Vapor Pressure Deficit (VPD) Calculator
Compute air VPD and leaf VPD in kPa from temperature & humidity using the Tetens equation, check your growth-stage zone (seedling, vegetative, flowering), get humidify/dehumidify actions, and read any condition off the full VPD chart.
Calculator
— kPa leaf VPD
Enter values
00.40.81.21.62.0+ kPa
Recommended actions
- Enter your grow-room readings above.
VPD chart (air VPD, kPa)
Rows = air temperature, columns = relative humidity. Cells are coloured for the selected growth stage; the outlined cell is your current reading.
Below stage range
In stage range
Above stage range
How it works
Saturation vapour pressure comes from the Tetens equation (Tetens 1930; Monteith & Unsworth 2008):
SVP = 0.6108 × e^(17.27·T / (T + 237.3)) kPa, T in °C
AVP = SVP × RH / 100 · VPD = SVP − AVP
Leaf VPD uses the leaf surface temperature for SVP while actual vapour pressure still comes from the air: VPDleaf = SVP(Tleaf) − AVP(Tair, RH). Leaves under grow lights typically run 1–3°C cooler than the air.
Stage bands follow conventional grow charts: seedling/clone 0.4–0.8, vegetative 0.8–1.2, early flower 1.0–1.3, late flower 1.2–1.6 kPa. Below ~0.4 kPa any stage risks mold; above ~1.6 kPa stomata close and growth stalls.
Scope note: this tool measures
vapor pressure deficit — how thirsty the air is for your plants. For how fast water evaporates off an open surface (pool, soil, fresh concrete), see our
evaporation rate calculator.
Frequently asked questions
What is vapor pressure deficit (VPD)?
VPD is the difference between how much water vapor the air could hold at saturation and how much it actually holds. It measures the “drying power” of the air in kilopascals (kPa): a high VPD pulls water out of leaves fast, a low VPD means the air is nearly saturated and transpiration stalls.
How is VPD calculated?
From air temperature T (°C) and relative humidity. Saturation vapour pressure uses the Tetens equation: SVP = 0.6108 × e^(17.27·T/(T+237.3)) kPa. Actual vapour pressure is AVP = SVP × RH/100, and VPD = SVP − AVP. Example: at 25°C and 60% RH, SVP = 3.17 kPa, so VPD = 3.17 × (1 − 0.60) = 1.27 kPa.
What is a good VPD for seedlings and clones?
Aim for 0.4–0.8 kPa. Young plants have small root systems and cannot replace water lost to dry air quickly, so a gentle VPD prevents wilting and transplant shock while still allowing some transpiration.
What is a good VPD for the vegetative stage?
Aim for 0.8–1.2 kPa. This moderate pull drives steady transpiration, which is how the plant moves water and nutrients (especially calcium) from the roots to the leaves.
What is a good VPD for flowering?
Aim for 1.0–1.3 kPa in early flower and 1.2–1.6 kPa in mid-to-late flower. The higher pull keeps dense buds dry enough to resist mold while driving nutrient density.
What is leaf VPD, and why is it different from air VPD?
Leaf VPD uses the leaf surface temperature instead of the air temperature. Under grow lights leaves typically run 1–3°C cooler than the air, so leaf VPD is lower than air VPD. Example: air at 25°C and 60% RH has an air VPD of 1.27 kPa, but with leaves at 23°C the leaf VPD is 0.91 kPa — a different growth zone. Measure leaf temperature with an IR thermometer for the most accurate reading.
What happens when VPD is too low or too high?
VPD below about 0.4 kPa (any stage) stalls transpiration, can cause calcium deficiency and edema, and sharply raises the risk of powdery mildew and botrytis. VPD above about 1.6 kPa forces stomata to close to conserve water, which cuts CO2 uptake and photosynthesis and causes heat and drought stress.
How do I convert VPD units (kPa, hPa, mbar)?
1 kPa = 10 hPa = 10 mbar, so multiply kPa by 10. A VPD of 1.27 kPa is 12.7 hPa (12.7 mbar). Some charts use inches of mercury: 1 kPa ≈ 0.295 inHg.
At 20°C and 60% humidity, what is the dew point?
About 12.0°C. The dew point is the temperature at which the air becomes saturated; surfaces colder than 12.0°C in that room will condense water, which is why VPD and dew point are tracked together in greenhouses.
Does this calculator work for greenhouses and grow tents?
Yes. The physics is the same for any indoor grow: enter your air temperature and relative humidity, pick your growth stage, and optionally enter leaf temperature for leaf VPD. The chart table shows the VPD for every temperature/humidity combination so you can plan humidifier or dehumidifier settings.
Free tool by NRTechStudio