A 2.4 by 1.2 metre bed filled to forty-five centimetres holds just under one and a half cubic metres of soil, which is around two tonnes, and three of them is nearly four cubic metres and five and a half tonnes. That is the number that decides how the job is done: whether it arrives in sixty-litre bags you carry, or in a bulk load tipped on the drive and shifted by barrow. This works out the volume, converts it to bags and to weight, and draws the soil in the bed with the depth marked, so you can see how much of the bed is actually rooting depth and how much is structure you are paying to fill.
Beds, depth and soil
A raised bed holds far more soil than it looks like it should.
1.2 m is the practical maximum if you reach in from both sides.
30 cm minimum for vegetables, 45 cm is comfortable, 60 cm for deep rooters.
Bagged topsoil and compost are commonly 60 L; some are 40 L.
Result
A raised bed is mostly not soil
The single most expensive mistake with raised beds is filling them with soil all the way down. A sixty-centimetre bed does not need sixty centimetres of bought soil: it needs thirty to forty-five centimetres of good growing medium on top, and the rest can be something cheaper that is not growing medium. The roots of almost every vegetable live in the top thirty centimetres; below that, the bed is structure.
The standard approach is a bottom layer of coarse material — branches, prunings, cardboard, turves upside down, rubble — which fills volume, drains freely and breaks down slowly, shrinking as it goes. That shrinking is why the first year needs a top-up: the bed settles by around ten percent as the bottom layer compacts, and the soil level drops visibly by spring.
Weight is the number people do not expect
A cubic metre of moist topsoil weighs between 1.2 and 1.6 tonnes depending on how wet it is and how much stone is in it. Three beds holding nearly four cubic metres is therefore over five tonnes, and that has two consequences. The first is structural: a raised bed is retaining that much material and the sides have to be built for it, because wet soil pushes hard against a timber wall and the usual failure is the sides bowing outwards in the first winter.
The second is logistical. Five tonnes is not a car boot. It is a bulk delivery tipped on the drive, or sixty-five sixty-litre bags, and either way it has to be moved to the beds by barrow. Sixty-five bags is a long afternoon; a tipped pile is a shorter one if you can get a barrow to it, and a nightmare if you cannot.
Bagged topsoil varies enormously in quality, and the cheap bags are frequently subsoil — orange, stony and lifeless — rather than topsoil.
Ask for a screened, peat-free topsoil with organic matter in it, and compost to blend. A blend of roughly two parts topsoil to one part compost is a good general bed mix.
Depth by what you are growing
Salads, herbs and most annual vegetables are content in twenty-five to thirty centimetres. Root vegetables want forty: carrots and parsnips need the length, and potatoes want more still. Deep rooters and perennial crops — asparagus, artichokes, fruit bushes — want sixty centimetres and will never thrive in less.
Deeper is not automatically better. A very deep bed of light compost dries out from the top down rather than pulling moisture up, and a deep bed costs proportionally more to fill for no gain. Forty-five centimetres is the comfortable middle: enough for almost everything annual, with enough mass to hold moisture through a dry week.
| Crop | Minimum depth | Mix | Note |
|---|---|---|---|
| Salads and herbs | 20 cm | Compost-rich | Shallow is fine, and dries fast |
| Most annual vegetables | 30 cm | Two topsoil to one compost | The general case |
| Carrots and parsnips | 40 cm | Sandy topsoil, no fresh manure | Forked, not forked-in |
| Potatoes | 40 cm | Topsoil plus compost | Earth up as they grow |
| Tomatoes and cucumbers | 45 cm | Compost-rich, free draining | Heavy feeders |
| Asparagus and fruit | 60 cm | Topsoil, compost, grit | Permanent, so build it once |
A worked example
- Volume per bed2.4 by 1.2 by 0.45 is 1.296 m³ per bed.
- Across three beds3.888 m³, which rounds to 3.89 and is about three thousand nine hundred litres.
