Grow science · 07
Ideal humidity
There is no single right humidity. The right number depends on your temperature.
Why % alone isn’t enough
60 % humidity at 22 °C is a completely different climate than 60 % at 28 °C. Warm air holds more water, so the same percentage leaves a bigger gap (a higher VPD) and pulls more water out of the leaves.
That’s why good targets start with VPD and turn it into humidity for your temperature.
Humidity by stage
| Stage | Target VPD | Humidity at 24 °C | at 26 °C |
|---|---|---|---|
| Seedlings & clones | 0.4–0.8 kPa | 73–87 % | 76–88 % |
| Veg | 0.8–1.2 kPa | 60–73 % | 64–76 % |
| Flower | 1.2–1.6 kPa | 46–60 % | 52–64 % |
Air VPD, calculated with the Magnus formula. In late flower stay at the dry end. Dense flowers trap moisture, and that’s where mold starts.
Find your number
Relative humidity for a target VPD at each air temperature:
| Air | 0.6 kPa | 0.8 | 1.0 | 1.2 | 1.4 |
|---|---|---|---|---|---|
| 20 °C | 74 % | 66 % | 57 % | 49 % | 40 % |
| 22 °C | 77 % | 70 % | 62 % | 55 % | 47 % |
| 24 °C | 80 % | 73 % | 66 % | 60 % | 53 % |
| 26 °C | 82 % | 76 % | 70 % | 64 % | 58 % |
| 28 °C | 84 % | 79 % | 74 % | 68 % | 63 % |
| 30 °C | 86 % | 81 % | 76 % | 72 % | 67 % |
Every value in between: free VPD chart (PDF) or the VPD calculator.
Under LEDs, go a little drier
LED-lit leaves usually run 1–3 °C cooler than the air, so the plant feels a lower VPD than the air numbers suggest. If you measure leaf temperature, aim the table at your leaf VPD. In practice that often means a few percent less humidity.
Humidity too high?
Seven fixes, from free to effective: how to lower humidity in a grow tent. And watch the night: humidity climbs on its own when the lights go off (dew point & mold).
Questions growers ask
What humidity should I run in veg?
Around 60–75 %, depending on temperature. At 24 °C that’s about 60–73 % for a VPD of 0.8–1.2 kPa, at 26 °C about 64–76 %.
What humidity is best for flowering?
Around 45–60 % at typical flower temperatures of 24–26 °C, which gives a VPD of about 1.2–1.6 kPa. In late flower stay at the dry end to protect dense flowers from mold.
What humidity do seedlings and clones need?
High humidity, around 70–85 %. Their roots are small or missing, so they need air that pulls very little water out of the leaves: a VPD of about 0.4–0.8 kPa.
Is 70 % humidity too high?
For seedlings and clones it’s fine, in early veg it can be fine at warm temperatures. In flower 70 % is too high. The VPD drops below 1.0 kPa and the risk of mold rises sharply, especially at night.