How Much Mulch Do I Need? Calculate Litres, Bags and Cubic Metres

Wood-chip mulch around garden shrubs and lavender, with a wheelbarrow and rake beside the bed.

Buying mulch is easier when you separate two questions: how deep should this material go, and how much volume will cover your bed?

The shortcut: area in m² × depth in mm = litres of mulch.

For cubic metres, divide that answer by 1,000. For bags, divide the litres by the bag size and round up. When topping up, calculate only the depth you need to add.

1. Choose the depth before the quantity

Check the mulch supplier’s directions for your material and plants. A fine mulch, coarse bark and compressed straw bale do not necessarily have the same recommended depth or coverage.

For an ordinary garden-bed estimate, ABC Gardening Australia uses 50 mm as a rule of thumb. That is the example depth below, not a universal instruction for every mulch or planting.

Gardening Australia’s depth advice also warns against overly thick layers around plants: water still needs to reach the soil. Do not use a deep path-mulching or no-dig-bed recipe as a guide for covering established plants.

2. Turn bed area into litres or cubic metres

Measure just the area to be mulched, excluding paving and other uncovered spaces. For curves or several separate beds, use our garden-area measuring guide, then add the areas together.

Area

6 m × 2 m = 12 m²
A rectangular garden bed.

Depth

50 mm = 0.05 m
The chosen example layer.

Volume

12 × 50 = 600 L
Or 12 × 0.05 = 0.6 m³.

The two calculations give the same answer. One cubic metre equals 1,000 litres. Keep the units together: multiplying square metres by metres gives cubic metres; multiplying square metres by millimetres gives litres.

Quick coverage check at 50 mm:
50 L covers 1 m².
100 L covers 2 m².
1 m³ covers 20 m².

These are calculated volumes for an even layer. They are not a product’s guaranteed bag coverage.

3. Convert the volume into an order

Loose-fill bags sold by litres: divide the required litres by the labelled bag volume. For 600 L using 50 L bags, that is 12 bags. For 620 L, 620 ÷ 50 = 12.4, so round up to 13 bags.
Bulk supply: give the supplier your volume in cubic metres, along with the material you want. Ask about their minimum order and delivery access.
Compressed bales or packs: use the manufacturer’s stated coverage at the recommended depth. Do not treat a compressed pack’s outside dimensions as its spread-out volume.
Supply by weight: ask the supplier to convert your volume for that specific material. A tonne is a mass, not a cubic metre, and different mulches have different densities.

Check uneven ground and the existing mulch layer before ordering. If you choose an allowance for unevenness or settling, keep it separate from the calculated volume rather than disguising it as an exact requirement.

4. Topping up? Subtract what is already there

Part the mulch and check its depth at several spots with a ruler. Measure the remaining mulch above the soil, not the soil itself. Rake it level before deciding where more is needed.

Target total depth

50 mm, with 30 mm still in place.
Add 20 mm, not another 50 mm.

For the 12 m² example

12 × 20 = 240 L.
That is five 50 L bags, rounded up.

This assumes the old layer is suitable to keep and reasonably even. Calculate bare patches separately. Spread only what is needed; rounding up your bag count is not a reason to bury plants with the remainder.

5. Spread, then check the soil underneath

Remove weeds and water dry soil first. Mulch is a surface covering, not a substitute for getting moisture into the root zone.
Rake an even layer. Check the finished depth at several points instead of judging the pile before spreading.
Keep stems and trunks clear. Leave a gap around plant bases and do not heap mulch against bark or cover low-growing crowns.
Check after watering. Lift a little mulch and feel the soil beneath. A damp surface alone does not tell you whether water reached the soil.

The Western Metropolitan Regional Council mulch factsheet supports weeding, watering dry soil and leaving stems clear. If water beads on the exposed soil, our water-repellent soil guide explains the next checks. Do not assume another layer of mulch will fix it.