What compost should I use for containers?

Multi-purpose peat-free compost for most plants. Ericaceous compost for acid-loving plants. John Innes for long-term plantings.

Should I account for drainage material?

Yes. If using crocks or gravel for drainage, reduce compost volume by about 15%. This calculator includes a drainage toggle.

How often should I replace pot compost?

Annual plants: fresh compost each year. Perennials: top-dress or repot every 2-3 years. Refresh the top 5 cm annually.

Can I reuse old compost?

Yes, mix old compost with fresh at 50/50. Remove old roots and add slow-release fertiliser. Do not reuse compost from diseased plants.

How often should I replace container compost?

Annual containers (bedding, veg) need fresh compost each year. Permanent shrubs in pots benefit from annual top dressing and full replacement every 3-4 years.

GardenCalcs

Container Compost Calculator — Compost for Pots

Work out how much compost you need to fill your containers.

Number of Pots

Average Pot Volume

Price per 40L Bag (optional)

Account for drainage layer (−15%)

How We Calculate This

Total compost = Number of pots × Average pot volume.

If drainage layers are included, volume is reduced by 15% to account for crocks or gravel at the bottom.

Results in 40L and 70L bag quantities for easy purchasing.

Frequently Asked Questions

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Drainage Layer Calculator

Work out crocks or gravel needed for container drainage layers.

Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.