Garden Frost Protection Calculator
Find how much frost cloth your beds need and whether your cover keeps plants above freezing on a cold night.
How it works
fabric = bed area × (1 + drape %) · effective low = forecast low + cloth protection
Frost cloth (floating row cover) works by trapping heat that the soil radiates back at night, so it must reach the ground on all sides to hold that warm air in — plan for 15–25% extra fabric beyond the bare bed area. Its rating tells you roughly how many degrees of protection it adds: light cloth buys 2–4°F, heavy cloth 6–8°F. Add that to the forecast low to judge whether plants clear freezing; a cover that only floats on top of the leaves protects far less.
Worked example
A 120 sq ft bed with 20% for drape needs 120 × 1.20 = 144 sq ft of cloth, about $21.60 at $0.15/sq ft. Medium 1.0 oz cloth adds roughly 5°F, so a forecast low of 30°F becomes about 35°F under the cover — safely above freezing.
Frequently asked questions
Does frost cloth actually keep plants warm?
It doesn’t make heat — it slows the loss of heat the ground stored during the day. That’s why it must drape to the soil and why a sunny day before a frost night helps: warm soil is the battery the cover holds in.
Can I just use bedsheets or plastic?
Old sheets and blankets work in a pinch for a light frost. Plastic is the worst choice touching foliage — where it touches leaves it conducts cold straight through and can cause more damage than no cover; keep plastic off the leaves with hoops.
When should I take the cover off?
Vent or remove breathable frost cloth in the morning once temperatures rise, especially in sun, so trapped heat and moisture don’t cook or rot the plants. Lightweight row cover can often stay on through mild days.
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This calculator is for educational purposes. Double-check important results before acting on them.