Soil and Mulch: How to Calculate Cubic Metres for Your Garden

Ordering soil or mulch sounds easy until you're standing at the nursery trying to work out whether you need half a cubic metre, 17 bags, or "roughly that much" with both arms spread wide. That method is popular, to be fair. It is also rubbish.

The good news is the maths is straightforward. Once you know your garden bed size and the depth you want, you can calculate the volume in cubic metres, compare bulk delivery with bags, and avoid paying for a mountain of mulch that ends up living on your driveway for three weeks.

If you want to skip the manual maths, use our Soil & Mulch Calculator to work it out in seconds. But it helps to understand what's going on underneath the numbers so you can sanity-check your order before you click buy.

The basic formula

For a rectangular garden bed, the formula is:

Volume (m³) = Length (m) × Width (m) × Depth (m)

The only catch is depth. In Australia, landscaping depth is often discussed in millimetres, but cubic metres use metres. So if your mulch depth is 75 mm, convert it first:

75 mm ÷ 1000 = 0.075 m

Then drop it into the formula.

Worked example: mulch for a garden bed

Let's say your garden bed is 6 metres long, 1.5 metres wide, and you want 75 mm of mulch.

• Length = 6 m
• Width = 1.5 m
• Depth = 75 mm = 0.075 m

6 × 1.5 × 0.075 = 0.675 m³

So you'd need 0.675 cubic metres of mulch before allowing extra for uneven spread, settling, or the bit that somehow ends up everywhere except the garden bed.

For real-world ordering, rounding up is sensible. You'd probably order around 0.75 m³ or use a unit converter if you want to double-check the litres equivalent.

Typical depths for soil, mulch, and topdressing

The right depth depends on the material and job:

These are rules of thumb, not sacred scripture. Some mulches settle more than others, and some soils compact once watered in. If you're refreshing an existing bed, the lower end often does the job. If you're starting from scratch, you usually need more than you think.

How to convert cubic metres into bags

Bulk landscape suppliers usually sell by the cubic metre. Bags are sold by litres. The link between them is simple:

1 cubic metre = 1000 litres

That means:

• 1 m³ = 40 bags of 25 L
• 1 m³ = 25 bags of 40 L

Using our 0.675 m³ example:

• 0.675 × 1000 = 675 litres
• 675 ÷ 25 = 27 bags of 25 L
• 675 ÷ 40 = 16.9 bags of 40 L, so round up to 17 bags

This is why bigger projects often make more sense with bulk delivery. Once the bag count starts looking like a small wall, cubic metres become your friend.

When bulk delivery is worth it

Bags are great for tiny jobs, top-ups, or places where getting a trailer involved would ruin the weekend. But if you're doing multiple beds, raised planters, or a proper backyard tidy-up, bulk tends to win on value and effort.

Bulk delivery often makes sense when:

• You need around 0.5 m³ or more
• You're filling a new raised bed
• You're mulching several beds at once
• You do not fancy lifting 20 to 40 bags one by one like you've offended the universe

Before ordering bulk, check access. There is no point calculating the perfect volume if the truck cannot get close enough and you still have to barrow everything uphill in the rain.

What about circular beds?

If your garden bed is round, use the area of a circle first:

Area = π × radius²

Then multiply by depth in metres.

Example: a circular bed with a 3 m diameter has a radius of 1.5 m. At a mulch depth of 75 mm:

Area = 3.1416 × 1.5² = 7.07 m²
Volume = 7.07 × 0.075 = 0.53 m³

So you'd need roughly 0.53 m³, and in real life you'd probably round that up a little.

Why you should add a wastage buffer

Even if your calculations are perfect, your garden probably is not. Beds taper. Edges are wonky. Material settles. Some gets compacted, some gets spilled, and some mysteriously vanishes into the same place as cable ties and 10 mm sockets.

A 10% buffer is a sensible default for soil and mulch. That is also what our calculator uses by default.

For the 0.675 m³ example:

0.675 × 1.10 = 0.7425 m³

Rounded, that becomes 0.75 m³.

Common mistakes that blow the order

1. Mixing up mm and m

This is the classic. Put 75 into the formula instead of 0.075 and suddenly you're ordering enough mulch to bury the mailbox.

2. Measuring the outside of the edging

Measure the actual fill area, not the decorative border. Timber sleepers and edging take up space too.

3. Forgetting the depth changes with the job

Mulch, topsoil, compost, gravel, and lawn topdressing all use different depths. One number does not fit everything.

4. Not rounding up

Running short is more annoying than having a little left over. Especially if the supplier has a minimum delivery fee for the top-up.

The easiest way to get it right

If you have a few garden beds, a circular patch, or a lawn topdressing job, use the Soil & Mulch Calculator. It handles rectangle beds, round beds, lawn topdressing, wastage, bag counts, and even a rough material cost estimate.

And if you're also tackling the rest of the yard, our Paint Coverage Calculator is handy for fences, screens, and touch-up jobs that somehow appear the second you start improving the garden.