VPD calculator
Vapour pressure deficit tells you how hard the air is pulling water out of your plants. Enter your room's temperature and humidity to see where you are, and what would put you on target.
Leaves usually run a little cooler than the air. Measure yours with an infrared thermometer; leave 0 to calculate air VPD.
What VPD is, in one paragraph
Air at a given temperature can hold only so much water vapour. VPD is the gap between that maximum, at the leaf's temperature, and the vapour actually in the air. A bigger gap pulls water through the plant faster. Too little, and transpiration slows and damp-loving diseases such as botrytis get easier; too much, and plants close their stomata to save water and growth stalls.
Common targets
| Stage | VPD |
|---|---|
| Propagation and early vegetative growth | 0.4 to 0.8 kPa |
| Late vegetative and early flowering | 0.8 to 1.2 kPa |
| Mid and late flowering | 1.2 to 1.6 kPa |
These are widely used rules of thumb, not laws. Cultivars differ, and moving between stages is gradual.
How it is calculated
Saturation vapour pressure uses the Tetens equation, with temperature T in °C:
SVP(T) = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa
Leaf VPD subtracts the vapour actually in the air from the saturation pressure at the leaf:
VPD = SVP(Tleaf) − SVP(Tair) × RH ÷ 100
The dew point uses the Magnus form with the same constants. With a leaf offset of 0 the result is air VPD.
GrowRoom calculates this live. Its air sensor reports temperature and humidity, and the app shows VPD next to them, with history. The air sensor pack