VPD Calculator for Grow Rooms
Temperature and humidity only mean something together. Enter your air temperature, relative humidity, and a leaf-temperature offset to get vapor pressure deficit — the number that actually describes how hard your plants are transpiring.
Transpiring leaves usually run cooler than the air — often −1 to −3 °C under lights. Set to 0 to calculate air VPD instead.
Formula & how it works
Saturation vapor pressure uses the Tetens equation: es(T) = 0.6108 × e^(17.27T ÷ (T + 237.3)) in kPa, with T in °C. The air's actual vapor pressure is ea = es(air) × RH ÷ 100.
Leaf VPD is es(leaf) − ea, where leaf temperature is air temperature plus your offset. Air VPD is es(air) − ea. Fahrenheit converts with °C = (°F − 32) × 5⁄9.
Worked example
At 24 °C and 60% RH, saturation pressure is about 2.98 kPa and the air holds 1.79 kPa, giving an air VPD of 1.19 kPa. Assume leaves run 2 °C cooler at 22 °C and their saturation pressure drops to 2.64 kPa, so leaf VPD is 0.85 kPa — comfortably in the vegetative range.
Working with VPD
Why not just watch humidity
Sixty percent humidity means something completely different at 18 °C than at 28 °C, because warm air holds far more moisture. VPD folds both into a single figure describing the drying pressure plants actually feel, which is why growers increasingly steer by it rather than by humidity alone.
Too low, too high
When VPD is very low, transpiration stalls, nutrient movement slows, and the damp, still conditions invite mould and mildew. When it is too high, plants lose water faster than roots supply it, stomata close, and growth stops even with a full reservoir. The useful range sits between those failures and shifts upward as plants mature.
Leaf temperature is the honest input
Because VPD is defined at the leaf surface, the offset matters. Under intense lighting leaves may sit closer to air temperature or even above it, while strong transpiration cools them further. An inexpensive infrared thermometer aimed at the canopy replaces the guess and makes the number meaningful.