A raised bed is a container pretending to be a garden, and the pretending is literal: every litre of soil in it was chosen, ordered and carried there by you. The ground-level gardener inherits whatever the plot offers; the raised-bed gardener specifies a growing medium from scratch, which is the whole appeal - and the whole arithmetic problem. Depth by crop, litres per bed, bags versus bulk, and the settling that quietly removes a tenth of it all in the first season.
This guide runs the numbers in that order. The worked example is three beds of 2.4 × 1.2 m at 30 cm of fill - 2 592 litres in total - carried through to the bag count, and it matches the arithmetic the Raised Bed Soil Calculator runs. Depths are metric-first with imperial in parentheses; mixes are described the way suppliers actually sell them.
Volume = internal length × width × fill depth, in litres (1 m³ = 1 000 L). Depth: 15–20 cm for leafy crops and herbs, 30 cm for the general vegetable bed, 40 cm+ for root crops on hard ground. Order 10–15% extra for settling. A common blend is roughly 50% topsoil and 30–50% compost, sometimes with 10–20% aeration material. Deep beds can be honestly underfilled with logs and leaves below the top 30–40 cm of real soil.
Depth: Match the Bed to the Crop
Depth is the first decision because it multiplies everything else - a bed that is 40 cm deep holds a third more soil than the same bed at 30 cm, and needs that much more to fill. The useful planning ranges are well established across cool- and warm-season vegetables:
| Crop group | Typical fill depth | Notes |
|---|---|---|
| Lettuce, spinach, herbs, spring onions | 15–20 cm (6–8 in) | Shallow rooted; the classic salad-frame depth |
| Beans, brassicas, cucumbers, peppers | 20–30 cm (8–12 in) | The productive middle of the range |
| Tomatoes, potatoes, squash | 30–40 cm (12–16 in) | Heavy feeders with extensive roots |
| Carrots, parsnips, long radishes | 40 cm+ (16 in+) | The depth the root actually reaches, especially over hard ground |
Two footnotes keep those numbers honest. First, an open-bottomed bed shares soil with the earth below it: roots continue into loosened ground, so 30 cm of fill over dug soil behaves like a deeper bed, while the same 30 cm over concrete or weed membrane is the entire rooting volume. Second, depth is water management as much as root space - a deep bed holds a bigger reservoir through a dry week, and a shallow one dries out on the first warm afternoon. Gardeners in hot-summer climates often choose one depth class deeper than the crop table alone suggests.
The 30 cm (12 in) figure works as a single planning default for a mixed bed: it grows almost everything at a slight compromise and nothing badly. The 15 cm salad bed and the 45 cm carrot bed are the deliberate exceptions at either end.
The Volume Arithmetic
Litres are the friendly unit - soil is sold by the 50 L bag in most markets and by the cubic metre in bulk - and the conversion is exact: 1 m³ = 1 000 L. The formula is the plain rectangular prism: volume = internal length × internal width × fill depth. The word doing the work is internal: a bed built from 30 mm timber to a 2.4 × 1.2 m outside footprint is only 2.34 × 1.14 m inside, about 8% less soil than the nominal numbers promise.
The worked example: three beds at 2.4 × 1.2 m internally, filled to 30 cm. Per bed: 2.4 × 1.2 × 0.3 = 0.864 m³, which is 864 litres. Three beds: 864 × 3 = 2 592 L ≈ 2.6 m³. At 50 L per bag that is 2 592 ÷ 50 = 51.84, so 52 bags - before settling. This is exactly the sum the Raised Bed Soil Calculator runs, including the bag-count rounding.
Irregular shapes submit to the same trick used for mulch and concrete: split them into rectangles, or average the width along the bed's length for the tapered kind. And where the bed sits on a slope with one side dug in, use the average fill depth rather than the tall side - the alternative over-orders by the difference.
What Actually Goes In: Common Mixes
Suppliers and gardening traditions disagree about the perfect raised-bed medium, but the working recipes cluster into a recognisable family: roughly half mineral soil, a large share of compost, and sometimes a small aerating fraction.
| Blend | Typical proportions | Character |
|---|---|---|
| Topsoil-and-compost | 50% topsoil / 50% compost | The common bulk-delivered default; forgiving and easy to recharge |
| Lighter growing mix | 60% topsoil / 30% compost / 10% coarse sand or perlite | Better drainage in wet climates and heavy-soil regions |
| Compost-rich | 40% topsoil / 50% compost / 10% leaf mould | Intensive vegetable growing; settles and needs annual feeding |
| Square-metre-gardening mix | Equal thirds peat-free alternative (coir), blended composts, vermiculite | Contains no mineral soil; light, productive, annual recharging essential |
The classic square-metre-gardening recipe deserves its footnote honestly: it was designed for shallow (15 cm) intensive beds, grows beautifully in year one, and contains no mineral soil at all - so it oxidises, settles and needs recharging every season. Where peat is restricted or undesired, coir or composted bark stands in. At the other end, a straight 50/50 topsoil-and-compost blend is what most bulk suppliers deliver when you ask for vegetable-bed soil, and it is the least maintenance of the family.
What no recipe fixes is bad inputs. Compost that is still hot or stringy, topsoil stripped from a building site full of rubble and couch grass, and "bargain" bagged mixes that compact into a brick are all common failures. If you can, open a bag or inspect the heap before committing to 2 600 litres of it.
Filling Deep Beds Honestly
A 45–60 cm bed filled end to end with growing mix is a generous use of purchased soil, and an unnecessary one: below the root zone, soil is doing drainage and structure work, not feeding. The old solution - older than the marketing name hugelkultur it now carries - is to bury logs, branches, autumn leaves, spent compost and plain cardboard in the base, reserving the top 30–40 cm for the real growing medium.
