Calculator Inputs
Formula Used
Saturation vapor pressure is the maximum vapor pressure possible at a given temperature when air is in equilibrium with water or ice. This calculator supports Buck, Magnus, and Tetens forms. Temperature is internally converted to Celsius before evaluation.
es = 6.1121 × exp[(18.678 − T/234.5) × (T/(257.14 + T))]Buck over ice
es = 6.1115 × exp[(23.036 − T/333.7) × (T/(279.82 + T))]Actual vapor pressure
e = RH × es / 100Vapor pressure deficit
VPD = es − eSaturation mixing ratio
ws = 0.62198 × es / (P − es)
Here, T is temperature in °C, es is saturation vapor pressure in hPa, RH is relative humidity in percent, and P is ambient pressure in hPa.
How to Use This Calculator
- Enter the known temperature and choose its unit.
- Select Buck, Magnus, or Tetens based on your preferred formulation.
- Choose water, ice, or auto phase switching around freezing.
- Optionally enter relative humidity to estimate actual vapor pressure and vapor pressure deficit.
- Optionally enter ambient pressure to estimate saturation mixing ratio and specific humidity.
- Set graph range and step size for the Plotly curve.
- Press Calculate to view results above the form.
- Use the export buttons to download the current result table as CSV or PDF.
Example Data Table
Sample values below use the Buck equation with automatic water and ice phase selection.
| Temperature (°C) | Temperature (°F) | Phase | Saturation Vapor Pressure (hPa) | Saturation Vapor Pressure (kPa) | Saturation Vapor Pressure (mmHg) |
|---|---|---|---|---|---|
| -20 | -4.0 | Ice | 1.0329 | 0.1033 | 0.7747 |
| -10 | 14.0 | Ice | 2.5995 | 0.2599 | 1.9498 |
| 0 | 32.0 | Water | 6.1121 | 0.6112 | 4.5845 |
| 10 | 50.0 | Water | 12.2786 | 1.2279 | 9.2097 |
| 20 | 68.0 | Water | 23.3834 | 2.3383 | 17.5390 |
| 30 | 86.0 | Water | 42.4513 | 4.2451 | 31.8411 |
| 40 | 104.0 | Water | 73.8236 | 7.3824 | 55.3723 |
Frequently Asked Questions
1. What is saturation vapor pressure?
It is the pressure exerted by water vapor when air is fully saturated at a specific temperature. At that point, evaporation and condensation are balanced, and no additional vapor can remain in the air without condensation.
2. Why does saturation vapor pressure rise with temperature?
Warmer molecules have more kinetic energy, so more water molecules can escape into the vapor phase. That raises the equilibrium vapor pressure and sharply increases the amount of moisture air can hold.
3. What is the difference between saturation vapor pressure and actual vapor pressure?
Saturation vapor pressure is the maximum possible vapor pressure at a temperature. Actual vapor pressure is the moisture pressure truly present in the air. Relative humidity compares these two quantities as a percentage.
4. Why are separate water and ice formulas available?
Below freezing, vapor pressure over ice differs from vapor pressure over liquid water. Separate equations improve accuracy for snow, frost, cold chambers, and atmospheric studies involving subzero surfaces.
5. Which equation should I choose?
Buck is widely used for practical meteorology and good accuracy. Magnus is simple and popular in calculators. Tetens is classic and still useful for teaching, screening, and quick engineering estimates.
6. What units does this calculator support?
It accepts Celsius, Fahrenheit, and Kelvin for temperature. It reports saturation vapor pressure in hPa, kPa, Pa, mmHg, psi, atm, and bar, making it useful across science and engineering workflows.
7. What is vapor pressure deficit?
Vapor pressure deficit is the difference between saturation vapor pressure and actual vapor pressure. It indicates drying power, plant stress potential, and evaporation demand in atmospheric, agricultural, and HVAC analysis.
8. Where is saturation vapor pressure used?
It is used in meteorology, thermodynamics, HVAC, climate work, drying studies, greenhouse control, cloud physics, storage design, and laboratory humidity calculations wherever moisture equilibrium matters.