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 tool
- Measure the area you're mulching or improving.
- Set the layer depth for your intended use; depth is a user-selected input, not a universal recommendation.
- Read the volume required and the equivalent number of bags.
Assumptions
- Geometric volume = area × entered depth, with the editable allowance applied. Bag count divides by the assumed 50L per bag and rounds up; this is exact only if the product volume matches that assumption. Check the package volume.
- The calculation does not model settling, compost type, soil condition or a suitable application rate.
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 you choose. The calculator converts the entered dimensions to volume; follow relevant product and gardening guidance for application depth.
- Waste allowance
wastePercent - Optional editable allowance for uneven ground or settling; the default is illustrative.
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 m, Width: 3 m, Depth: 5 cm, Waste allowance: 10%
Result: 0.66 m³ required, 14 bags
Practical guidance
- For mulching, choose a depth suited to the compost, plants and purpose; product instructions and local horticultural guidance may differ.
- For larger quantities, compare the supplier's bulk and bagged options, including volume supplied and delivery cost.
Common mistakes
- Applying compost too thickly against plant stems, which can encourage rot.
- Treating the assumed 50L bag as universal; check package volume or supplier bulk quantity.
FAQs
How much compost do I need to mulch a border?
For example, a 4m × 3m border at a chosen 5cm depth is 0.6m³ before allowance, or 0.66m³ with the example 10% allowance. Choose depth for the compost and purpose; 5cm is not a universal rate.
How often should I mulch with compost?
Frequency depends on compost, planting and soil conditions. Follow product instructions and the needs of the bed rather than assuming a fixed annual schedule.
Last reviewed: 2026-10-11
A simple rectangular-area estimate — an irregular bed shape needs a rough area equivalent instead.
This tool provides an indicative estimate, based on the assumptions stated above.
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