Vapor Pressure Calculator

Enter air temperature and relative humidity to calculate actual vapor pressure, saturation vapor pressure, vapor pressure deficit, dew point, and more.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter the air temperature
Input the current air temperature in Celsius, Fahrenheit, or Kelvin, which significantly influences the saturation vapor pressure.
2
Provide relative humidity
Enter the relative humidity as a percentage (0-100%), representing the ratio of actual to saturation vapor pressure.
3
Select temperature unit
Choose the appropriate unit for your temperature input: Celsius (°C), Fahrenheit (°F), or Kelvin (K).
4
Review your results
The calculator will display the actual vapor pressure, saturation vapor pressure, vapor pressure deficit (VPD), dew point, specific humidity, and mixing ratio.

Example Calculation

A meteorologist needs to determine the atmospheric conditions when the air temperature is 20°C with 65% relative humidity.

Temperature

20 °C

Relative Humidity

65%

Temperature Unit

Celsius (°C)

Results

15.204 hPa

Tips

Monitor VPD for Plant Health

For optimal plant growth in greenhouses, aim to keep the Vapor Pressure Deficit (VPD) between 0.8 and 1.2 kPa (approximately 8-12 hPa), as this range balances transpiration and nutrient uptake effectively.

Dew Point Indicates Comfort

A dew point below 10°C generally indicates dry and comfortable air, while a dew point above 16°C often signifies muggy conditions, impacting human comfort and the likelihood of condensation.

Temperature's Strong Influence

Even a small increase in temperature dramatically increases the air's capacity to hold moisture, almost doubling the saturation vapor pressure for every 10°C rise, making accurate temperature input critical.

Understanding Atmospheric Moisture with the Vapor Pressure Calculator

The Vapor Pressure Calculator provides a comprehensive analysis of atmospheric moisture, allowing users to determine actual vapor pressure, saturation vapor pressure, vapor pressure deficit (VPD), dew point, specific humidity, and mixing ratio from temperature and relative humidity inputs. This tool is indispensable for meteorologists, agriculturalists, and industrial engineers who require precise data on air's moisture content. Understanding these metrics is fundamental to predicting weather, optimizing greenhouse environments, and controlling processes like drying or humidification, where even small changes can significantly impact outcomes.

Calculating Key Atmospheric Moisture Metrics

The core of atmospheric moisture calculations relies on the Magnus formula, which accurately estimates saturation vapor pressure (es) based on temperature. From this, the actual vapor pressure (e) is derived using relative humidity. The Vapor Pressure Deficit (VPD) then quantifies the drying power of the air, while the dew point reveals the temperature at which condensation begins. These interconnected calculations provide a holistic view of the air's moisture dynamics.

es = 6.112 × e^((17.67 × tempC) / (tempC + 243.5))
e = es × (relative humidity / 100)
vpd = es - e

Where:

es is saturation vapor pressure (hPa)
e is actual vapor pressure (hPa)
tempC is temperature in Celsius
vpd is vapor pressure deficit (hPa)

💡 To understand how the presence of solutes affects vapor pressure, our Number of Moles in Solution Calculator can help analyze solution concentrations.

Analyzing Air Conditions for a Controlled Environment

Consider a scenario where an environmental scientist is monitoring a controlled growth chamber with an air temperature of 20°C and a relative humidity of 65%. They need to understand the precise moisture characteristics of this environment.

Calculate Saturation Vapor Pressure (es): Using the Magnus formula: es = 6.112 × e^((17.67 × 20) / (20 + 243.5)) es = 6.112 × e^(353.4 / 263.5) es = 6.112 × e^(1.3419) es ≈ 6.112 × 3.8267 = 23.391 hPa
Calculate Actual Vapor Pressure (e): e = 23.391 hPa × (65 / 100) = 15.204 hPa
Calculate Vapor Pressure Deficit (VPD): vpd = 23.391 hPa - 15.204 hPa = 8.187 hPa
Calculate Dew Point: (Using the inverse Magnus formula, which is more complex but yields a value) dewPoint ≈ 13.6°C

Thus, at 20°C and 65% RH, the actual vapor pressure is 15.204 hPa, the saturation vapor pressure is 23.391 hPa, and the VPD is 8.187 hPa. The dew point of 13.6°C indicates a comfortable moisture level in the air.

💡 When studying gas mixtures, understanding the molecular weight of organic components can be important; use our Organic Molecular Weight Calculator for related analyses.

Atmospheric Chemistry and Microclimates

Vapor pressure is a cornerstone in atmospheric science, directly influencing phenomena like cloud formation, fog, and the general humidity levels that define local microclimates. At sea level, standard atmospheric pressure is approximately 1013.25 hPa, and the partial pressure contributed by water vapor is a critical factor in determining weather stability. In agriculture, especially in controlled environments like greenhouses, maintaining the Vapor Pressure Deficit (VPD) within an optimal range, typically between 0.8 and 1.2 kPa, is crucial for plant transpiration and nutrient uptake, impacting crop yield and health. Industrial processes, from pharmaceutical drying to food preservation, also rely on precise vapor pressure control to ensure product quality and efficiency.

Typical Vapor Pressure Ranges Across Environments

Vapor pressure values vary significantly depending on environmental conditions and specific applications. For instance, in an optimal greenhouse environment designed for plant growth, the Vapor Pressure Deficit (VPD) is typically maintained between 0.8 and 1.2 kPa (8-12 hPa) to promote healthy transpiration without causing undue stress. For human comfort, a dew point below 16°C (61°F) is generally considered pleasant, indicating a lower actual vapor pressure, while dew points exceeding 20°C (68°F) often lead to muggy and uncomfortable conditions, signaling high moisture content. In industrial drying processes, engineers might target very low actual vapor pressures, sometimes below 1 hPa, to accelerate moisture removal from materials, while in steam generation, saturation vapor pressures can reach hundreds of atmospheres, corresponding to superheated temperatures.

Frequently Asked Questions

What is vapor pressure and why is it important in atmospheric science?

Vapor pressure is the pressure exerted by water vapor in a gas mixture, such as the atmosphere. It is crucial in atmospheric science because it directly influences cloud formation, precipitation, and overall weather patterns. Higher vapor pressure indicates more moisture in the air, increasing the likelihood of condensation and subsequent weather phenomena, making it a key metric for meteorologists and climate scientists to understand and predict atmospheric behavior.

How does relative humidity relate to actual and saturation vapor pressure?

Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure, expressed as a percentage. Actual vapor pressure is the amount of water vapor currently in the air, while saturation vapor pressure is the maximum amount of water vapor the air can hold at a given temperature. When actual vapor pressure equals saturation vapor pressure, relative humidity is 100%, and the air is fully saturated, leading to condensation.

What is Vapor Pressure Deficit (VPD) and its significance?

Vapor Pressure Deficit (VPD) is the difference between the saturation vapor pressure and the actual vapor pressure at a given temperature. It represents the 'drying power' of the air. In agriculture, VPD is critical for greenhouse management as it dictates the rate of plant transpiration. An optimal VPD range, typically between 0.8 and 1.2 kPa, ensures healthy plant growth by balancing moisture loss with nutrient uptake from the roots.