What this calculator does

This vapor pressure calculator estimates the partial pressure of water vapor in the air from air temperature and relative humidity. Vapor pressure is a direct moisture quantity that sits behind many other humidity measures, including wet-bulb temperature, humidex, and some apparent-temperature formulas.

Relative humidity is familiar, but it is not a direct measure of the amount of water vapor in the air. Vapor pressure is closer to a physical state variable because it describes the share of total pressure contributed by water vapor. That makes it valuable when you want a more explicit moisture quantity for weather interpretation or thermodynamic estimates.

Inputs explained

• Air temperature: Enter the current dry-bulb temperature.
• Relative humidity: Enter the moisture percentage relative to saturation.
• Output units: The page reports both hPa and kPa so you can move easily between weather and engineering conventions.

How it works / method

The engine first estimates saturation vapor pressure from temperature using a compact exponential approximation. It then scales that saturation pressure by relative humidity to estimate actual vapor pressure. The page presents the result in two common pressure units for easier reuse in related calculations.

Formula used

T is air temperature in C and e is reported in hPa before the page also converts it to kPa. This is a practical saturation-vapor-pressure approximation rather than a full property-table lookup.

Step-by-step example

Suppose the air temperature is 28 C and the relative humidity is 70 percent. Vapor pressure shows the moisture term more directly than RH alone.

1. Enter 28 for air temperature.
2. Enter 70 for relative humidity.
3. The page estimates the saturation vapor pressure at 28 C and then takes 70 percent of that value.
4. The result is displayed in hPa and kPa so it can be reused in comfort or engineering estimates.
5. If temperature rises without adding moisture, relative humidity may fall even while vapor pressure stays similar.

Use cases

• Supporting humidity-state interpretation behind dew point, wet-bulb, and apparent-temperature calculations.
• Comparing atmospheric moisture using a direct pressure term rather than only RH percentages.
• Checking moisture-related calculations in weather education, greenhouse monitoring, or HVAC interpretation.
• Providing a reusable moisture value for humidex or other thermal comfort estimates.

Assumptions and limitations

• The page estimates vapor pressure with a compact saturation formula and does not replace full property tables.
• At extremes of temperature or pressure, specialized formulations may be more appropriate than a simple online approximation.
• Relative humidity measurements often carry their own uncertainty, which feeds directly into the pressure result.
• The calculator assumes an ordinary air-water context and is not intended for full thermodynamic phase-equilibrium modeling.

For weather comfort interpretation, pair this tool with dew point, wet bulb, or humidex. For exact thermodynamic property work, consult NIST or another reference dataset.

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