How Much Soil Do I Need?

Most people order soil twice. The first order is a guess, the guess is wrong, and a week later there's a second delivery to finish the job (or half a cubic metre sitting in the driveway going nowhere). You can skip all of that. The maths takes about two minutes, and one number is all you walk away with.

One formula does it

Length × width × depth. Measure everything in metres, multiply the three together, and you get cubic metres. That's the number you're after.

That's the whole thing. The only place people come unstuck is depth, and there's a fix for that below.

Say you're filling a new garden bed, 4 metres long and 1.5 metres wide, to a depth of 30 centimetres. Turn the depth into metres (0.3) and multiply:

4 × 1.5 × 0.3 = 1.8 cubic metres

Order 1.8. Done. And if you'd rather skip the sums, use the calculator on this page and it'll do the multiplying for you.

Depth is where it goes wrong

Everyone measures length and width fine. Depth is the guess, and it's the number that swings your order the most, because doubling the depth doubles the soil.

Here's roughly what each job wants:

• Topdressing a lawn: 1–2 cm. Just enough to level and feed.
• Topping up an existing bed: 3–5 cm. Beds sink a bit every year; this brings them back.
• A brand-new garden bed: 20–30 cm. Enough depth for roots to actually do something.
• Raised beds: fill to a few centimetres below the rim.
• Under new turf: around 10 cm of underlay.
• Planting a tree or shrub: ignore the whole-bed maths and just dig the hole about twice the width of the rootball.

Round pots and circular beds

Same idea, slightly different sum. Measure the radius (half the width across the middle) and use:

radius × radius × 3.14 × depth

A round planter a metre across, filled 40 cm deep: the radius is 0.5, so 0.5 × 0.5 × 3.14 × 0.4 comes to about 0.31 cubic metres. Call it a third of a metre.

Order a little more than the number

Fresh soil settles. It compacts over the first few weeks, and if you've ordered the exact figure you'll end up short. Add about 10 percent. On the 1.8-metre bed above, that's an extra 0.18, so round up to 2.

Coming up short is the annoying outcome, the one that costs you a second delivery. A little left over just goes into pots or tops up a bed somewhere else.

Turning that into bags

Bagged products are labelled in litres, and there are 1,000 litres in a cubic metre. So to turn your cubic-metre figure into a number of bags, multiply by 1,000 and divide by the bag size.

Say you worked out 0.3 cubic metres and you're buying 30-litre bags: 0.3 × 1,000 = 300 litres, then ÷ 30 = 10 bags. Bigger jobs stack up fast, which is handy to know before you're loading a trailer, but the maths never changes.

Once you've got your number, that's what to buy, not a rough band around it. Work it out once, get the right amount in the door, and get on with the planting.