Dew Point Calculator

Enter an air temperature and relative humidity to calculate the dew point temperature instantly. The dew point tells you the temperature at which water vapor in the air will begin to condense. Knowing this value helps with weather planning, HVAC decisions, aviation preflight, and everyday comfort.

This calculator uses the Magnus formula to convert your inputs into a dew point estimate in both °C and °F.

Calculate Dew Point Temperature From Relative Humidity

To calculate dew point temperature, you need two values: the current air temperature and the relative humidity (RH). The calculator applies these to the Magnus approximation and returns the dew point in seconds.

A higher relative humidity pushes the dew point closer to the air temperature. When the two match, the air is fully saturated and condensation begins. A large gap between them means the air can still hold more moisture before dew or fog forms.

How the Dew Point Calculator Works

Temperature and Dewpoint Inputs

The calculator asks for two inputs:

• Air temperature in °C or °F (the current temperature of the surrounding air)
• Relative humidity as a percentage (how saturated the air is with water vapor compared to its maximum capacity at that temperature)

Enter values from a weather station, a hygrometer, or a forecast. The tool handles the Celsius to Fahrenheit conversion automatically and displays the dew point in both units.

Calculate Relative Humidity From Dewpoint

If you already know the dew point and the air temperature, you can work backward to calculate relative humidity. The relationship between these three values is fixed by the Magnus equation.

Relative humidity (RH) expresses the current water vapor content as a percentage of the maximum the air could hold at that temperature. When the dew point equals the air temperature, RH is 100%.

Humidity and Dew Point: What the Values Mean

Relative Humidity as a Percentage of Saturation

Relative humidity describes how close the air is to being fully saturated with water vapor. An RH of 50% means the air holds half of the moisture it could carry at that temperature.

RH changes with temperature even if the actual moisture content stays constant. Warmer air can hold more water vapor, so the same amount of moisture produces a lower RH on a hot day than on a cool one. That is why dew point is often a better comfort indicator than RH alone.

Water Vapor, Atmospheric Pressure, and Dew Point Formation

Water vapor is a gas mixed into the atmosphere. Its partial pressure contributes to total atmospheric pressure, though it makes up a small fraction. The dew point is the temperature at which that partial pressure equals the saturation vapor pressure and the vapor begins to condense into liquid water.

Several factors influence the dew point:

• Moisture content in the air. More water vapor raises the dew point.
• Atmospheric pressure. At standard pressure (1013.25 hPa), the Magnus formula gives reliable estimates. Extreme altitudes or pressures can shift results slightly.
• Air temperature. Higher temperatures allow more vapor before saturation.

Dew Point Temperature in °C and °F (Celsius and Fahrenheit)

The calculator reports the dew point in both Celsius and Fahrenheit. The conversion between them is straightforward:

• °F = (°C × 9/5) + 32
• °C = (°F − 32) × 5/9

Most meteorological sources report dew point in °C. Everyday weather apps in the US typically show °F. This tool gives you both so you can use whichever unit fits your situation.

Humidity Calculator for Different Conditions

Dry and Wet Bulb Temperature Measurement

A sling psychrometer measures two temperatures at once:

• Dry bulb temperature is the standard air temperature.
• Wet bulb temperature comes from a thermometer wrapped in a damp cloth. Evaporation cools it below the dry bulb reading.

The difference between dry and wet bulb values lets you derive relative humidity and dew point. A small gap means humid air (less evaporation). A large gap means dry air. If you have both bulb readings instead of RH, you can use psychrometric tables to find the RH, then enter it into this calculator.

Condensation, Fog, and Frost Formation Conditions

When the air temperature drops to the dew point, condensation occurs. Where and how it appears depends on conditions:

• Dew forms on grass and surfaces that cool faster than the surrounding air, typically on clear, calm nights.
• Fog forms when a large volume of air near the ground cools to its dew point.
• Frost forms when the dew point is at or below freezing (0 °C / 32 °F). Water vapor deposits directly as ice crystals instead of liquid.

Understanding these thresholds helps with agriculture, outdoor event planning, and building maintenance.

