Frost point calculator
Below freezing, moisture skips the dew and deposits straight tofrost. Enter temperature and humidity to get the frost point — and whether tonight's surfaces will dew up or ice over.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (Magnus formula over water and over ice). Last verified July 2026 — our methodology.
Frost point
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- Dew point
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- What forms
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- Air temp
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The frost point is the temperature at which the air is saturated with respect to ice, computed with the Magnus coefficients for ice rather than water. When a surface is below freezing, vapour deposits directly as frost, and the frost point sits a little above the dew point. Above freezing, dew forms at the dew point as usual.
Frost point vs dew point
Gardeners watch the frost point to know when a clear, still night will leave ice on the grass, car windshields, or tender plants; it is also why freezers and cold coils build up frost. Because saturation pressure over ice is slightly lower than over liquid water, frost can start a degree or two warmer than plain dew would. Above freezing the two match — use the dew point calculatorthere. To see how cooling air drives toward these points, try thetemperature-change humidity calculator, or read dew point vs relative humidity.