Vapour Pressure Deficit (VPD) Calculator
Work out the vapour pressure deficit (VPD) from temperature and relative humidity — the single number growers use to judge how hard the air is pulling moisture out of a plant.
Enter Values
How to use this calculator
- Enter the air or leaf temperature in °C (negatives allowed).
- Enter the relative humidity as a percentage between 0 and 100.
- Read the VPD in kPa, plus the saturation and actual vapour pressures behind it.
How it works
The saturation vapour pressure (SVP) — the most water vapour the air can hold at that temperature — is found from the Tetens relation SVP = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) in kPa. The actual vapour pressure is SVP × RH ÷ 100, and the deficit is what is left: VPD = SVP × (1 − RH ÷ 100). A higher VPD means drier-feeling air and stronger transpiration pull.
Worked example
Worked example. At 25 °C and 50% relative humidity, SVP = 0.6108 × exp(17.27 × 25 ÷ 262.3) = 3.17 kPa, so VPD = 3.17 × (1 − 0.50) = 1.58 kPa.
Common mistakes
- Entering relative humidity as a fraction (0.5) instead of a percentage (50).
- Assuming leaf temperature equals air temperature — transpiring leaves are often cooler, which changes the true leaf-level VPD.
- Reading VPD as humidity; a low VPD means humid, low-demand air and a high VPD means dry, high-demand air.
Frequently asked questions
What is a good VPD for plants?
Many crops are managed toward roughly 0.8 to 1.2 kPa. Too low and transpiration stalls (raising disease risk); too high and plants close their stomata and stress.
Why compute VPD instead of just using relative humidity?
Relative humidity alone ignores temperature. The same RH feels very different at 15 °C versus 30 °C; VPD combines both into the actual drying power of the air.
Related tools
- Rainfall Intensity Calculator
- Apparent Temperature Calculator
- Degree Days Calculator
- Evapotranspiration (Hargreaves) Calculator
- Absolute Humidity Calculator
- Air Density Calculator
Explore more in Weather & Environmental Monitoring.
Tip: Enter any known values to calculate the remaining results.
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