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Cold Frame Calculator -- How Many Cold Frames Do I Need
Calculate cold frame size and capacity for hardening off plants.
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
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.
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.
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.
Yes. Cold frames are excellent for winter salads, radishes, and early carrots. They warm the soil 2-4 weeks ahead of open ground.
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.
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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.