Hydroponicity
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VPD calculator

Vapor pressure deficit (not RH alone) is what drives transpiration. The same 60% RH is gentle at 20 °C and punishing at 30 °C. Leaves run cooler than the air, so leaf VPD is the number worth steering.

Room conditions

Result

Deficit between leaf and air moisture

Leaf VPDIn target range
0.91kPa
Air VPD
1.27kPa
Dew point
16.7°C
Surfaces colder than this will condense: keep lights-off temps above it.
To sit mid-band (1.00 kPa) at this air temperature you'd need about 68% RH.

Stage bands are rules of thumb, and the leaf offset default (−2 °C) is typical under LEDs; HPS canopies often run warmer than the air. VPD moves fast with temperature, so re-check after any lighting or HVAC change.

RH too high to hit your target? Size a dehumidifier → Too low? Size a humidifier →

What is a good VPD for hydroponics?

Target 0.4-0.8 kPa during veg and 0.8-1.2 kPa during flower, measured as leaf VPD when possible. Going below the range risks mold and poor transpiration; going above it stresses plants and slows growth. Enter your temp, humidity, and leaf offset above to check your room against these stage targets.

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