VPD Calculator & VPD Chart for Cannabis
Calculate the ideal VPD (vapor pressure deficit) from temperature and humidity — matched to your plant’s growth stage. Below the calculator you’ll find a VPD chart for every stage.
Calculated with PlantClue
Why VPD matters more than temperature or humidity alone
Why?
VPD (vapor pressure deficit) combines temperature and humidity into a single number that shows how strongly the air is currently “pulling” moisture out of the leaves. Two rooms at the same temperature but with different humidity can feel completely different to the plant — VPD makes them comparable.
Why does it matter?
VPD controls how much the plant transpires through its stomata and, with that, how much it pulls nutrients up from the roots. Too low a VPD slows this transport and invites mold; too high a VPD stresses the plant and makes leaves droop.
What to do when you’re off target
VPD too low: lower the humidity (dehumidifier, more air exchange) or raise the temperature slightly. VPD too high: raise the humidity (humidifier, less exhaust) or lower the temperature. Change only one factor at a time and measure again.
VPD Chart: Temperature & Humidity at a Glance
This chart shows the VPD (in kPa) for common temperature and humidity combinations — calculated with the same formula as the calculator above, using the standard leaf temperature offset of −3.6 °F (−2 °C). Use it for a quick overview and the calculator above for your exact readings.
| Humidity \ Temperature | 64 °F (18 °C) | 68 °F (20 °C) | 72 °F (22 °C) | 75 °F (24 °C) | 79 °F (26 °C) | 82 °F (28 °C) | 86 °F (30 °C) | 90 °F (32 °C) |
|---|---|---|---|---|---|---|---|---|
| 40 % | 0.99 | 1.13 | 1.28 | 1.45 | 1.64 | 1.85 | 2.08 | 2.34 |
| 50 % | 0.79 | 0.89 | 1.02 | 1.15 | 1.30 | 1.47 | 1.66 | 1.87 |
| 60 % | 0.58 | 0.66 | 0.75 | 0.85 | 0.97 | 1.09 | 1.23 | 1.39 |
| 70 % | 0.37 | 0.43 | 0.49 | 0.56 | 0.63 | 0.72 | 0.81 | 0.91 |
| 80 % | 0.17 | 0.19 | 0.22 | 0.26 | 0.29 | 0.34 | 0.39 | 0.44 |
| 90 %* | ≈0.0 | ≈0.0 | ≈0.0 | ≈0.0 | ≈0.0 | ≈0.0 | ≈0.0 | ≈0.0 |
*At 90% humidity, the calculated leaf temperature drops below the dew point of the air — VPD drops to about 0 kPa. In practice, that means a risk of condensation on the leaves and a very high risk of mold, no matter the growth stage.
| Growth stage | VPD target range |
|---|---|
| Seedling / clone | 0.4–0.8 kPa |
| Vegetative stage | 0.8–1.2 kPa |
| Early flower | 1.0–1.4 kPa |
| Late flower | 1.2–1.6 kPa |
What can go wrong when you measure VPD
- Judging temperature or humidity on their own — VPD combines both, and a single number says nothing about the actual stress on the plant
- Ignoring the leaf temperature offset: transpiration keeps leaves cooler than the air, by about −3.6 °F (−2 °C) as a standard — closer to −1.8 °F (−1 °C) with strong airflow, −5.4 °F (−3 °C) with weak airflow
- Using the same targets for every stage — seedlings (0.4–0.8 kPa) and late flower (1.2–1.6 kPa) need fundamentally different ranges
- Placing the sensor right under the light or close to the exhaust fan — always measure at canopy height, in the plant’s natural airflow
Cannabis VPD — FAQ
What is a good VPD for cannabis?
It depends on the growth stage: seedling/clone 0.4–0.8 kPa, veg 0.8–1.2 kPa, early flower 1.0–1.4 kPa, late flower 1.2–1.6 kPa. After you enter your readings, the calculator above shows where you stand and what that means for your plant.
What happens when VPD is too low?
Too low a VPD (air too humid for the temperature) reduces the plant’s transpiration. The result: slower nutrient transport from the roots and a higher risk of mold — especially dangerous in flower. The fix: lower the humidity (dehumidifier, more air exchange) or raise the room temperature slightly.
What happens when VPD is too high?
Too high a VPD (air too dry) forces the plant to close its stomata to save water. Visible signs: drooping leaves, curled leaf edges, stalled growth. The fix: raise the humidity (humidifier, less exhaust airflow) or lower the room temperature.
Why does the calculator have a leaf temperature offset?
Active transpiration keeps leaves about 2–5 °F (1–3 °C) cooler than the surrounding air. Technically, VPD should be calculated against leaf temperature, not air temperature alone. The calculator’s standard offset is −3.6 °F (−2 °C). With strong airflow, use about −1.8 °F (−1 °C); with weak airflow, up to −5.4 °F (−3 °C).
Do I need special equipment for VPD?
No. A simple thermo-hygrometer (temperature + humidity) is all you need — you can get one for about $10–15. Place it at canopy height, not under the light or right next to a fan. The calculator above handles the formula.