The woody layer slowly decays into a sponge that holds water through dry spells and hosts the fungal life a productive bed wants, and it displaces a remarkable volume of purchased fill: a 45 cm bed with a 15 cm log base orders two-thirds of the soil of a fully filled one. Three rules keep it honest. Keep the mineral-and-compost root zone genuinely on top, where roots will actually shop for nutrients. Expect the base to settle a few centimetres a year as it breaks down. And keep invasive weeds, treated timber and diseased material out of the base - everything down there is in contact with your food crop's root run.
Bags or Bulk?
The worked 2.6 m³ order sits right on the practical dividing line. As bags, it is 52 × 50 L - three to four car boots' worth, or a pallet of bagged mix, with the per-litre arithmetic of bagged soil applying to all of it. As bulk, it is a single tipper delivery of blended vegetable soil, dropped as close to the beds as the truck can reach, usually at a notably lower per-litre rate and the labour of a wheelbarrow rather than a trowel.
The rule of thumb that falls out of the arithmetic: a single bed (about 0.85 m³) is bag territory; two beds and up (1.7 m³+) increasingly justify bulk, if the delivery truck can get within barrow distance and you have an hour and a strong back. Bulk also lets you order the blend itself rather than mixing blends bag by bag - the supplier does the homogenising.
Settling and Season One
Fresh fill always settles - water and time consolidate the loose heap, and the organic fraction begins oxidising immediately. The planning allowance is 10–15%: on the worked 2 592 L, that is roughly 260–390 L, or five to eight further 50 L bags. The practical move is not to buy that extra volume upfront but to hold it back as compost and top-dress through the first season as the level drops - the bed needs the organic matter anyway, and the timing matches the settling.
After year one, the annual cycle is simple: each spring, 2–3 cm of compost raked onto the surface, and the bed recharged both in level and in nutrients. Deep re-digging is unnecessary and arguably counterproductive in a no-dig bed; the top-dress is the whole maintenance programme. Mulching through the growing season - see the mulch depth guide - protects the surface and feeds the same biology, at depths shallower than the decorative mulch values because a vegetable bed's needs differ from a shrub border's.
Frequently Asked Questions
How deep should a raised bed be?
Most vegetables are productive at 30 cm (12 in) of fill. Shallow-rooted crops - lettuce, spinach, herbs, spring onions - manage at 15-20 cm; deep-rooted crops - carrots, parsnips, tomatoes, potatoes - want 40 cm or more, especially where the bed sits on pavement or hardpan. Two honest footnotes: roots of many crops happily continue into the ground below an open-bottomed bed, so 30 cm of fill over loosened soil grows a deeper crop than 30 cm over concrete; and deeper beds hold more water, which is a benefit in summer and a risk in wet winters.
How much soil do I need for a 2.4 x 1.2 m bed?
Volume is internal length x width x fill depth: 2.4 x 1.2 x 0.3 m = 0.864 cubic metres, which is 864 litres. At 50 L per bag that is 18 bags per bed - and the honest order adds 10-15% for settling, bringing a single bed to about 20 bags or a bulk order of 1 cubic metre. Measure the internal dimensions: timber thickness eats a surprising share of a small bed's stated footprint.
What is a good soil mix for raised beds?
Common blends run roughly half mineral and half organic: about 50% screened topsoil with 30-50% compost and, in lighter recipes, 10-20% of an aerating material such as coarse sand, perlite or leaf mould. The widely-copied square-metre-gardening recipe - equal thirds of peat (or a peat-free alternative such as coir), blended compost and vermiculite - grows intensively but needs annual recharging because it contains no mineral soil at all. What matters more than the exact recipe: a mix that drains in a day after rain and still holds moisture a week into a dry spell.
Can I fill the bottom of a deep bed with other material?
Yes - a well-established practice. Below the top 30-40 cm of real soil, deep beds are commonly underfilled with logs, branches, leaves, spent compost or cardboard (the hugelkultur idea). The woody layer slowly decays, holds water and saves a surprising volume of purchased soil. Three rules keep it honest: keep the mineral-and-compost root zone on top; expect the fill to settle a few centimetres a year as the base breaks down; and avoid invasive or treated material in the base layer.
Why does the soil level in my bed drop every year?
Because the organic fraction is being consumed. Compost, coir and leaf mould break down into carbon dioxide and water, and the microbes driving that also oxidise some of the fine soil fraction - a bed can settle 2-5 cm per season with nothing washing away. The standard cure is an annual 2-3 cm top-dress of compost, raked in before sowing; deeper replenishment is only needed after a few seasons. That annual top-up is also the bed's feeding programme, which is why productive beds never quite stay level.
Planning guidance for home vegetable beds. Local climate, water quality and what your supplier actually blends all vary - treat the depth and mix tables as starting points, and the settling allowance as a floor, not a ceiling.
Where to Read Next
- The Raised Bed Soil Calculator — litres per bed and in total, bag counts by bag size, for any number of beds.
- How much mulch do I need? — the same depth-to-volume arithmetic for the surface layer, bags or bulk.
- How deep should mulch be? — application depths for beds, borders and paths, and the mulch-volcano mistake.
- Mulch vs. stone — the surface-covering trade-offs, from soil health to summer heat.
- The Garden & Outdoor category — every garden guide and calculator in one place.
The bottom line: pick depth by crop - 30 cm as the general default, 40+ for roots on hard ground - then order internal volume in litres plus 10–15% for settling. Half topsoil and half compost is the workhorse blend; deep beds can hide a log base under the top 30–40 cm. Three 2.4 × 1.2 m beds at 30 cm is 2 592 L, or 52 bags of 50 L - and a wheelbarrow argues for bulk.