Article — Vapor Pressure Deficit (VPD) Calculator
Vapor pressure deficit (VPD): the moisture demand that drives transpiration
Vapor pressure deficit (VPD) is the difference between how much moisture the air currently holds and how much it could hold at saturation. The formula is VPD = SVP × (1 − RH/100), where SVP is the saturation vapor pressure at the given temperature (Tetens formula) and RH is the relative humidity. VPD is the thermodynamic driver of plant transpiration — higher VPD pulls water out of leaves faster. Optimal range for most crops is 0.8 to 1.2 kPa during vegetative growth. Below 0.4 kPa you get fungal disease and calcium deficiency; above 1.6 kPa you get stomatal closure and stunted growth. The vapor pressure deficit calculator above runs the Tetens formula, supports air and leaf VPD modes, and classifies the result into a growth-stage band.
VPD is the metric that replaces "humidity" in serious greenhouse and indoor-cultivation work. The same RH at different temperatures gives wildly different VPDs, so RH alone is misleading. VPD captures the actual moisture demand the plant experiences.
What VPD measures
VPD is a pressure. Specifically, it is the difference between the saturation vapor pressure of water (the maximum pressure water vapor can exert at a given temperature before condensing) and the actual vapor pressure in the air at the current temperature and humidity. The SI unit is the pascal; the practical unit for atmospheric work is the kilopascal (kPa). One kPa equals about 7.5 mmHg or 10 mbar.
The physical meaning is straightforward: VPD measures how thirsty the air is. Saturated air (RH 100%) has VPD zero — it cannot accept any more water. Dry hot air has high VPD — it pulls water out of any wet surface, including leaves.
The VPD formula
The standard VPD calculation has two steps. First, calculate saturation vapor pressure (SVP) from air temperature using the Tetens formula. Second, multiply SVP by (1 − RH/100) to get VPD.
Tetens SVP 0.6108 × exp(17.27T/(T+237.3))VPD SVP × (1 − RH/100)Leaf VPD SVP(T_leaf) − SVP(T_air) × RH/1001 kPa 10 hPa = 7.5 mmHgOptimal vegetative 0.8 to 1.2 kPaLate flower (cannabis) 1.4 to 1.6 kPaMold risk VPD < 0.4 kPaStomatal closure VPD > 1.6 kPaTetens (1930) is accurate to better than 0.5 percent over −40 to +50°C — fine for any biological or HVAC use. For ultra-precise scientific work, the Buck (1981) formula or Wagner equation give 0.05 percent accuracy at the cost of a more complex expression.
Optimal VPD by growth stage
Plants need different VPDs at different life stages. Seedlings and clones have limited root systems and cannot replace water as fast as it leaves — they need low VPD (0.4 to 0.8 kPa) to avoid wilting. Vegetative growth wants moderate VPD (0.8 to 1.2 kPa) to drive transpiration and nutrient uptake. Late-stage flowering or fruiting often benefits from slightly drier air (1.2 to 1.6 kPa for cannabis) to harden tissue and reduce fungal pressure on dense flowers.
Cannabis growers were among the first hobby cultivators to adopt VPD as a control variable. Commercial greenhouse operators tracked it for decades, but the Cannabis Cultivation Science boom of the 2010s spread VPD into mainstream indoor cultivation literature. Modern grow controllers — TrolMaster, Pulse, Argus — log VPD in real time and adjust dehumidifiers and AC to keep VPD within a programmed band. Pre-VPD growers controlled "humidity" — usually meaning RH alone — and got inconsistent results at different temperatures.
VPD for cannabis cultivation
Cannabis VPD targets are some of the most refined in horticulture because the plant is grown to very specific quality endpoints. Clones and seedlings: 0.4 to 0.8 kPa with high RH (70 to 85 percent) at 22 to 26°C. Vegetative: 0.8 to 1.2 kPa with 60 to 70 percent RH at 22 to 26°C. Early flower: 1.0 to 1.4 kPa with 50 to 60 percent RH at 22 to 26°C. Late flower: 1.4 to 1.6 kPa with 40 to 50 percent RH at 18 to 24°C. The progressively drier conditions over the bloom cycle protect dense buds from gray mold (Botrytis cinerea) and powdery mildew.
