How many seedlings fit in a standard cold frame?

A standard cold frame (about 1.2m x 0.6m) holds approximately 20 seedlings in 9cm pots or one standard seed tray. Smaller module trays will fit more.

When should I use a cold frame?

Use cold frames from March to May for hardening off greenhouse-raised seedlings before planting out. They are also useful in autumn for overwintering tender herbs and salads.

What is hardening off?

Hardening off is the process of gradually acclimatising indoor-grown seedlings to outdoor conditions over 7-14 days. Place plants in a cold frame, opening the lid more each day.

Can I grow crops directly in a cold frame?

Yes. Cold frames are excellent for winter salads, radishes, and early carrots. They warm the soil 2-4 weeks ahead of open ground.

Which direction should a cold frame face?

South-facing is best in the UK to catch maximum sunlight. Angle the lid towards the south at roughly 30-45 degrees for optimal light.

GardenCalcs

Cold Frame Calculator -- How Many Cold Frames Do I Need

Calculate cold frame size and capacity for hardening off plants.

Seedlings to Accommodate

Total number of seedlings or plants you need to fit

Pot / Tray SizeSmaller pots and module trays fit more seedlings per frame

PurposeHow you plan to use the cold frame affects management advice

How We Calculate This

This calculator estimates cold frame requirements based on seedling count, pot size, and intended use.

A standard cold frame (1.2m x 0.6m) holds about 20 seedlings in 9cm pots. Module trays hold more per frame. The calculator adjusts capacity based on your pot size and provides a hardening schedule based on purpose.

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.