Tools / Environment

VPD calculator

Vapor pressure deficit is the documented horticulture measure of the moisture-pulling force between leaf and air, expressed in kilopascals. The calculator below reads three inputs — air temperature, relative humidity, and a leaf-temperature offset that accounts for transpiration cooling — and returns the live VPD value along with the cannabis growth stage that figure matches. The formula is derived from the Tetens approximation of saturation vapor pressure, the same equation used in commercial HVAC and greenhouse climate controllers. Use the result as a planning target alongside a room thermometer and hygrometer rather than a single source of truth.

How the calculation works

Saturation vapor pressure is the documented amount of water vapor air can hold at a given temperature before condensation begins. The Tetens approximation expresses it in kilopascals as SVP(T) = 0.61078 × exp((17.27 × T) / (T + 237.3)), where T is temperature in degrees Celsius. The calculator evaluates this equation twice — once for the leaf surface temperature and once for the ambient air temperature — and then applies the relative humidity reading to the air value to find the actual vapor pressure in the room. VPD is the difference between the two: VPD = SVP(leaf) − (RH / 100) × SVP(air). A negative or near-zero result describes a saturated room where transpiration effectively stalls, and a large positive number describes a room aggressive enough to close stomata and stunt nutrient uptake.

Leaf temperature is documented in horticulture literature as running roughly 2 °C cooler than the surrounding air under active transpiration, which is why the default offset is −2 °C. Under direct high-intensity light, the offset is documented as shrinking toward zero or even reversing positive; under dim light or sealed-room conditions, the documented offset deepens. Adjust the slider when an infrared leaf thermometer is available to read the actual differential.

Reference ranges

Documented cannabis VPD targets are commonly split into three stage windows. Seedlings and freshly cut clones are documented as wanting 0.4 to 0.8 kPa, a low-deficit environment that keeps fragile root systems from being out-pulled by leaf demand. Vegetative plants are documented as performing best at 0.8 to 1.2 kPa, the published sweet spot for fast leaf and stem expansion. Flowering plants are documented as benefiting from a tighter 1.2 to 1.5 kPa range, with the higher deficit pulling more aggressively against the canopy and discouraging mold pressure on dense buds.

Values below 0.4 kPa are documented as risking sluggish transpiration, slow nutrient transport, and elevated bud-rot pressure in late flower; values above 1.5 kPa are documented as risking stomatal closure, photosynthesis stalls, and the wilted-without-deficit appearance described in grower reports. Use the calculator output as a setpoint, then verify with leaf-surface readings before accepting it as accurate.

Related references: Tent setup · Flowering stage · Common beginner mistakes

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