Cannabis growers calculate leaf VPD, not air VPD, because indoor lighting heats the leaf surface differently than ambient air. Under high-pressure sodium or LED lighting at canopy distance, leaf temperatures can run 1 to 3°C above ambient air; under cooler LED at greater distance, leaves can run 1 to 2°C below ambient because transpiration cooling dominates. The leaf-air offset matters — using the wrong sign on the offset can move VPD by 0.2 kPa, enough to push from optimal into the stress zone.
VPD in greenhouses
Commercial greenhouse climates target 0.8 to 1.2 kPa as a generic vegetative target across most crops. Tomato and pepper growers run slightly higher (1.0 to 1.4 kPa) because warm-season fruiting crops produce best with strong transpiration. Cool-season crops (lettuce, spinach, brassicas) target 0.6 to 1.0 kPa because they tolerate humid air. Hydroponic propagation chambers run 0.4 to 0.8 kPa with very high RH to prevent transplant shock.
Day-night VPD swings stress plants. Most controllers run a slightly lower VPD setpoint at night (when the plant is not photosynthesizing and benefits from reduced water loss). Night VPD of 0.5 to 0.8 kPa is standard, even when daytime target is 1.2 kPa. The transition should be gradual — over 1 to 2 hours at lights-on and lights-off — to avoid condensation on cold leaf surfaces, which invites disease.
Leaf VPD vs air VPD
Air VPD is calculated from air temperature alone. Leaf VPD uses leaf temperature for the saturation pressure term but air conditions for the actual vapor pressure. Plants transpire from leaf surfaces, so leaf VPD is the more physiologically accurate metric. Leaf temperature can be measured with an IR thermometer pointed at the canopy or estimated from a leaf-air offset typical of the lighting setup.
For a transpiring leaf 2°C cooler than air, leaf VPD is lower than air VPD at the same humidity — because the leaf is cooler and its saturation vapor pressure is lower. A typical correction: at 25°C air and 60 percent RH, air VPD = 1.27 kPa, but leaf VPD (leaf at 23°C) = 0.91 kPa. Use leaf VPD when working with cannabis charts; air VPD is fine for HVAC sizing and general agriculture.
What bad VPD does to plants
Very low VPD (under 0.4 kPa) slows transpiration so much that the calcium-carrying transpiration stream cannot keep up with leaf demand. Result: blossom-end rot in tomatoes, tip burn in lettuce, deformed leaves in cannabis. Low VPD also creates persistent surface moisture on leaves and stems, ideal conditions for Botrytis, powdery mildew, downy mildew, and Pythium.
When VPD climbs above 1.6 kPa, plants close their stomata to conserve water. Closed stomata also stop CO2 entry, which halts photosynthesis. Sustained high VPD (above 2.0 kPa) causes leaf curling, tip burn, and wilting even with adequate root-zone water — because the roots cannot move water fast enough to keep up with leaf evaporation. The fix is to reduce temperature or raise RH, not to water more. Many indoor growers misdiagnose high-VPD wilting as drought and overwater, drowning the roots and making the problem worse.
How to control VPD
To lower VPD: raise RH (humidifier, wet the floor, group plants tightly), lower air temperature, or both. To raise VPD: dehumidify, raise air temperature, increase ventilation. Most professional grow rooms use a combination — a humidifier paired with a dehumidifier, both controlled by a VPD setpoint, gives stable VPD across daily temperature swings.
Sensors matter. Use a quality temperature-humidity sensor (Sensirion SHT3x or Honeywell HumidIcon) placed at canopy height, not near the floor or near a humidifier vent. Sensors in dead air corners give misleading readings. For commercial work, two or three sensors averaged together give a more reliable canopy-level VPD reading.
- Tetens formula = standard SVP estimate (0.5% accurate)
- 1 kPa = 10 mbar = 7.5 mmHg
- Optimal vegetative VPD = 0.8 to 1.2 kPa
- Cannabis late flower = 1.4 to 1.6 kPa
- Mold risk below 0.4 kPa
- Stomatal closure above 1.6 kPa
- Leaf-air offset = typically −2 to −3°C transpiring
- Day-night swing = 0.5 to 0.8 kPa at night