The VPD calculator works out leaf-to-air vapour pressure deficit from air temperature, humidity and leaf temperature, then tells you which humidity window keeps the current stage inside its band.
Formula — VPD calculator
VPD is the gap between the water vapour pressure the leaf surface could hold and what the air actually holds. Because transpiration is driven by the leaf, the number that matters uses leaf temperature for saturation and air temperature for the actual vapour pressure. Saturation vapour pressure comes from the Magnus–Tetens equation.
- SVP(T) [kPa] = 0.61078 × e^((17.27 × T) ÷ (T + 237.3))
- actual VP = SVP(air) × RH ÷ 100
- VPD_leaf = SVP(leaf) − actual VP
- VPD_air = SVP(air) − actual VP
- leaf temp = air temp − offset (1–3 °C under LED, ~0 with strong IR)
- absolute humidity [g/m³] = 216.7 × actual VP × 10 ÷ (T + 273.15)
Worked example — VPD calculator
Air at 25 °C and 60% RH, with leaves 2 °C cooler. SVP(25) = 3.17 kPa, so the actual vapour pressure is 3.17 × 0.60 = 1.90 kPa. SVP(23) = 2.81 kPa, so leaf VPD = 2.81 − 1.90 = 0.91 kPa. Air VPD would read 3.17 − 1.90 = 1.27 kPa — a 0.36 kPa difference that is the whole reason two calculators can disagree about the same room.
0.91 kPa sits inside the vegetative band (0.8–1.2 kPa), so nothing needs to change. To move the same room into a flowering band of 1.2–1.6 kPa you would have to drop humidity to roughly 38–51% RH at the same temperatures, or raise air temperature. Dew point here is 16.7 °C, nearly 8 °C below air temperature, so there is no condensation risk; once the gap falls below 3 °C, water starts forming on cool surfaces and botrytis becomes the real problem rather than VPD.
Typical ranges — VPD calculator
Leaf offset is the single biggest source of error. Under LED with no radiant heat, leaves typically run 1–3 °C below air; under HPS or in strong airflow they can match or exceed air temperature. An infrared thermometer aimed at a large fan leaf costs little and removes the guesswork. GrowLog also computes this automatically for every event that records temperature and humidity, and flags values that fall outside the band for that growth phase.
| Stage | VPD band |
|---|---|
| Seedling / clone / early root | 0.4–0.8 kPa |
| Vegetative | 0.8–1.2 kPa |
| Flowering | 1.2–1.6 kPa |
| Late flower / ripening | 1.2–1.5 kPa with 45–50% RH |
| Drying | 0.8–1.0 kPa (about 18–20 °C and 58–62% RH) |
| Danger zones | below 0.4 kPa mould risk · above 1.6 kPa stomatal closure |