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). The classic base is 7 parts loam to 3 parts peat-substitute to 2 parts coarse sand by loose volume, giving good structure and nutrient retention for longer-term plantings.

Why is the orchid mix bark-based, not compost?

Epiphytic orchids like Phalaenopsis grow on tree branches in the wild, with roots exposed to air. Standard compost holds too much water around the roots and quickly rots them. A free-draining bark-led mix (roughly 60% medium orchid bark with perlite and grit or charcoal) keeps roots aerated, which is why this calculator outputs bark — not compost — for the orchid recipe.

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

Litres of finished mix. A 30cm pot holds ~14 L; a 1m × 1m bed at 20cm deep needs ~200 L.

Mix TypePick by plant: seedlings = fine/light, cacti = high grit, orchids = bark-led, John Innes = soil-based for long-term planting.

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 & succulent: 30% compost, 40% grit, 30% perlite. Orchid: 60% medium orchid bark, 20% perlite, 20% grit/charcoal — never general compost, which holds too much water and rots orchid roots. John Innes style: 58% loam, 25% compost, 17% coarse grit (the classic 7:3:2 loam:peat-substitute:sand by loose volume).

Enter your desired total volume and the calculator multiplies it by each component's fraction, so the parts always add up to the volume you asked for.

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.