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Dew Point Calculator
This dew point calculator finds the dew point from the air temperature and the relative humidity, using the Magnus formula. Enter the temperature in Fahrenheit or Celsius and the humidity as a percentage, and it shows the dew point in both units, a plain-English comfort note, and the frost point whenever the dew point drops below freezing.
The dew point is never higher than the air temperature. If the two numbers are close together, the air is near saturation and feels muggy.
How it works
The dew point is the temperature the air would have to cool to before its water vapor starts condensing into liquid. Warm air can hold more water vapor than cold air, so on a warm afternoon the air can carry a lot of moisture without feeling damp. As the air cools — overnight, or against a cold window — it can hold less and less, until at the dew point it is full. Any further cooling squeezes the extra vapor out as dew, fog, or condensation on a cold surface.
That is why the dew point is a better measure of mugginess than relative humidity. Relative humidity tells you how full the air is at its current temperature, so the same amount of moisture reads as a low percentage on a hot day and a high percentage on a cold day. The dew point does not move with the temperature in that way: a dew point of 70 degrees feels muggy whether the air is 75 or 95 degrees, because the actual amount of moisture in the air is the same. Weather forecasts quote the dew point for exactly this reason.
This calculator uses the Magnus formula. First it turns the temperature and humidity into a single intermediate value, then it converts that value back into a temperature — the dew point. The two inputs are all it needs: the air temperature, in Fahrenheit or Celsius, and the relative humidity as a percentage from 1 to 100.
Two extra numbers help you read the result. The temperature–dew point spread is the gap between the air temperature and the dew point: a small spread means the air is close to saturation, which is why fog forms on mornings when the spread is only a few degrees. The comfort note sorts the dew point into plain bands — below 55 degrees feels comfortable, 55 to 59 slightly sticky, 60 to 64 humid, 65 to 69 very humid, and 70 or above oppressive. When the dew point falls below 32 degrees Fahrenheit, the calculator also shows the frost point, the temperature at which vapor turns straight into ice. The frost point sits about a degree above the dew point, and it is the number that matters for frost on windshields, plants, and roofs.
Worked example
It is a summer afternoon in Atlanta, Georgia. The thermometer reads 75 °F and the relative humidity is 60%. What is the dew point, and how will the air feel?
Step 1: Convert 75 °F to Celsius: (75 − 32) × 5 ÷ 9 = 23.9 °C.
Step 2: Apply the Magnus formula with T = 23.9 °C and RH = 60%: α = (17.27 × 23.9 ÷ (237.7 + 23.9)) + ln(0.60) = 1.58 − 0.51 = 1.07.
Step 3: Dew point = 237.7 × 1.07 ÷ (17.27 − 1.07) = 15.6 °C.
Step 4: Convert back to Fahrenheit: 15.6 × 9 ÷ 5 + 32 = 60.2 °F. The spread is 75 − 60.2 = 14.8 degrees, and 60.2 °F falls in the humid band.
The tool gives exactly this result: dew point 60.2 °F (15.6 °C), with the note that the air feels humid and sticky. A cold-morning check works too: at 30 °F and 70% humidity the dew point is 21.4 °F and the frost point is 22.6 °F, so expect frost on the windshield.
FAQ
What is the dew point?
The dew point is the temperature at which the air becomes fully saturated with water vapor, so the vapor starts to condense into liquid water. If the air cools to its dew point overnight, dew forms on grass, cars, and windows. It is always equal to or lower than the air temperature, and the closer the two numbers are, the more humid the air feels.
What dew point feels comfortable?
Most people find a dew point below 55 degrees Fahrenheit comfortable and dry-feeling, and 55 to 59 degrees slightly sticky but still fine. From 60 to 64 degrees the air feels humid, 65 to 69 degrees feels very humid and oppressive, and 70 degrees or higher feels extremely muggy. These are comfort guidelines, not health limits — a dew point of 62 degrees on a summer afternoon in Florida is normal, even if it feels sticky.
How is the dew point calculated from temperature and humidity?
This calculator uses the Magnus formula, which works from the air temperature and the relative humidity. It first computes an intermediate value from the temperature and the humidity, then converts that value back into a temperature. The formula is accurate to within about half a degree for the temperatures and humidity levels people normally meet outdoors, which is plenty for planning a run, a paint job, or a camping trip.
What is the difference between dew point and frost point?
They are the same idea at different temperatures. The dew point is the temperature where water vapor condenses into liquid dew, and the frost point is the temperature where it turns directly into ice crystals. When the dew point is below freezing, the frost point is the more useful number, and it sits slightly above the dew point — usually by about one degree. This calculator shows the frost point whenever the dew point is below 32 degrees Fahrenheit.
Why does my bathroom mirror fog up if the dew point is low?
A hot shower adds a lot of water vapor to a small room, which raises the dew point fast, while the mirror stays cool from the room air. When the mirror's surface drops below the new, higher dew point, vapor condenses on it. Opening a window or running the exhaust fan brings in drier air and drops the dew point, which is why the mirror clears. The same effect fogs car windows on a cool morning.