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Vapor Pressure Deficit Calculator — result sheet
Estimate saturation vapor pressure, actual vapor pressure, and vapor pressure deficit from air temperature and relative humidity using a transparent FAO-style approximation.
Inputs used
Results
Visual chart
Breakdown
Calculation steps
Returned data table
Formula and methodology
Formula: Saturation vapor pressure e_s = 0.6108 × exp(17.27T ÷ (T + 237.3)) kPa; actual vapor pressure e_a = e_s × RH ÷ 100; VPD = e_s − e_a.
Vapor pressure deficit describes the difference between saturation vapor pressure and actual vapor pressure at the entered temperature. This page uses a single-temperature and mean-relative-humidity approximation for planning and teaching; daily FAO-56 calculations can instead use maximum and minimum temperatures, humidity, dew point, wind, and radiation.
This result follows the calculator's declared inputs, precision, validation boundaries, and model limits.
Input contract
- Air temperature — °C; minimum -80; maximum 70
- Relative humidity — %; minimum 0; maximum 100
Worked example
| Input | Value |
|---|---|
| Air temperature | 25 |
| Relative humidity | 50 |
At 25 °C and 50% relative humidity, saturation vapor pressure is about 3.168 kPa, actual vapor pressure is about 1.584 kPa, and VPD is about 1.584 kPa.
Assumptions and limits
- Temperature is entered in degrees Celsius and the saturation-pressure approximation is evaluated over the bounded input range.
- Relative humidity is entered as a percentage from 0 to 100.
- The calculation uses the FAO-style saturation vapor pressure relation over water and does not switch to an ice-surface equation below freezing.
- Actual vapor pressure is estimated as saturation vapor pressure multiplied by mean relative humidity.
- The result is an air-humidity metric and does not calculate evapotranspiration, irrigation demand, plant stress, or a crop-specific target.
- Measurements, sensor placement, time of day, and averaging period can materially change a real VPD value.
Calculator note
Source and methodology
Use the official WorldCalculate methodology policy for the source, formula, precision, and boundary standards behind this calculator.
Planning estimate, not financial, medical, legal, or professional advice. © WorldCalculate — reuse with attribution. Built and curated by Hassan ALRowaie.
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