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Soil & Mulch Calculator
Cubic metres or yards, bags and bulk bags.
What it does. Volume is area times depth, which makes depth the number that matters — and it is the one people guess. Mulch at 75mm uses three times what 25mm does over the same bed, and the difference between the two is whether it actually suppresses weeds.
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How to use Soil & Mulch Calculator
- Enter bed dimensions, or the area if you know it.
- Set the depth for what you are spreading.
- Read volume, bags, bulk bags and approximate weight.
Depth is the whole calculation
| Material | Depth | Purpose |
|---|---|---|
| Mulch, weed suppression | 75–100mm | Blocks light to weed seeds |
| Mulch, decorative | 50mm | Appearance only |
| Compost, soil improvement | 50–75mm | Dug in or as a top dress |
| Topsoil, new bed | 200–300mm | Root depth for planting |
| Turf, base preparation | 100–150mm | Root establishment |
| Gravel path | 50mm over sub-base | Deeper sinks underfoot |
The mulch depths are the ones worth respecting. Below about 50mm, light still reaches the soil and weeds germinate straight through — a thin layer of mulch is decoration, not weed control.
Bulk bags are not a fixed volume
A "bulk bag" or "tonne bag" is sold by weight, not volume, so what you receive in cubic metres depends on the density of the material.
A tonne of bark mulch is a substantially larger heap than a tonne of gravel — bark is light and gravel is not.
Rough densities: bark mulch around 250 to 350 kg per cubic metre, compost 400 to 600, topsoil 1,200 to 1,600, and gravel 1,500 to 1,700. Buying by volume where the supplier offers it removes the guesswork.
Settlement
Loose material compacts. Freshly spread mulch settles by roughly 10 to 20% over a few weeks of rain, and topsoil in a new bed settles more than that if it has not been consolidated in layers.
Spread a little proud of your target depth so the settled result is what you wanted.
For a raised bed, filling to the brim guarantees disappointment. Fill, water thoroughly, let it settle for a week and top up before planting.
Frequently asked questions
How deep should mulch be?
75 to 100mm for weed suppression. Below about 50mm light still reaches the soil and weeds germinate straight through, so a thin layer is decoration rather than weed control.
How much does a bulk bag cover?
It depends on the material, because bulk bags are sold by weight not volume. A tonne of bark mulch is a much bigger heap than a tonne of gravel — bark is roughly a fifth the density.
Does soil settle after spreading?
Yes, typically 10 to 20% for loose mulch over a few weeks of rain, and more for topsoil in a new bed. Spread slightly proud of your target depth.
How much topsoil does a new bed need?
200 to 300mm for general planting, which gives roots usable depth. Turf needs less — 100 to 150mm is enough for establishment.
Should I fill a raised bed to the top?
No. Fill, water thoroughly, let it settle for about a week, then top up before planting. Filling to the brim always ends up short.