Carburetor Icing and Aviation Dewpoint

Pilots monitor the dew point closely. Carburetor icing can occur even when outside air temperatures are well above freezing, sometimes in conditions as warm as 20 °C to 25 °C (roughly 68 °F to 77 °F), when the dew point spread is small.

As fuel vaporizes inside the carburetor, the temperature drop can push local conditions below the dew point and then below freezing. Ice forms on the throttle plate and restricts airflow. Aviation weather reports (METARs) always include the dew point so pilots can assess this probability during preflight.

A dew point spread (the gap between temperature and dew point) of 3 °C or less signals high moisture and elevated risk of fog, low ceilings, or icing conditions.

How to Measure Dew Point and Relative Humidity

Using a Hygrometer to Measure Water Vapor

A hygrometer measures the moisture content of the air. Common types include:

• Capacitive sensors. Used in most digital weather stations and indoor monitors. They measure changes in electrical capacitance caused by absorbed moisture.
• Chilled mirror hygrometers. Cool a mirror until condensation forms, then read the temperature of the mirror surface directly as the dew point. These are highly accurate and used in lab and industrial settings.
• Sling psychrometers. Spin dry and wet bulb thermometers through the air, then use the temperature difference to calculate RH.

For home use, an inexpensive digital hygrometer gives RH and temperature readings you can plug straight into this calculator.

Weather Forecasting and Atmospheric Temperature

Meteorologists use dew point data extensively in weather forecasting. A rising dew point signals increasing moisture flowing into an area, which raises the probability of precipitation, thunderstorms, or fog.

Dew point values also help forecasters assess atmospheric stability. When the dew point and air temperature at various altitudes converge, clouds form at those levels. Surface dew points above 18 °C (65 °F) often indicate a humid, potentially uncomfortable air mass, while values above 21 °C (70 °F) signal oppressive humidity for most people.

Dew Point Calculator Formula and Equation

Magnus Formula to Calculate Dew Point Temperature

This calculator is built on the Magnus (or Magnus-Tetens) approximation. The formula calculates the dew point temperature (Td) from air temperature (T, in °C) and relative humidity (RH, in %):

1. Compute the intermediate value γ: γ(T, RH) = (b × T) / (c + T) + ln(RH / 100)
2. Solve for the dew point: Td = (c × γ) / (b − γ)

The constants typically used are:

• b = 17.67
• c = 243.5 °C

These constants are valid for air temperatures between roughly 0 °C and 60 °C, covering most everyday conditions.

Saturation Vapor Pressure of Water

The saturation vapor pressure is the maximum pressure water vapor can exert at a given temperature. The Magnus formula approximates it as:

es(T) = 6.112 × exp((b × T) / (c + T))

Where es is in hectopascals (hPa). Actual vapor pressure (e) is then:

e = es × (RH / 100)

The dew point is simply the temperature at which es equals e. These equations are widely accepted in meteorology, HVAC engineering, and environmental monitoring. They provide reliable estimates, though specialized instruments or the more detailed Sonntag equation may be used where laboratory-grade precision is required.

Humidity Calculator Conditions and Limitations

Apparent Temperature, Perspiration, and Comfort

The dew point is one of the best single indicators of how humid the air will feel. Here is a general comfort guide:

A dew point of 70 °F (about 21 °C) means the air holds enough moisture that your body's cooling system (perspiration evaporation) becomes much less effective. Apparent temperature rises well above the actual thermometer reading.

When This Calculator Gives an Estimate, Not an Exact Value

This dew point calculator provides estimates suitable for planning and general reference. Results may vary slightly from laboratory instruments for several reasons:

• The Magnus formula uses simplified constants. More complex equations (such as the Sonntag equation) add decimal precision but require additional parameters.
• Extreme temperatures below 0 °C or above 60 °C push outside the range where these constants are most accurate.
• Atmospheric pressure variations at high altitude or in unusual weather patterns can affect real-world dew point formation.
• Sensor accuracy matters. The estimate is only as good as the temperature and RH values you enter.

For HVAC design, aviation operations, or industrial process control, always verify results with calibrated instruments or consult a qualified professional. This calculator is a planning aid, not a replacement for professional measurement.