Actual Vapor Pressure Calculator: Formula and Use

An actual vapor pressure calculator estimates the partial pressure of water vapor in air from temperature and relative humidity. Enter air temperature and relative humidity; the calculator returns actual vapor pressure, typically in kilopascals (kPa), along with saturation vapor pressure for context.

Formula

Actual vapor pressure is the fraction of saturation vapor pressure indicated by relative humidity:

e = (RH / 100) × e_s

For temperatures near ordinary atmospheric conditions, this calculator estimates saturation vapor pressure with the commonly used Tetens approximation:

e_s = 0.6108 × exp[(17.27 × T) / (T + 237.3)]

• e is actual water-vapor pressure, in kPa.
• RH is relative humidity, entered as a percentage from 0 to 100.
• e_s is saturation vapor pressure at the air temperature, in kPa.
• T is air temperature in degrees Celsius (°C).
• exp means the exponential function, e^x.

The input accepts Celsius. For Fahrenheit, first convert with T = (°F − 32) × 5/9. The coefficient 0.6108 gives the pressure in kPa; the same pressure can be converted to hectopascals (hPa) by multiplying by 10, or to millibars by the same factor. One kPa is approximately 0.1450 pounds per square inch (psi).

Worked Example

Using the calculator’s defaults, air temperature is 20°C and relative humidity is 50%.

1. Calculate saturation vapor pressure: e_s = 0.6108 × exp[(17.27 × 20) / (20 + 237.3)] ≈ 2.338 kPa.
2. Apply relative humidity: e = (50 / 100) × 2.338 ≈ 1.169 kPa.
3. Rounded to the calculator’s three decimal places, actual vapor pressure is 1.169 kPa.

The corresponding saturation vapor pressure is 2.338 kPa. At the same temperature, the air’s water-vapor pressure is half the saturation value because relative humidity is 50%.

What Actual Vapor Pressure Means

Air can contain water vapor, and that vapor exerts pressure. Actual vapor pressure describes the pressure contributed by water vapor currently present in a sample of air. It is a partial pressure, not the total atmospheric pressure.

Saturation vapor pressure is the equilibrium vapor pressure when air is saturated with water at a specified temperature. It rises as temperature rises: warmer air has a higher saturation vapor pressure. Relative humidity compares actual vapor pressure with the saturation value at the same temperature:

RH = 100 × e / e_s

This relationship explains why relative humidity can change when air warms or cools even if its actual water-vapor content changes little. Cooling air raises its relative humidity as saturation vapor pressure falls. If the air reaches saturation, condensation may begin, depending on surfaces and conditions.

When to Use the Calculator

Actual vapor pressure is useful when a calculation or comparison needs humidity expressed as pressure rather than as a percentage. It appears in meteorology, environmental measurements, and some engineering calculations involving moist air. It can also help compare moisture conditions across temperatures: relative humidity alone can be misleading because its reference value changes with temperature.

• Use it when you know air temperature and relative humidity and need vapor pressure in kPa or a convertible pressure unit.
• Use the saturation output to see the maximum vapor pressure represented by the approximation at that temperature.
• For dew-point calculations, actual vapor pressure is an intermediate quantity; a separate dew-point equation or measurement is needed.

For example, two locations can both have 50% relative humidity but different actual vapor pressures if their temperatures differ. The warmer location generally has the higher saturation vapor pressure and therefore the higher actual vapor pressure at that same relative humidity.

Accuracy, Units, and Common Mistakes

The Tetens equation shown here is an approximation, not a direct humidity measurement. It is widely used for ordinary atmospheric temperatures, but specialized work may require a more precise formulation, such as one that distinguishes saturation over liquid water from saturation over ice. Near freezing, in very cold conditions, or at high pressures, check whether the required reference and equation match your application.

• Enter temperature in Celsius. Putting a Fahrenheit number into the temperature field produces a substantially incorrect result.
• Enter relative humidity as a percentage. Use 50 for 50%, not 0.50. The formula divides the entered value by 100.
• Keep the units consistent. The formula’s output is kPa. Convert only after calculating if your project uses hPa, mbar, psi, or another pressure unit.
• Do not confuse actual and saturation pressure. Actual pressure is the humidity-adjusted result; saturation pressure depends on temperature alone in this calculation.
• Do not overinterpret rounded digits. The output is rounded to three decimal places, while sensor uncertainty and the approximation may be larger than the last displayed digit.

Relative humidity is normally bounded between 0% and 100% for air at equilibrium, although measurements can briefly report values above 100% in supersaturated conditions or because of sensor limitations. This calculator does not diagnose sensor errors or model supersaturation; treat unusual inputs cautiously.

How It Differs from Dew Point and Humidity Ratio

These humidity measures describe related but different properties. Actual vapor pressure is a pressure. Dew point is the temperature to which air must be cooled, at roughly constant pressure and water-vapor content, to reach saturation. Humidity ratio is the mass of water vapor relative to the mass of dry air, commonly expressed in kilograms per kilogram or pounds per pound.

A calculator for one measure cannot automatically substitute for another. Actual vapor pressure can be used in conversions to dew point or humidity ratio, but those require additional equations and, for some calculations, total air pressure. If a report asks for dew point, do not label the vapor-pressure result as dew point.

Frequently Asked Questions

How do I calculate actual vapor pressure from relative humidity?
Calculate saturation vapor pressure at the air temperature, then multiply it by relative humidity divided by 100: e = (RH/100) × e_s.

What is actual vapor pressure at 20°C and 50% humidity?
Using the approximation in this calculator, saturation vapor pressure is about 2.338 kPa, so actual vapor pressure is about 1.169 kPa.

Is actual vapor pressure the same as relative humidity?
No. Actual vapor pressure is a pressure, while relative humidity is a percentage comparing actual vapor pressure with saturation vapor pressure at the same temperature.

Can I enter Fahrenheit?
Not directly in this calculator. Convert Fahrenheit to Celsius first: °C = (°F − 32) × 5/9.

How do I convert kPa to hPa or millibars?
Multiply kPa by 10. For example, 1.169 kPa is 11.69 hPa, also approximately 11.69 millibars.

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in t | Air temperature | °C | 20 | 
in rh | Relative humidity | % | 50 | 0
out e | Actual vapor pressure | kPa | (rh / 100) * 0.6108 * exp((17.27 * t) / (t + 237.3)) | 3
out es | Saturation vapor pressure | kPa | 0.6108 * exp((17.27 * t) / (t + 237.3)) | 3