VPD Calculator for Cannabis Cultivation
Enter your grow room's air temperature and relative humidity to get instant vapor pressure deficit (VPD) in kPa, mapped against typical target ranges for clone, veg, and flower.
What is VPD?
Vapor pressure deficit is the gap between how much moisture the air can hold at a given temperature and how much moisture it's actually holding. A wider gap pulls water out of the plant faster through the leaf's stomata; a narrower gap slows that pull down. Cannabis growers use VPD instead of humidity alone because RH by itself doesn't account for temperature -- 60% RH at 65°F and 60% RH at 85°F put very different stress on a plant, even though the humidity reading is identical.
Ideal VPD ranges by growth stage
| Stage | Target VPD (kPa) |
|---|---|
| Clone / Seedling | 0.4 – 0.8 |
| Early Veg | 0.8 – 1.0 |
| Late Veg | 1.0 – 1.2 |
| Early Flower | 1.0 – 1.3 |
| Mid–Late Flower | 1.2 – 1.5 |
| Late Flower / Ripen | 1.3 – 1.6 |
These are commonly used starting points, not hard rules -- strain, canopy density, and airflow all shift where a specific room actually performs best.
How VPD is calculated
The calculator above uses the standard Tetens formula to get saturation vapor pressure from air temperature, then applies your relative humidity to get the deficit: SVP = 0.6108 × e^(17.27 × T / (T + 237.3)), then VPD = SVP × (1 − RH / 100), where T is air temperature in °C. The result is in kilopascals (kPa), the standard unit for VPD in cultivation.
Eyeballing one reading only tells you the moment you took it.
CannaKPI logs VPD -- and every other environmental metric -- against your own facility's grower-defined Min/Target/Max bands, week over week, across every room.