Dew Point Calculator

1. Enter the air temperature, in Fahrenheit or Celsius.
2. Enter the relative humidity, as a percentage from 1 to 100.
3. Read the dew point, the temperature air must cool to for moisture to condense.
4. See the comfort level and how far the dew point sits below the air temperature.

Here is what each result means:

The dew point is the temperature the air would need to cool to for water vapour to start condensing into dew, fog, or droplets. It is a direct measure of how much moisture is actually in the air.

A higher dew point means more moisture and a more humid, sticky feel. Because it does not depend on the current temperature the way relative humidity does, meteorologists treat the dew point as the more honest comfort measure.

It uses the Magnus-Tetens approximation, the standard meteorological formula that links temperature, relative humidity, and dew point.

1. Convert the temperature to Celsius if entered in Fahrenheit.
2. Compute the intermediate term alpha from temperature (T) and relative humidity (RH), using a = 17.27 and b = 237.7 degrees C.
3. Solve for the dew point temperature (Td).
4. Convert the result back to your chosen display unit.

The dew point can never be higher than the air temperature. At 100 percent humidity the two are equal, and the calculator clamps the result accordingly.

Find the dew point at 85 degrees F with 70 percent relative humidity.

Calculation: converting 85 degrees F to about 29.4 degrees C, feeding T = 29.4 and RH = 70 into the Magnus formula gives a dew point of about 74 degrees F, roughly 11 degrees below the air temperature. A dew point in the mid-70s feels very humid and sticky.

Comfort tracks the dew point closely. The bands below follow the widely used National Weather Service dew point comfort scale.

Relative Humidity Changes With Temperature

Relative humidity is the percentage of moisture the air holds compared with the most it could hold at that temperature. As the air warms through the day, the same amount of moisture gives a lower relative humidity, even though nothing about the moisture changed.

Dew Point Is Absolute

The dew point reflects the actual amount of water vapour in the air. It barely shifts as the temperature rises and falls, so a dew point of 70 degrees F feels muggy whether the air temperature is 75 or 95 degrees.

Why Meteorologists Prefer Dew Point

Because it does not depend on the current temperature, the dew point is a more reliable guide to how humid it actually feels than relative humidity on its own.

Judging Real Comfort

Use the dew point to tell how muggy it will actually feel, especially in summer, when relative humidity alone can be misleading.

Predicting Fog and Dew

When the air temperature falls close to the dew point overnight, fog or dew becomes likely. A small gap is an early warning sign.

Managing Indoor Humidity

Knowing the outdoor dew point helps you decide when to ventilate or run a dehumidifier to keep indoor air comfortable and condensation-free.

1. Treating Humidity and Dew Point as the Same Thing

A relative humidity of 90 percent on a cold morning holds far less moisture than 50 percent on a hot afternoon. The dew point tells you the actual moisture; humidity does not.

2. Expecting Dew Point Above Air Temperature

The dew point can never exceed the air temperature. At 100 percent humidity they are equal, and it cannot go higher.

3. Entering Zero Humidity

A relative humidity of exactly zero has no defined dew point, since perfectly dry air never condenses. Enter a value of at least 1 percent.

4. Ignoring the Comfort Scale

People often watch the temperature and ignore the dew point. Yet a modest 80 degree day with a 74 degree dew point can feel far worse than a hot, dry one.

The Magnus-Tetens approximation is accurate to within about 0.4 degrees C across the normal range of weather conditions.

The dew point uses the Magnus-Tetens approximation, which combines air temperature and relative humidity. At 85 degrees F with 70 percent humidity, the dew point works out to about 74 degrees F.

Relative humidity is the percentage of moisture the air holds compared with its maximum at that temperature, so it changes as the air warms and cools. The dew point measures the actual moisture in the air and barely changes, which makes it a more reliable comfort guide.

A dew point below about 60 degrees F feels comfortable to most people. Between 60 and 65 degrees it becomes noticeable, above 70 degrees it feels very humid, and 75 degrees or higher feels oppressive.

No. The dew point can never exceed the air temperature. When the air is fully saturated at 100 percent humidity, the dew point equals the air temperature, and it cannot go any higher.

When the air temperature falls close to the dew point, the air is near saturation, so fog or dew is likely to form. Overnight, a small gap is a good early sign of morning fog.

Relative humidity drops as the day warms, even when moisture stays the same. The dew point does not, so a high dew point explains why an afternoon can feel muggy despite a modest humidity reading.

The heat index is calculated from temperature and relative humidity, not dew point directly. However, a high dew point usually means a high heat index, since both reflect abundant moisture in the air.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere, unless you choose Save, which keeps the result only on this device.

• Vapor Pressure and Dew Point (National Weather Service, El Paso).
• Dew Point vs Humidity (National Weather Service, La Crosse).