- Bags or bulkAt sixty litres that is 65 bags. Delivered in bulk it is just under four cubic metres.
- WeightAt 1,400 kg per cubic metre the beds will hold about 5,443 kg — five and a half tonnes.
- Cost and top-upSixty-five bags at $8 is $520, and allow about another 0.39 m³ for the first year's settling.
Five hundred and twenty dollars of soil for three beds is the sort of figure that makes people reconsider the number of beds. Two well-filled beds outperform three thin ones, because depth beats area for almost every vegetable.
Filling and aftercare
Fill in layers and water as you go. A bed filled dry and then watered from the top will have dry pockets that never wet through, and you will not find them until something wilts. Fill a third, soak it, fill a third, soak it, and finish with the soil slightly mounded, because it will settle.
Mulch the surface once planted, which reduces the watering enormously, and the mulch figure sizes it. Watering a raised bed is a different proposition from watering open ground: it dries faster because it drains faster and has more exposed side, and the irrigation run time is worth running against the bed area rather than the whole plot.
Plan the paths at the same time. Beds at 1.2 m wide with paths of 0.6 to 0.9 m between them is the standard layout: wide enough to reach the centre from either side, narrow enough that the path is not wasted ground. The walkway width figure is the one that decides whether a barrow fits down it.
Do
- Fill the bottom third with cheap bulky material rather than bought soil.
- Build deep enough for the deepest crop you will ever grow.
- Water in layers as you fill, and mound it slightly for settling.
- Check that the sides are built to retain wet soil, not dry.
- Budget a ten percent top-up for the first year.
Avoid
- Filling the whole depth with expensive bought topsoil.
- Building sides too light and watching them bow in the first winter.
- Filling dry and watering from the top afterwards.
- Ordering sixty-five bags without somewhere to put them.
- Accepting cheap subsoil because the bag says topsoil.
Where the numbers stop being true
The weight figure uses 1,400 kg per cubic metre, which is a moist topsoil. Peat-free compost is much lighter, sand and gravel heavier, and anything wet is heavier still — water is a tonne a cubic metre on its own. If the bed is going on a roof, a balcony or a suspended structure, get a proper weight figure for the actual material.
Volume is computed as length times width times depth, which assumes square corners and vertical sides. Tapered beds, which are common with timber sides that lean, hold slightly less. Beds with thick internal framing hold less again.
Nothing here accounts for the soil already in place. A bed built straight onto good ground, with the base broken up rather than lined, is effectively deeper than its timber suggests, because roots go down into the subsoil. A bed lined with a membrane is exactly as deep as it looks and not a centimetre more.
How the figures are produced
Length times width times depth gives the volume of one bed, multiplied by the number of beds for the total. Litres are a thousand times cubic metres, and bags are litres divided by bag size, rounded up. Weight uses 1,400 kilograms per cubic metre, which is a moist topsoil. The top-up figure is ten percent of the volume, which is what a new bed typically settles in its first year.
Questions people ask
How deep should a raised bed be?
Thirty centimetres minimum for vegetables, forty-five is comfortable, sixty for perennial crops. Below thirty, most vegetables struggle and the bed dries out.
Can I fill the bottom with something cheap?
Yes, and you should. Coarse prunings, cardboard, turves upside down or rubble fill volume and drain freely. Only the top thirty to forty-five centimetres needs to be growing medium.
How much does the soil weigh?
About 1,400 kg per cubic metre for moist topsoil, so three beds at nearly four cubic metres is over five tonnes.
Why does the level drop after a year?
The bottom layer compacts and breaks down. Allow about ten percent and top it up in spring.
Should I line the bed?
Only to keep out persistent weeds or to protect timber from wet soil. Lining cuts the bed off from the ground and makes it exactly as deep as it looks.
Bags or bulk delivery?
Bulk from about two cubic metres upwards, if you can get a barrow to it. Bags are easier to handle and much more expensive per litre.