VPD chart and calculator
Enter your temperature and humidity. The chart plots where your room sits, the calculator gives leaf VPD in kPa, and the target zones show where the crop wants to be. Free, no signup.
Leaf VPD
0.85
kPa
In range
Vegetative target is 0.8–1.2 kPa.
- Air VPD
- 1.19 kPa
- Leaf temp
- 22.0 °C
- Dew point
- 15.8 °C
- Abs. humidity
- 13.1 g/m³
Leaf VPD by temperature and humidity
Each cell is the leaf VPD in kPa at that temperature and humidity, using your leaf offset. The marker is your room. Tap or drag the chart to set conditions directly.
VPD targets by growth stage
General ranges for leaf VPD. Cultivar, light intensity, and CO₂ all shift the ideal, so treat these as a starting point and refine against your own crop response.
| Stage | Leaf VPD | Typical climate | Why |
|---|---|---|---|
| Propagation, clones | 0.4–0.8 | 24–26 °C, 75–85 % RH | No roots yet, so the cutting cannot replace water it loses. |
| Early vegetative | 0.8–1.0 | 24–27 °C, 65–75 % RH | Pushes transpiration up as the root system establishes. |
| Late vegetative | 1.0–1.2 | 24–28 °C, 60–70 % RH | Maximum growth rate with stomata still fully open. |
| Early flower | 1.2–1.4 | 24–27 °C, 55–60 % RH | Higher nutrient uptake, lower disease pressure in the canopy. |
| Late flower | 1.4–1.6 | 22–26 °C, 45–55 % RH | Drier air protects dense flower from botrytis. |
What VPD actually measures
Vapour pressure deficit is the gap between the water vapour the air currently holds and the most it could hold at that temperature. It is measured in kilopascals, and it tells you how hard the air is pulling water out of the leaf.
That single number predicts transpiration better than temperature or humidity alone, because the two readings only mean something together. 70 % humidity at 20 °C and 70 % at 30 °C are completely different growing conditions.
The formula
Saturation vapour pressure comes from the Magnus equation, with temperature T in degrees Celsius:
SVP = 0.61078 × e(17.27 T / (T + 237.3))
Actual vapour pressure is the air’s SVP multiplied by relative humidity. Leaf VPD is then the SVP at leaf temperature minus that actual vapour pressure — which is why the leaf offset matters.
Why leaf temperature changes the answer
Water evaporates from the leaf surface, not from the air, so the saturation term belongs at leaf temperature. Transpiration cools leaves, typically to 1–3 °C below air temperature under lights.
Skip that correction and you overstate VPD by roughly 0.2–0.4 kPa — enough to run the room drier than intended for an entire cycle. An infrared leaf sensor removes the guess.
Where the number goes wrong
An uncalibrated humidity sensor is the usual culprit. A 5 % relative-humidity error moves VPD by about 0.15 kPa, which is enough to leave the target zone while the display still looks correct.
Sensor placement matters just as much: a probe in dead air by the wall does not describe the canopy. Measure where the plants are, with references you can trace to a standard.
VPD questions, answered
What is a good VPD range?
As a general guide: 0.4–0.8 kPa for propagation and young plants, 0.8–1.2 kPa for vegetative growth, and 1.2–1.6 kPa for flowering. Below roughly 0.4 kPa transpiration stalls and disease pressure rises; above roughly 1.6 kPa plants close their stomata and growth slows.
What is the difference between leaf VPD and air VPD?
Air VPD uses air temperature for both terms. Leaf VPD uses leaf temperature for the saturation term, because the water actually evaporates from the leaf surface. Leaves under lights typically sit 1–3 °C below air temperature through transpirational cooling, so leaf VPD is the number that matters for the plant.
Why is my VPD wrong even though temperature and humidity look correct?
Two common causes. First, an uncalibrated humidity sensor: a 5 % relative-humidity error shifts VPD by roughly 0.15 kPa, enough to move you out of the target zone. Second, using air temperature instead of leaf temperature, which typically overstates VPD by 0.2–0.4 kPa under lights.
How do I control VPD in a grow room?
VPD is controlled by coordinating heating, cooling, humidification, and dehumidification against a single VPD target rather than separate temperature and humidity setpoints. Buddaban Tech’s climate module runs that loop automatically on the equipment already installed in the facility.
Stop reading VPD off a chart. Hold it automatically.
A chart tells you where the room is right now. Buddaban Tech’s climate module holds the target through the night, through lights-on, and through every stage change — on the equipment you already own.
Or see the climate control module.