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Saturation Vapor Pressure Calculator

Enter an air temperature in °C to calculate the saturation vapor pressure using the Magnus formula, plus conversions to kPa, mmHg, psi, and more.
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Luis GonzalezCreated by Luis GonzalezLast updated:

How to Use This Calculator

  1. 1

    Enter Temperature (°C)

    Input the air temperature in degrees Celsius. The Magnus formula is accurate for roughly -40°C to 60°C.

  2. 2

    Review your results

    The calculator will display the saturation vapor pressure in multiple units (hPa, kPa, mmHg, psi) and saturation absolute humidity.

Example Calculation

A meteorologist needs to determine the saturation vapor pressure at a temperature of 25°C to assess atmospheric conditions.

Temperature (°C)

25

Results

31.670 hPa

Tips

Understand Relative Humidity

Saturation vapor pressure is the maximum possible. Actual vapor pressure divided by saturation vapor pressure (times 100) gives relative humidity.

Note Temperature Sensitivity

Vapor pressure increases exponentially with temperature. Even a small change in temperature can significantly alter the saturation point.

Consider Phase Changes

Below 0°C, the saturation vapor pressure over ice is slightly different from over supercooled liquid water. This calculator uses the water formula.

The Saturation Vapor Pressure Calculator is a critical tool for meteorologists, HVAC professionals, and chemists, providing precise calculations of saturation vapor pressure from temperature using the Magnus formula. This tool delivers results in various units, including hPa, kPa, mmHg, and psi, along with saturation absolute humidity. Understanding this metric is vital for predicting condensation, cloud formation, and humidity levels in 2025. For example, at a temperature of 25°C, the saturation vapor pressure is approximately 31.670 hPa, indicating a moderate capacity for atmospheric moisture.

The Thermodynamics of Water Vapor in Air

Saturation vapor pressure is a fundamental concept in physical chemistry and atmospheric science, describing the maximum partial pressure that water vapor can exert at a given temperature before it begins to condense into liquid water or sublimate into ice. This phenomenon is governed by the kinetic energy of water molecules; as temperature rises, molecules move faster, and more are able to escape the liquid phase into the air, increasing the vapor pressure. It's a key factor in understanding phase transitions, cloud formation, and the dew point. At the triple point of water (0.01 °C and 6.1166 hPa), all three phases (solid, liquid, gas) coexist in equilibrium.

Calculating Saturation Vapor Pressure with Magnus's Formula

The Saturation Vapor Pressure Calculator primarily uses the Magnus formula, an empirical equation widely adopted for its accuracy in meteorological applications. The formula, specifically the Alduchov & Eskridge 1996 coefficients, is as follows:

es_hPa = 6.1078 × exp((17.2694 × T) / (T + 237.29))

Where:

  • es_hPa = Saturation Vapor Pressure in hectopascals (hPa)
  • exp = The exponential function (e^x)
  • T = Temperature in degrees Celsius (°C)

This formula is valid for a wide range of temperatures, typically from -40 °C to 60 °C. The result in hPa can then be converted to other units such as kilopascals (kPa), Pascals (Pa), millimeters of mercury (mmHg), inches of mercury (inHg), and pounds per square inch (psi) using standard conversion factors.

💡 For other gas-related calculations, our Volume to Mole Gas Calculator can help determine the amount of substance in a given gas volume.

Determining Vapor Pressure at 25°C

Let's calculate the saturation vapor pressure at a common room temperature of 25°C using the Magnus formula:

  1. Input Temperature: T = 25 °C.
  2. Apply Magnus Formula: es_hPa = 6.1078 × exp((17.2694 × 25) / (25 + 237.29)) es_hPa = 6.1078 × exp(431.735 / 262.29) es_hPa = 6.1078 × exp(1.6460) es_hPa = 6.1078 × 5.1856 es_hPa ≈ 31.670 hPa
  3. Unit Conversions:
    • es_kPa = 31.670 hPa / 10 = 3.1670 kPa
    • es_mmHg = 31.670 hPa × 0.750062 = 23.754 mmHg
    • es_psi = 31.670 hPa × 0.0145038 = 0.45934 psi

The primary result, the Saturation Vapor Pressure, is 31.670 hPa. This value indicates a moderate capacity for water vapor in the air at this temperature.

💡 For understanding other water properties, our Water Hardness Converter (ppm to dGH) can translate different units of water hardness.

The Thermodynamics of Water Vapor in Air

Saturation vapor pressure is a fundamental concept in physical chemistry and atmospheric science, describing the maximum partial pressure that water vapor can exert at a given temperature before it begins to condense into liquid water or sublimate into ice. This phenomenon is governed by the kinetic energy of water molecules; as temperature rises, molecules move faster, and more are able to escape the liquid phase into the air, increasing the vapor pressure. It's a key factor in understanding phase transitions, cloud formation, and the dew point. At the triple point of water (0.01 °C and 6.1166 hPa), all three phases (solid, liquid, gas) coexist in equilibrium.

Comparing Saturation Vapor Pressure Formulas

While the Magnus formula (Alduchov & Eskridge 1996 coefficients) is widely used for its balance of accuracy and simplicity, several other formulas exist for calculating saturation vapor pressure, each with slightly different coefficients or ranges of applicability. For instance, the Antoine equation is another common empirical formula, often used for various substances, including water, and typically expressed as log10(P) = A - B/(C+T). Different sets of A, B, and C coefficients are used depending on the temperature range and desired precision. The Clausius-Clapeyron equation, derived from thermodynamic principles, provides a more theoretical basis for the relationship but requires knowing the latent heat of vaporization. This calculator uses the Magnus formula for its specific application in meteorology, but for high-precision scientific work or for other substances, these alternative models or more complex polynomial equations might be employed.

Frequently Asked Questions

What is saturation vapor pressure?

Saturation vapor pressure is the maximum amount of water vapor that can exist in the air at a given temperature before condensation or evaporation occurs. It represents the equilibrium point where the rate of water molecules evaporating from a liquid surface equals the rate of water molecules condensing back into the liquid. This pressure increases exponentially with rising temperature, meaning warmer air can hold significantly more moisture.

Why is saturation vapor pressure important in meteorology?

Saturation vapor pressure is a fundamental concept in meteorology, essential for understanding weather phenomena like cloud formation, dew point, and humidity. It directly influences relative humidity and helps predict when air will become saturated, leading to precipitation. Meteorologists use it to model atmospheric processes and forecast weather patterns globally.

How does temperature affect saturation vapor pressure?

Temperature is the primary driver of saturation vapor pressure; as temperature increases, the kinetic energy of water molecules rises, allowing more molecules to escape into the vapor phase, thus increasing the maximum vapor pressure the air can sustain. Conversely, lower temperatures result in lower saturation vapor pressure, reducing the air's capacity to hold moisture and making condensation more likely.