Why make your own potting mix?

Homemade potting mix is often cheaper in bulk and lets you tailor the mix to specific plants. Cacti need more grit; seedlings need finer, lighter mixes.

What is perlite and why add it?

Perlite is expanded volcanic glass that improves drainage and aeration. It prevents compaction and keeps roots healthy. Use 10-30% in most mixes.

Can I use garden soil in pots?

Garden soil alone is too dense for containers. It compacts, drains poorly, and may contain weed seeds or diseases. Always mix with other materials.

What is John Innes compost?

John Innes is a soil-based compost recipe (loam, peat-free compost, and grit). It provides good structure and nutrient retention for longer-term plantings.

Can I reuse old potting mix?

Yes, refresh it by mixing in 30-50% new compost, add perlite for drainage, and a slow-release fertiliser. Discard any mix from pots where plants had disease.

GardenCalcs

Potting Mix Calculator — Make Your Own Potting Compost

Work out the right ratios of compost, perlite, and vermiculite for potting mix.

Total Volume Needed

Mix Type

How We Calculate This

This calculator provides standard potting mix ratios for different plant types.

General purpose: 60% compost, 20% perlite, 10% vermiculite, 10% grit. Seed starting: 50% compost, 30% perlite, 20% vermiculite. Cactus: 30% compost, 40% grit, 30% perlite.

Enter your desired total volume and the calculator works out each component quantity.

Frequently Asked Questions

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Last updated: February 2026

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