Dew Point Calculator

The Formula

The Magnus formula is a widely used approximation for calculating the dew point temperature from air temperature and relative humidity. The dew point is the temperature at which air becomes saturated with water vapor and condensation begins. A higher dew point indicates more moisture in the air, directly affecting human comfort. The constants a and b are empirically derived for temperatures above freezing and provide accurate results for typical weather conditions.

Variable Definitions

How to Use This Calculator

1. 1

   Enter the current air temperature in degrees Fahrenheit.

2. 2

   Enter the relative humidity percentage (0-100%).

3. 3

   The calculator converts to Celsius, applies the Magnus formula, and converts the dew point back to Fahrenheit.

4. 4

   Review the comfort level description and HVAC relevance.

5. 5

   A dew point below 55°F is comfortable; above 70°F feels oppressive.

Dew point is an absolute measure of moisture, unlike relative humidity

Understanding the Concept

The dew point is a critical meteorological measure that directly indicates the moisture content of the air. Unlike relative humidity, which changes with temperature, the dew point is an absolute measure of atmospheric moisture. This makes it a more reliable indicator of human comfort. A dew point below 55°F feels dry and comfortable, while a dew point above 70°F feels oppressive. The Magnus formula provides an accurate approximation of the dew point for temperatures above freezing. Understanding the dew point is essential for HVAC system design, agricultural frost prediction, fog forecasting, and assessing mold growth risk in buildings. When the temperature drops to the dew point, fog, dew, or frost will form depending on conditions. Practical example: if the temperature is 85°F and relative humidity is 70%, the Magnus formula gives a dew point of approximately 74°F. This means the air contains as much moisture as air at 74°F would hold at 100% humidity. According to the comfort scale, 74°F is in the "oppressive" range — the air feels very humid and sticky. If the temperature drops to 74°F overnight, fog will form. Edge cases: when the dew point is below 32°F (freezing), frost forms instead of dew. If the temperature is also below freezing, the saturation vapor pressure over ice differs from that over liquid water, so the Magnus constants change (a = 21.875, b = 265.5 for ice). For indoor humidity management, the dew point should ideally be between 45-55°F to prevent both mold growth (which requires dew points above 55°F) and excessive dryness (which can cause respiratory irritation and static electricity). For instrument calibration and industrial applications (e.g., compressed air systems), the dew point is measured directly with a chilled mirror hygrometer. In tropical climates, dew points of 75-80°F are common and can be dangerous when combined with high temperatures because the body cannot cool itself through sweat evaporation.

Frequently Asked Questions

Sources & References