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Cold Frame Calculator -- How Many Cold Frames Do I Need
Calculate cold frame size and capacity for hardening off plants.
Total seedlings or young plants to harden off or grow on at once. Sow ~10-20% extra to cover germination and losses
Smaller 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)
Hardening 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
Number of 9cm pots per standard frame (default 20)
Width of each cold frame in centimetres (default 120)
Depth (front to back) of each cold frame in centimetres (default 60)
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.