What this calculator does

This calculator estimates relative humidity when you already know the air temperature and dew point. It is useful because dew point and dry-bulb temperature together define the moisture state of the air well enough to solve for RH.

People often know dew point from weather data or field observations but still need relative humidity for reporting, equipment checks, or comfort comparisons. This page bridges that gap by converting both temperatures into vapor pressure terms and then computing the ratio between actual vapor pressure and saturation vapor pressure.

Inputs explained

• Air temperature: Enter the current dry-bulb air temperature.
• Dew point temperature: Enter the dew point associated with the same air sample.
• Internal units: The page evaluates the saturation relationships in Celsius and then reports the RH percentage.

How it works / method

The engine applies the same compact vapor-pressure-style approximation to both the dew point and the air temperature. Actual vapor pressure comes from the dew point, while saturation vapor pressure comes from the air temperature. Relative humidity is then calculated as 100 times the ratio of those two values.

Formula used

T is air temperature in C and Td is dew point in C. Because this uses an approximation for vapor pressure, the result is a practical estimate rather than a high-precision psychrometric inversion.

Step-by-step example

Suppose the air temperature is 25 C and the dew point is 15 C. The air contains a fixed moisture amount, but RH depends on how close 25 C is to saturation.

1. Enter 25 for air temperature.
2. Enter 15 for dew point.
3. The calculator estimates actual vapor pressure from the dew point and saturation vapor pressure from the air temperature.
4. The resulting RH is a little above 50 percent.
5. If the same dew point occurred at a cooler air temperature, RH would be higher because the air would be closer to saturation.

Use cases

• Converting forecast or observed dew point into relative humidity for reports and quick comparisons.
• Explaining why RH can change through the day even when moisture content changes very little.
• Supporting classroom psychrometric exercises and humidity-state interpretation.
• Checking indoor or outdoor moisture conditions when dew point is the primary available measurement.

Assumptions and limitations

• The result depends on the approximation used for vapor pressure and on the accuracy of both temperatures.
• Relative humidity can be a useful display value but is not always the best standalone indicator of moisture comfort.
• The page does not explicitly model barometric pressure or specialty psychrometric edge cases.
• Rounding both temperatures too aggressively can introduce visible RH differences in near-saturation situations.

If you want a more stable description of moisture content across changing temperatures, compare this page with the dew point and vapor pressure calculators.

Sources & references

• NOAA National Weather Service - Dew point glossary
• NOAA National Weather Service - Relative humidity glossary
• NOAA National Weather Service - Dry bulb, wet bulb, and dew point definitions
• NOAA National Weather Service - Vapor pressure glossary
• American Meteorological Society - Stull wet-bulb approximation
• NIST Chemistry WebBook - Water data
• Schema.org - FAQPage and WebApplication vocabulary