🌱 VPD Calculator (Vapor Pressure Deficit)
Enter air temperature, relative humidity and an optional leaf-temperature offset to get the VPD in kPa — with the ideal range for each growth stage.
Vapour pressure deficit
1.58 kPa
Flowering / fruiting
VPD is the gap between how much moisture the air holds and how much it could hold at leaf temperature — the "drying power" that drives transpiration. VPD = SVP(leaf) − SVP(air)×RH. Growers target roughly 0.4–0.8 kPa for clones/seedlings, 0.8–1.2 for veg and 1.2–1.6 for flower. Set a leaf offset (leaves often run 1–3° cooler than the air). 🔒 In your browser.
How the vpd calculator (vapor pressure deficit) works
Vapor pressure deficit is the difference between how much moisture the air could hold (at leaf temperature) and how much it actually holds — the "drying power" that pulls water out of a leaf and drives transpiration. VPD = SVP(leaf) − SVP(air) × RH ÷ 100, using the Magnus saturation vapour pressure, expressed in kilopascals. Leaves usually sit a degree or two below air temperature; enter that offset for a more accurate figure.
VPD is the metric serious growers tune instead of bare humidity, because it accounts for temperature. Rough targets: 0.4–0.8 kPa for clones and seedlings, 0.8–1.2 kPa for vegetative growth, and 1.2–1.6 kPa for flowering/fruiting. Too low invites mould and weak transpiration; too high stresses plants and closes stomata.
Frequently asked questions
What is VPD (vapor pressure deficit)?
The gap between the water vapour the air is holding and the maximum it could hold at leaf temperature. It's the drying demand on the plant — higher VPD pulls more water through the plant via transpiration.
How do you calculate VPD?
VPD = saturation vapour pressure at leaf temperature − (saturation vapour pressure at air temperature × relative humidity ÷ 100), in kPa. The saturation pressures come from the Magnus formula. This tool does it from your temperature, RH and leaf offset.
What is a good VPD for plants?
It depends on the stage: about 0.4–0.8 kPa for clones and seedlings, 0.8–1.2 kPa for vegetative growth, and 1.2–1.6 kPa for flowering. The tool shows which zone your value falls in.
Why use VPD instead of humidity?
Because the same relative humidity means very different drying power at different temperatures. VPD combines temperature and humidity into the single number that actually governs transpiration, so it's a better control target for greenhouses and grow rooms.
What is leaf temperature offset?
Leaves are often 1–3 °C cooler than the surrounding air (from transpiration) or warmer under intense light. Because VPD depends on leaf temperature, entering this offset gives a more accurate result than assuming leaf equals air temperature.