Tools · Climate
VPD calculator
for cannabis growers
Enter air temperature, humidity and leaf temperature. Get the VPD in kPa, the growth stage it fits and the humidity to aim for.
Inputs
Units
Under LEDs leaves usually run 0–2 °C below the air. Measure them with an IR thermometer.
Growth stage
Result
VPD at your leaf offset
Tap a cell to use it. Rows: air temperature, columns: humidity.
Method and sources. Saturation vapour pressure from the Tetens equation (Murray, 1967, Journal of Applied Meteorology 6:203–204); dew point from the Magnus formula. Leaf VPD = SVP(leaf) − SVP(air) × RH ÷ 100, with leaf temperature = air temperature + the offset you set. Stage bands as in our VPD chart.
How the calculator works
VPD, vapour pressure deficit, is the difference between the water vapour the air inside the leaf holds (saturated, at leaf temperature) and the water vapour the room air actually holds. The calculator uses the Tetens equation for saturation vapour pressure:
SVP(T) = 0.61078 × e17.27·T / (T + 237.3) kPa
VPD = SVP(leaf) − SVP(air) × RH / 100
Leaf temperature matters because water leaves the plant at the leaf, not in the air. At 26 °C and 60% RH, a leaf 2 °C cooler than the air lowers the VPD from 1.34 to about 0.97 kPa: from the flowering band to the veg band, with the same thermostat and hygrometer readings.
Target VPD by growth stage
| Stage | VPD (kPa) | Why |
|---|---|---|
| Clones and seedlings | 0.4–0.8 | High humidity while roots form and transpiration is small. |
| Vegetative | 0.8–1.2 | Strong transpiration pulls water and nutrients up for fast growth. |
| Flowering | 1.2–1.6 | Drier air limits mould in dense buds; many growers go to the upper end in late flower. |
Getting reliable numbers
- Measure at the canopy, not on the wall or by the door. Climate can differ by several degrees across a room.
- Use the leaf temperature when you can. An infrared thermometer pointed at a few top leaves is enough.
- Watch the dew point at lights off. When the temperature drops towards the dew point, water condenses on buds, walls and pipes, and that is where bud rot starts.
- Log it over the day, not once. One reading at noon tells you little about the night.
For the full chart, the leaf temperature effect and the science behind the bands, read our VPD chart for cannabis. To match light and climate, see PPFD and DLI for cannabis and CO₂ enrichment.