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

The number of healthy plants you want to end up with — one strong seedling per cell.

Tray TypeLarge seeds (beans, squash, sweetcorn) suit 6-cell modules; most veg and flowers suit 24- to 40-cell.

Germination Rate

Fresh seed germinates at 80-95%; older or difficult species can be 50-70% (check the seed packet).

How We Calculate This

You want one strong seedling per cell, so the number of cells (and trays) is driven by the number of plants you want — not by the raw seed count.

Sowing one seed per cell: germination insurance comes from sowing a few spare cells. Cells needed = Seedlings wanted ÷ (Germination Rate ÷ 100), rounded up, and Seeds to sow = Cells needed (one per cell).

Multi-sowing (2 or more seeds per cell): the extra seeds in each cell are the insurance against non-germination, and you thin to the strongest seedling. Cells needed = Seedlings wanted, and Seeds to sow = Cells needed × Seeds per cell. The germination rate is not applied a second time, otherwise you would over-order seed.

Trays needed = Cells needed ÷ Cells per tray, rounded up.

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.