Calculator Inputs
Choose a mode, equation, phase, units, and optional humidity data.
Temperature Comparison Graph
The graph compares selected equations across your temperature range.
Calculation History
Completed calculations appear here automatically.
Formula Used
Saturation pressure rises quickly as temperature increases. Each equation estimates equilibrium water-vapor pressure. Select a method matching your temperature range carefully.
- Magnus: compact meteorological approximation for water or ice.
- Tetens: simple atmospheric approximation near ordinary temperatures.
- Buck: refined empirical equation for common weather conditions.
- Antoine: coefficient equation suited to defined temperature intervals.
- Goff–Gratch: logarithmic reference relation for atmospheric calculations.
- IAPWS/Wagner: high-quality liquid-water vapor-pressure correlation.
How to Use This Calculator
- Enter temperature and select its unit.
- Choose water, ice, or automatic phase selection.
- Select the preferred vapor-pressure equation.
- Add relative humidity for dew point and deficit.
- Add atmospheric pressure for mixing-ratio estimates.
- Choose output precision and pressure units.
- Set chart limits for method comparisons.
- Press Calculate and review warnings carefully.
Example Data
| Temperature | Phase | Approximate pressure | Common interpretation |
|---|---|---|---|
| −20°C | Ice | 0.103 kPa | Cold, low moisture capacity |
| 0°C | Water | 0.611 kPa | Freezing-point reference |
| 20°C | Water | 2.34 kPa | Typical indoor condition |
| 25°C | Water | 3.17 kPa | Warm room condition |
| 50°C | Water | 12.35 kPa | High moisture capacity |
| 100°C | Water | 101.33 kPa | Boiling near standard pressure |
Frequently Asked Questions
What is saturation vapor pressure?
It is the equilibrium vapor pressure above water. Temperature strongly controls this pressure. Warmer surfaces support much more water vapor overall.
How does actual vapor pressure differ?
Saturation pressure is the maximum equilibrium value. Actual pressure depends on present humidity. Relative humidity links both pressure values directly together.
Should I select water or ice?
Use water for liquid-surface equilibrium. Use ice for frozen-surface equilibrium. Automatic mode switches phases at zero degrees Celsius.
Which equation should I use?
Buck works well for common weather calculations. IAPWS suits liquid water across broader temperatures. Always respect each equation's published validity range carefully.
Why do methods give different answers?
Every equation uses different fitted coefficients. Their valid ranges and reference data differ. Small differences are normal within overlapping ranges generally.
What is vapor pressure deficit?
Deficit measures unsatisfied atmospheric moisture capacity. It equals saturation pressure minus actual pressure. Larger deficits usually indicate stronger drying demand nearby.
How is dew point estimated?
The calculator first finds actual vapor pressure. It then inverts a Magnus relationship. Dew point marks the temperature where saturation begins.
Can pressure exceed atmospheric pressure?
That condition signals boiling or invalid assumptions. Liquid stability then depends on ambient pressure. Review atmospheric units before accepting such results carefully.
What do custom Antoine coefficients require?
Coefficients must match water and selected units. They also require a documented temperature range. Incorrect coefficients can produce severely misleading pressures quickly.
Why use a logarithmic chart scale?
Pressure changes span several orders of magnitude. A logarithmic scale reveals low-temperature differences. Linear scales better emphasize larger high-temperature changes clearly.
Are these results suitable for design?
Results support education and preliminary engineering checks. Critical designs require approved standards and data. Verify every assumption before making safety decisions independently.