Relative Humidity to Mole Fraction Calculator
Estimate vapor composition from humidity data fast. Compare saturation pressure, partial pressure, and mole fraction. Download clean results for charts, reports, and classwork instantly.
Estimate vapor composition from humidity data fast. Compare saturation pressure, partial pressure, and mole fraction. Download clean results for charts, reports, and classwork instantly.
Enter humidity, temperature, pressure, and equation settings. Then calculate the mole fraction of water vapor.
These examples use the Buck equation and an enhancement factor of 1.
| RH (%) | Temperature (C) | Pressure (kPa) | Saturation Pressure (kPa) | Partial Pressure (kPa) | Mole Fraction | ppmv |
|---|---|---|---|---|---|---|
| 50 | 25 | 101.325 | 3.168531 | 1.584266 | 0.01563549 | 15635.49 |
| 75 | 30 | 100 | 4.245126 | 3.183844 | 0.03183844 | 31838.44 |
| 60 | 10 | 95 | 1.22786 | 0.736716 | 0.00775491 | 7754.91 |
| 40 | 35 | 101.325 | 5.626752 | 2.250701 | 0.02221269 | 22212.69 |
Water vapor partial pressure:
Pw = (RH / 100) × Psat(T) × F
Water vapor mole fraction:
xw = Pw / P
Dry air mole fraction:
xd = 1 - xw
Humidity ratio:
W = 0.621945 × Pw / (P - Pw)
RH is relative humidity. Psat is saturation vapor pressure at temperature T. F is the optional enhancement factor. P is total pressure. The result assumes ideal gas behavior.
This calculator changes relative humidity into water vapor mole fraction. It uses air temperature, total pressure, and saturation pressure. The output shows how much of the gas mixture is water vapor. That value is useful in chemistry, HVAC, drying, meteorology, and process design. A mole fraction is dimensionless. It compares moles of one component with total gas moles. In this case, the component is water vapor.
Temperature controls the saturation vapor pressure of water. Warm air can hold more vapor before saturation. Cold air reaches saturation at a lower vapor pressure. So the same relative humidity can give different mole fractions. For example, fifty percent humidity at thirty degrees is not the same as fifty percent humidity at ten degrees. The warmer case usually has a larger vapor mole fraction.
Total pressure is also important. Mole fraction equals vapor partial pressure divided by total pressure. A lower total pressure gives a higher mole fraction for the same vapor pressure. This is common in high altitude conditions. It is also important in vacuum drying and sealed vessels. Always enter absolute pressure. Do not enter gauge pressure unless it is converted first.
The Buck equation is a strong general choice. It works well for many atmospheric calculations. The Tetens equation is simple and popular. It is often used in quick environmental estimates. The Antoine equation is common in chemical engineering. It is best used inside its valid temperature range. The calculator allows all three choices. This helps compare results for study or design checks.
A mole fraction of 0.020 means two percent of gas moles are water vapor. The calculator also gives ppmv. That is mole fraction multiplied by one million. It also reports mole percent and humidity ratio. Humidity ratio is mass of water vapor per mass of dry air. These extra values make the result easier to use in reports.
Check every unit before trusting the result. Use kPa, atm, bar, mmHg, psi, or pascal carefully. Make sure pressure is total absolute pressure. Keep relative humidity between zero and one hundred for normal air. Values above one hundred can show supersaturation. The calculator warns you when values need review. Use the export buttons when you need a saved record.
It is the fraction of total gas moles that are water vapor. A value of 0.01 means one percent of the gas molecules are water vapor molecules.
Relative humidity gives the share of saturation pressure. The calculator multiplies relative humidity by saturation pressure, then divides water vapor partial pressure by total pressure.
Enter total absolute pressure. Standard atmospheric pressure is 101.325 kPa. Do not enter gauge pressure unless you first convert it to absolute pressure.
You can enter Celsius, Fahrenheit, or Kelvin. The calculator converts the value to Celsius before applying the saturation pressure equation.
It is the water vapor pressure at saturation for a given temperature. It rises quickly as temperature increases.
ppmv means parts per million by volume. For ideal gases, it equals mole fraction multiplied by one million.
Mole fraction is partial pressure divided by total pressure. If vapor pressure stays fixed and total pressure drops, mole fraction increases.
It is a multiplier for saturation pressure. Keep it at 1 for most ideal calculations. Change it only when your method requires correction.
Yes. A relative humidity near zero gives a very small water vapor mole fraction. At exactly zero, water vapor partial pressure is zero.
It can occur in supersaturated conditions, but it is not common for standard calculations. The calculator accepts high values and gives a warning.
Humidity ratio is the mass of water vapor per mass of dry air. It is often used in psychrometric and HVAC calculations.
Use Buck for general air calculations. Use Tetens for quick estimates. Use Antoine when you need a chemical engineering style vapor pressure equation.
The calculator rejects cases where water vapor partial pressure is equal to or higher than total pressure. That would make the gas mixture impossible.
Yes. Use the CSV button for spreadsheet work. Use the PDF button for a simple printable calculation report.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.