Calculator Inputs
Enter temperature, total pressure, and optional relative humidity. The calculator returns saturation mixing ratio, actual moisture values, deficit values, and a temperature-response graph below.
Plotly Graph
The graph compares saturation mixing ratio with the actual mixing ratio curve implied by your selected relative humidity and pressure.
Formula Used
Saturation Vapor Pressure
T is in °C and es is returned in hPa.
Saturation Mixing Ratio
ε = 0.621945, P is total pressure in hPa, and ws is the saturation mixing ratio in kg/kg.
Actual Vapor Pressure and Actual Mixing Ratio
These values help compare the chosen humidity state with full saturation.
Specific Humidity
Specific humidity is another common way to describe water vapor content in moist air.
How to Use This Calculator
- Enter the air temperature and choose its unit.
- Enter the total air pressure and choose its pressure unit.
- Optionally enter relative humidity to compare actual and saturated conditions.
- Select auto, water, or ice mode for the vapor pressure equation.
- Set graph sweep minimum, maximum, and step values in Celsius.
- Press the calculate button to display the result block above the form.
- Review the metrics table, graph, formula notes, and sample cases.
- Use the CSV and PDF buttons to export your result summary.
Example Data Table
| Case | Temp (°C) | Pressure (hPa) | RH (%) | Saturation Vapor Pressure (hPa) | Saturation Mixing Ratio (g/kg) | Actual Mixing Ratio (g/kg) |
|---|---|---|---|---|---|---|
| Humid lab air | 25.00 | 1,013.25 | 60.00 | 31.6743 | 20.0694 | 11.8882 |
| Cool room sample | 10.00 | 1,000.00 | 75.00 | 12.2717 | 7.7271 | 5.7774 |
| Freezer chamber | -5.00 | 980.00 | 90.00 | 4.0174 | 2.5601 | 2.3031 |
| Warm process stream | 35.00 | 950.00 | 45.00 | 56.3116 | 39.1890 | 17.0443 |
Frequently Asked Questions
1) What is saturation mixing ratio?
Saturation mixing ratio is the maximum mass of water vapor air can hold per unit mass of dry air at a given temperature and pressure. It represents full saturation, where additional cooling or moisture would trigger condensation.
2) Why does temperature strongly affect the result?
Warmer air supports much higher saturation vapor pressure. Because the mixing ratio formula depends on that vapor pressure, the saturation mixing ratio rises quickly with temperature. This is why warm air can store much more water vapor than cold air.
3) Why does pressure matter in this calculator?
Total pressure appears in the denominator of the mixing ratio equation. Lower pressure usually allows a larger mixing ratio for the same vapor pressure. That is why altitude, chamber conditions, and process pressure can change the final result.
4) When should I use the ice equation?
Use the ice option when moisture equilibrium is referenced to an ice surface instead of liquid water. This matters most below freezing. The auto mode switches based on temperature and is practical for general laboratory or environmental use.
5) Is saturation mixing ratio the same as specific humidity?
No. Mixing ratio compares water vapor mass to dry-air mass, while specific humidity compares water vapor mass to total moist-air mass. They are closely related, but they are not identical and should not be interchanged without conversion.
6) Why does the calculator ask for relative humidity too?
Relative humidity is optional here. Saturation mixing ratio itself only needs temperature and pressure. The added humidity field helps compare the actual moisture state with the saturation limit, which makes the tool more useful for analysis and reporting.
7) What units are best for reporting results?
g/kg is common for weather, HVAC, and practical reporting because it is easy to read. kg/kg is better for direct scientific equations and coding workflows. This page shows both so you can use the format that fits your work.
8) Can I use this for lab, weather, or industrial air studies?
Yes. The calculation is useful anywhere moist-air behavior matters, including meteorology, climate studies, drying systems, clean rooms, storage analysis, and chemistry labs. Just make sure your pressure and temperature inputs match the actual operating conditions.