How many seedlings fit in a standard cold frame?

A standard cold frame (about 1.2m x 0.6m, so 0.72 m² of floor) holds roughly 20 seedlings in 9cm pots once you leave room to lift trays in and out. It also takes about six full 344 x 214mm seed trays, so module growers can fit far more cells per frame -- around 140 in 24-cell trays or 240 in 40-cell trays. The figures here allow working space rather than packing pots edge to edge.

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 seedlings or young plants to harden off or grow on at once. Sow ~10-20% extra to cover germination and losses

Pot / Tray SizeSmaller pots pack tighter: 7cm fits ~1.5x and 11cm ~0.65x the 9cm count. Module trays count individual cells, allowing space to lift trays in and out (~140 cells in 24-cell, ~240 in 40-cell per frame)

PurposeHardening off acclimatises plants over 7-14 days; overwintering protects tender herbs and salads from frost; direct growing warms soil 2-4 weeks ahead; staging holds potted-on plants before final positions

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 = 0.72 m² of floor) holds about 20 seedlings in 9cm pots with room to handle them. Smaller pots and module trays fit more per frame -- the table assumes around six standard 344 x 214mm trays per frame, so a 24-cell user gets roughly 140 cells and a 40-cell user roughly 240. Capacities allow working space rather than tight edge-to-edge packing. The calculator scales capacity to your chosen pot or tray size and adds a management note 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.