How much compost does a garden bed need?
Unlike mulch, compost is meant to be mixed into the soil, so the depth you enter is the layer you spread before digging or forking it through the top 15–20 cm. The right amount depends on what the soil is like now:
| Situation | Compost layer | What to do |
|---|---|---|
| Established bed, annual refresh | 25–50 mm | Spread in autumn or early spring, fork lightly into the top 10 cm, mulch over. |
| Tired, compacted or sandy soil | 50–75 mm | Dig through the top 20 cm; repeat next season. Sandy Perth and coastal soils benefit most – see improving sandy soil. |
| New bed on heavy clay | 75–100 mm + gypsum | Break up the clay first, add compost (and gypsum if the clay disperses), mix well; do not bury pure compost in a hole. See improving clay soil. |
| Raised bed layer | 100–150 mm | Sits between drainage and the garden-mix top layer – use the raised bed calculator. |
| Lawn top-dress with fine compost | 5–10 mm | Screened compost only; rake in so grass shows through. See the lawn calculator. |
| Planting a tree or shrub | 10–20 % of backfill | Mix a little compost into the native soil; too much makes a "pot" the roots will not leave. |
Compost is around 20–30 % of a good soil by volume, not 100 %. A bed filled entirely with compost shrinks by a third within a year, dries out fast and can lock up nitrogen while it finishes breaking down.
Worked example
You have a 5 m × 2 m vegetable bed that grew poorly last season on sandy soil. You decide on a 50 mm layer of compost dug through, plus 5 mm of screened compost over a 60 m² lawn:
- Bed: 5 × 2 = 10 m² × 0.05 m = 0.5 m³ (500 L). In 30 L bags: 500 ÷ 30 = 16.7 → 17 bags.
- Lawn: 60 m² × 0.005 m = 0.3 m³ (300 L) → 10 bags of 30 L.
- Total 0.8 m³. Bagged at $9: 27 bags = $243. Bulk compost at $80/m³ + $70 delivery = $134. Bulk saves $109, and you can order a "compost blend" and a "lawn top-dress" on the same truck at many yards.
Which compost? Reading the bag
- AS 4454 certified compost has been tested for maturity, contaminants and weed seeds. In Australia the mark is worth paying for when you are digging compost through a food garden.
- "Soil conditioner" is usually a coarser, cheaper compost intended for digging in. It does the same job for bulk improvement.
- Mushroom compost is alkaline (high pH) and can be salty when fresh – great on clay and vegetable beds, poor around acid-lovers such as azaleas, blueberries and most Australian natives.
- Manure-based composts (cow, sheep, chicken) are richer; use thinner layers (25–40 mm) and keep well-composted chicken manure away from direct contact with seedlings.
- Homemade compost is usually the best of all; the compost C:N ratio calculator balances the greens and browns. If you make your own, measure the finished heap: a 1 m × 1 m × 1 m bin holds a cubic metre when full, but only about 0.4 m³ of finished compost by the time it has broken down.
Compost questions
How many bags of compost for a 3 × 1.5 m bed?
4.5 m² at 50 mm = 225 L, so 8 × 30 L bags or 6 × 40 L bags. At 75 mm it is 338 L, or 12 × 30 L bags.
How much does a cubic metre of compost weigh?
Roughly 500–700 kg depending on moisture – about half the weight of soil. A 30 L bag weighs 15–20 kg, so 17 bags is about 300 kg to lift and carry.
Should I add compost before or after mulching?
Before. Compost goes on or in the soil; mulch goes on top of the compost to protect it. If the bed is already mulched, rake the mulch back, spread the compost, water, then put the mulch back.
Sources and method
- Standards Australia, AS 4454:2012 Composts, soil conditioners and mulches. standards.org.au
- Application depths follow common Australian and New Zealand horticultural advice for soil improvement (25–100 mm dug in, 5–10 mm as lawn top-dress) – see for example the ABC Gardening Australia fact sheets on soil improvement.
- Volumes are geometry only; bag counts round up; prices are illustrative.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error