Compost calculator
Work out how much compost you need to mulch a bed or border, or to mix into a new planting area, from its area and the depth of the layer.
Result
Compost required
Bags needed (50L)
How to use this calculator
- Measure the area you're mulching or improving.
- Set the depth — a mulch top-up is usually 5cm, a new bed mix-in is deeper.
- Read the volume required and the equivalent number of bags.
Assumptions
- Bag count assumes a standard 50L bag — bulk-bag suppliers are usually cheaper for larger areas.
- Compost settles and breaks down over a season, so an annual top-up is normal even without disturbing the bed.
Technical details: what the fields mean
- Length
length - The longest side of the bed or border.
- Width
width - The other side of the bed or border.
- Depth
depth - Layer thickness — 5cm suits an annual mulch top-up, 10–15cm suits mixing into a new bed.
- Waste allowance
wastePercent - Extra to cover an uneven surface and settling.
Methodology
Volume = length × width × depth, increased by the waste allowance. Bags assume a standard 50L bag.
Worked example
4m × 3m border, 5cm mulch layer, 10% waste
Inputs: length=4, width=3, depth=5, wastePercent=10
Result: 0.66 m³ required, 14 bags
Practical guidance
- Mulching in spring or autumn suppresses weeds and locks in soil moisture through the following season.
- For a large area, a bulk bag or loose-tipped load works out cheaper than bagged compost.
Common mistakes
- Applying compost too thickly against plant stems, which can encourage rot.
- Under-ordering because settling and breakdown over the season aren't accounted for.
FAQs
How much compost do I need to mulch a border?
At a typical 5cm depth, roughly 0.05 cubic metres per square metre of border — this calculator does the multiplication for your exact area.
How often should I mulch with compost?
Once a year is typical for most beds and borders, usually in spring or autumn.