Vapor Pressure Deficit Calculator
Vapor pressure deficit (VPD) measures the difference between how much moisture the air can hold at a given temperature and how much it actually holds. It is the primary variable controlling plant transpiration and stomatal behaviour. Greenhouse growers, irrigation managers and agronomists use VPD to optimise crop growth, prevent disease and improve water use efficiency. This calculator uses the Tetens saturation vapour pressure formula recommended by the FAO in their Penman-Monteith reference evapotranspiration standard. Enter air temperature and relative humidity to get VPD in kPa.
VPD formula (Tetens / FAO 56)
es(T) = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa]
ea = es(T) × RH / 100
VPD = es(T) - ea
Where T is temperature in deg C and RH is relative humidity in percent. This is the form used in FAO Irrigation and Drainage Paper No. 56, the international standard for reference evapotranspiration.
VPD zones for crop management
- 0.0 to 0.4 kPa: Very low demand. Risk of condensation, fungal disease (botrytis, powdery mildew). Ideal for germination only.
- 0.4 to 0.8 kPa: Low demand. Good for propagation, clones and young seedlings.
- 0.8 to 1.2 kPa: Optimal for vegetative growth. Stomata open freely; good nutrient uptake.
- 1.0 to 1.5 kPa: Moderate-high demand. Suitable for flowering and fruiting crops.
- Above 1.5 kPa: High demand. Stomata close to limit water loss; growth slows. Above 2.0 kPa causes stress in most crops.
VPD calculator: frequently asked questions
What is vapor pressure deficit (VPD)?
VPD is the difference between the maximum amount of water vapour air can hold at a given temperature (saturation vapour pressure) and the actual amount it currently holds. It is measured in kilopascals (kPa) and quantifies the evaporative demand of the atmosphere. A higher VPD means the air is drier and plants must transpire more to maintain water balance.
What formula is used to calculate VPD?
VPD = es(T) - ea, where es(T) is saturation vapour pressure at temperature T = 0.6108 x exp(17.27 x T / (T + 237.3)) kPa, and ea is actual vapour pressure = es(T) x RH / 100. This is the Tetens formula used by the FAO in their Penman-Monteith evapotranspiration standard (FAO Irrigation and Drainage Paper No. 56).
What is an ideal VPD for greenhouse cannabis or tomatoes?
According to University of Connecticut Extension and USDA NRCS guidelines: 0.4 to 0.8 kPa is ideal for propagation and early seedlings; 0.8 to 1.2 kPa is optimal for vegetative growth; 1.0 to 1.5 kPa suits flowering and fruiting; above 1.5 kPa causes plant stress, wilting and reduced yield in most crops.
Why is VPD better than relative humidity for crop management?
Relative humidity is temperature-dependent, so a 70% RH reading means different things at 20 deg C versus 30 deg C. VPD directly measures the drying power of the air regardless of temperature, giving growers a consistent metric to optimise transpiration, stomatal opening and nutrient uptake in controlled environments.
What is the difference between VPD at leaf and VPD at air?
Air VPD is calculated from air temperature and RH. Leaf VPD uses the leaf surface temperature (often 1 to 3 deg C cooler than air due to transpirational cooling) and the same RH. Leaf VPD is a more accurate driver of stomatal behaviour but requires leaf temperature measurement. This calculator computes air VPD, which is the standard for most agricultural monitoring systems.
Official sources
- FAO Irrigation and Drainage Paper No. 56, Penman-Monteith ETo: fao.org.
- USDA NRCS National Engineering Handbook, Irrigation: nrcs.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.