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VPD (vapour pressure deficit)

The drying pull air exerts on a leaf, in kPa; stage targets run roughly 0.4–1.6 kPa from propagation to late flower.

GlossaryBeginnerEnvironment and lighting termsDraft
For home and commercial growers1 min readUpdated Reviewed by Horus editorial

Vapour pressure deficit (VPD) is how hard the air is pulling water out of a leaf, in kilopascals: the saturation vapour pressure at leaf temperature, minus the vapour pressure the air already holds.

VPD = SVP(leaf) − RH/100 × SVP(air), using the Tetens approximation for saturation vapour pressure [1]:

SVP(T) = 0.61078 × exp(17.27 × T / (T + 237.3))      kPa, T in °C

Because SVP curves upward with temperature rather than climbing in a straight line, the same 60% RH gives a leaf about 0.94 kPa at 20 °C but about 1.51 kPa at 28 °C. A hygrometer reading alone never tells you the deficit; you need the temperature too.

PropagationPropagation: ideal 0.4–0.8 kPa, on a scale of 0.0 to 2.0 kPa.PropagationIdeal 0.4–0.8 kPaToo lowToo high0.00.51.01.52.0kPaVegetativeVegetative: ideal 0.8–1.2 kPa, on a scale of 0.0 to 2.0 kPa.VegetativeIdeal 0.8–1.2 kPaToo lowToo high0.00.51.01.52.0kPaEarly flowerEarly flower: ideal 1.0–1.5 kPa, on a scale of 0.0 to 2.0 kPa.Early flowerIdeal 1.0–1.5 kPaToo lowToo high0.00.51.01.52.0kPaLate flowerLate flower: ideal 1.2–1.6 kPa, on a scale of 0.0 to 2.0 kPa.Late flowerIdeal 1.2–1.6 kPaToo lowToo high0.00.51.01.52.0kPa
Fig. 1Grower and manufacturer consensus corridors, not a proven optimum: the target tightens as the plant moves from an unrooted cutting to a dense late-flower canopy.Horus

Grow-room controllers and printed charts work to four rough corridors that tighten as the plant matures: propagation 0.4–0.8 kPa, vegetative growth 0.8–1.2 kPa, early flower 1.0–1.5 kPa, late flower 1.2–1.6 kPa.

The formula needs leaf temperature, not the reading on the wall. Under LED, leaves commonly run 1–3 °C below the air, which lowers the true deficit below what an air-only calculation implies. Run the deficit below its corridor and stomata stay open while transpiration slows anyway, so calcium, which moves only in the transpiration stream, arrives late at new growth, and condensation risk climbs. Run it above the corridor and stomata narrow to save water: photosynthesis drops with them even though the room thermometer looks unremarkable [2].

See VPD explained for leaf-temperature measurement and the worked maths, the VPD calculator to turn a reading into a humidity target, and dew point for the condensation risk at the low end.

Sources

  1. Murray FW (1967). On the computation of saturation vapor pressure. Journal of Applied Meteorology 6(1):203–204 Accessed 2026-09-27.
  2. Grossiord C, Buckley TN, Cernusak LA, et al. (2020). Plant responses to rising vapor pressure deficit. New Phytologist 226(6):1550–1566 Accessed 2026-09-27.
ExplainerBeginner6 minVPD explained: the number behind transpirationVPD is the drying pull on a leaf, not the room's humidity reading. What it measures, why it drives calcium uptake, and the stage targets to start from.CalculatorBeginner5 minVPD calculator: leaf vapour pressure deficit from temperature and humidityEnter air temperature, RH and a leaf-to-air offset to get leaf VPD in kPa, checked against propagation, veg and flower targets.