Which size seed tray should I use?

6-cell trays: large seeds (beans, squash). 24-40 cell: most vegetables and flowers. Open trays: broadcast sowing of fine seeds for later pricking out.

How do I account for germination rates?

Sow 20-50% more seeds than plants needed. Fresh seed: 80-95% germination. Older seed or difficult species: may be 50-70%.

Should I sow one seed per cell?

For large seeds with good germination, yes. For small or unreliable seeds, sow 2-3 per cell and thin to the strongest seedling.

When should I start seeds indoors?

Most vegetables: 6-8 weeks before last frost (mid-March to mid-April in most of the UK). Check individual crop guidance.

What size seed tray modules should I use?

Large seeds (beans, sweetcorn): 6-cell or individual pots. Medium (brassicas, lettuce): 24-cell. Small seeds (carrots, lettuce) can go in open trays and be pricked out later.

GardenCalcs

Seed Tray Calculator — How Many Seed Trays Do I Need

Calculate how many seed trays and cells you need for your seedlings.

Seedlings Wanted

Tray Type

Germination Rate

How We Calculate This

Seeds to sow = Seedlings wanted ÷ (Germination Rate / 100), rounded up.

Trays needed = Seeds to sow ÷ Cells per tray, rounded up.

This ensures you sow enough seeds to achieve the desired number of healthy seedlings.

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.