth. Leave a few centimetres of freeboard at the top so soil and mulch don’t spill over when watered.