The quick answer: length x width x depth
Soil for a raised bed is just a volume calculation. Measure the inside length, the inside width and how deep you want to fill it, all in metres, and multiply the three together. That gives you the volume in cubic metres. To turn cubic metres into litres, the unit garden compost is sold in, multiply by 1,000, because one cubic metre is exactly 1,000 litres. Then divide the litres by your bag size to get the number of bags.
As a worked headline: a bed 2 metres long by 1 metre wide, filled to 30 centimetres (0.3 metres) deep, is 2 x 1 x 0.3 = 0.6 cubic metres. That is 0.6 x 1,000 = 600 litres. With standard UK 50 litre bags, 600 / 50 = 12 bags. Everything else in this guide is detail on top of those three steps. If you would rather not do the arithmetic by hand, the raised bed soil calculator takes your dimensions and returns litres and bag counts directly.
A full worked example
Picture a classic vegetable bed: 2 metres by 1 metre, the most common kit size you will find in UK garden centres. You decide on a 30 centimetre fill, which is a sensible depth for most crops, deep enough for carrots, beans and brassicas to root well without needing a tonne of fill.
- Volume in cubic metres: 2 x 1 x 0.3 = 0.6 cubic metres.
- Volume in litres: 0.6 x 1,000 = 600 litres.
- In 50 litre bags: 600 / 50 = 12 bags.
- In 60 litre bags: 600 / 60 = 10 bags.
Now change one thing at a time so you can feel how the numbers move. Keep the footprint but go shallower, to 20 centimetres, and you need 0.4 cubic metres, or 400 litres, which is eight 50 litre bags. Keep 30 centimetres deep but double the length to 4 metres and you are at 1.2 cubic metres, 1,200 litres, or twenty four bags. Depth and footprint both scale the total directly, which is why an extra few centimetres of depth across a large bed quietly adds several bags. If you are buying loose soil by the bulk bag instead, a builder's bulk bag holds very roughly 0.6 to 0.8 cubic metres, so the example bed is close to a single bulk bag.
The right soil mix
Filling a raised bed with one material rarely works well. Pure topsoil can be heavy and low in nutrients, while pure multipurpose compost is rich and fluffy but holds too much water at first and then collapses as it breaks down. A blend gives you the best of both. A reliable general purpose recipe is about 60 percent good quality topsoil, 30 percent compost and 10 percent horticultural grit or coarse sand.
The topsoil is the backbone: it holds structure and moisture and stops the bed drying out in a week of sun. The compost feeds the plants and keeps the soil open and alive. The grit earns its place on heavier ground or for crops that hate wet feet, improving drainage so roots are not sitting in water. Apply those proportions to the worked example and the 600 litre bed wants roughly 360 litres of topsoil, 180 litres of compost and 60 litres of grit. For very hungry crops like courgettes or pumpkins you can lift the compost share, and for a permanent ornamental bed you can lean more on topsoil to keep it stable for longer.
Why fresh compost settles, and how to plan for it
Do not be surprised when a freshly filled bed sinks. New compost and loosely tipped topsoil are full of air pockets and partly broken down material. Over the first few weeks, as you water and as the organic matter continues to rot down, the whole lot compacts. A drop of 10 to 20 percent of the depth is normal, so a bed you filled to the brim can end up several centimetres low.
Two simple habits handle this. First, overfill slightly when you build, mounding the soil a little proud of the top edge rather than levelling it flush. Second, water the bed thoroughly once filled and leave it a few days, which settles the material far faster than waiting for rain, then top up with spare mix before you plant. This is exactly why buying one or two extra bags beyond the calculated figure is wise: it is annoying to be a single bag short when the bed has slumped and your plants are waiting. Each year the level will also drop a touch as the organic content is used up, so an annual top dressing of compost in spring keeps the bed full and fed.
Beyond the bed: paths and mulch
The same length by width by depth method covers most garden filling jobs, just with a shallower depth. For a bark or gravel mulch over a border you might use 5 to 7.5 centimetres rather than 30, but the arithmetic is identical: area in square metres times depth in metres gives cubic metres, times 1,000 gives litres. A 5 square metre border mulched 5 centimetres deep is 5 x 0.05 = 0.25 cubic metres, or 250 litres. If you are topping beds or paths with bark, gravel or wood chip, the mulch calculator works the depth out in the same way and saves you a trip back for one more bag.