VPD explained: the number behind transpiration

Vapour pressure deficit (VPD) is the number that states how hard the air is pulling water out of a leaf, in kilopascals, and it is not the same thing as the percentage on your hygrometer. Read a 26 °C, 60% RH room off an ordinary air-temperature wall chart and you get 1.34 kPa. Measure the actual leaf, running 2 °C cooler under an LED fixture, and the honest figure is about 0.97 kPa: solidly vegetative, not pushed toward the top of early flower. The gap between those two numbers is the whole subject of this page.

Two pressures make the deficit: saturation at the leaf, minus what the air already holds

VPD comes from two numbers. The first is saturation vapour pressure (SVP): the maximum vapour pressure the air could hold at a given temperature before water starts condensing out of it. The second is the vapour pressure the air actually holds right now: relative humidity multiplied by the saturation pressure at air temperature. Subtract the second from the first, but take the saturation term at leaf temperature, not air temperature, because it is the leaf's surface the water is evaporating from. The standard approximation, Murray's 1967 reworking of Tetens' 1930 formula, is the one used throughout this library ^[1]:

SVP(T) = 0.61078 × exp(17.27 × T / (T + 237.3))      kPa, T in °C
AVP    = RH / 100 × SVP(T_air)
VPD    = SVP(T_leaf) − AVP

Everything past this point is what that subtraction means for a grower, not more algebra.

Why 60% RH means something different at 20 °C than at 28 °C

Saturation pressure does not rise in a straight line with temperature; it curves upward, so the same relative-humidity reading sits at a different deficit depending on how warm the room is. Hold RH at 60% and warm the room from 20 °C to 28 °C: saturation pressure climbs from about 2.34 kPa to about 3.78 kPa, roughly 60% higher, while the number on the hygrometer has not moved at all. The deficit rises with it, from about 0.94 kPa to about 1.51 kPa, for the same "60% humidity". A hygrometer alone cannot tell you the deficit; it needs the temperature too, which is why two rooms at the same RH can feel completely different to a plant.

The stage corridors, and whose numbers these are

Grow-room controllers and most VPD charts work to four rough corridors, one per stage: propagation 0.4–0.8 kPa, vegetative growth 0.8–1.2 kPa, early flower 1.0–1.5 kPa, and late flower 1.2–1.6 kPa. These track the plant's changing relationship with water and disease risk. Young, unrooted cuttings need a small deficit so they lose almost no water before roots exist to replace it, while dense late-flower colas need a firmer pull to keep their interior dry.

Too little pull leaves calcium behind and wets the air

When the deficit sits below its corridor, the driving force pulling water out of the leaf is small, so transpiration slows even though the stomata have no reason to close. That matters because calcium travels to new growth only by riding the transpiration stream; slow flow means slow calcium delivery, and new leaves or bud tips can show the same tip-burn and clawing as an outright deficiency even when the feed recipe is correct. A low deficit also means the room sits closer to its dew point, raising the odds of condensation on cool surfaces and deep inside a dense canopy. The fix is rarely more calcium; it is more pull.

Too much pull shuts the stomata, even if the thermometer looks fine

Push the deficit above its corridor and the plant does the opposite: stomata narrow to limit water loss, and because carbon dioxide enters through the same pores, photosynthesis drops with them ^[2]. A room can show a perfectly ordinary air temperature on the wall thermometer and still be running a leaf deficit high enough to throttle growth, because the number that matters is the one at the leaf surface in low humidity, not the one on the wall. Leaves tacoing upwards under a fixture that otherwise looks healthy, or growth that stalls despite good light and feed, is worth checking against a leaf VPD figure before anything else changes.

The room thermometer is not reading your leaf

Leaf temperature is not air temperature. Leaves gain heat from whatever radiation reaches them and lose it by transpiring, so under an LED fixture they commonly run 1–3 °C below the air, and under HPS the gap can close or reverse. The full method for measuring that offset with an infrared thermometer, and how much it moves with fixture type and distance, lives in the leaf-temperature article and the VPD calculator; this page only needs the consequence.

Take the worked example from the opening: a room reading 26 °C and 60% RH, leaves running 2 °C cooler. Saturation pressure at 26 °C is 3.36 kPa, so the air already carries 0.60 × 3.36 = 2.02 kPa. Saturation pressure at the leaf's 24 °C is 2.98 kPa, and the difference is about 0.97 kPa: solidly vegetative. Assume no offset at all, the mistake most wall charts make by printing a single air-temperature scale, and the same room reads 3.36 − 2.02 = 1.34 kPa, into the top of early flower on the same corridor table. Nothing in the room changed; only which temperature fed the saturation term.

What the deficit sets in motion next

VPD sits upstream of the rest of the system: the deficit sets how wide the stomata open, and stomatal aperture governs both water loss and how much carbon dioxide reaches the leaf's interior, which is why VPD, CO2 concentration and photosynthesis end up linked rather than three separate settings ^[2]. That interaction, and what it means for CO2 enrichment specifically, is covered in CO2, VPD and stomata. The condensation risk from running the deficit too low is covered in full in dew point and condensation, and the temperature half of every number on this page is in temperature targets by growth stage. Once you trust the number, the next practical decision is sizing equipment to it rather than to tent volume: see sizing a dehumidifier from plant water use.