🪴Compost & soil calculator

Compost, topsoil and mulch are sold by volume, but gardeners think in bags and bakkie loads, and the two rarely line up in your head at the nursery. The result is either five trips in a hatchback or a heap of leftover bags going hard in the sun. This calculator turns an area and a depth into the volume you actually need, then into bags and bakkie loads.

How the compost and soil calculator works

Volume is just area multiplied by depth. Spread 30 mm of compost over 10 square metres and you need 0.3 cubic metres, which is 300 litres, or about 10 bags of the common 30 litre size. The calculator does this for you, and because a small bakkie carries roughly one cubic metre, it also tells you whether this is a bag job or a delivery job.

How deep should you go?

• ✓Top-dressing existing beds with compost: about 20 to 30 mm.
• ✓Making a new bed by digging compost in: 100 to 150 mm before you fork it through.
• ✓Bark mulch over beds: 50 to 75 mm, thick enough to actually suppress weeds and hold moisture.
• ✓Lawn dressing over grass: around 10 mm, and no more, or you smother it.
• ✓Filling a raised bed: the full internal depth of the bed.

Compost, topsoil or mulch: which do you need?

They are not interchangeable. Compost is decomposed organic matter that feeds the soil and its life, so it is what you dig in and top-dress with. Topsoil is mineral soil that gives you body and structure to fill and build beds, and it is usually improved with compost rather than used alone. Mulch, most often bark, is a protective blanket laid on top: it locks in moisture, keeps the soil cool and starves weeds of light. A healthy bed often uses all three in their proper place.

Why it is sold by volume, not weight

Bagged and bulk garden materials are sold by litres or cubic metres because their weight swings with moisture and material. A cubic metre is always 1,000 litres, so at 30 litres a bag that is about 33 bags. Working in volume keeps the maths honest whether the compost is wet or dry.

Common mistakes buying compost and soil

• ✓Spreading a layer so thin it makes no real difference to the soil.
• ✓Digging in fresh, unrotted compost, which can rob young plants of nitrogen as it breaks down.
• ✓Using bark mulch as if it were soil, or planting straight into it.
• ✓Forgetting that loose material settles, so a fluffy heap shrinks once it beds down and rain compacts it.
• ✓Buying everything in bags when a bulk delivery would be cheaper for anything over about a cubic metre.

We deliver compost and garden supplies, and bring the right materials as part of any landscaping or maintenance job. For what to plant into all that lovely soil, see our guide to water-wise and indigenous plants.