Free calculator

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

Air temperature°CRelative humidity%Leaf temperature offset°CLeaves usually run 1–3 °C cooler than the air under LEDs. Measure with an IR thermometer if you can.Growth stage (target band)

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 →