free calculator
VPD Calculator
Enter the air temperature and humidity — the calculator shows the air and leaf VPD, and whether it's within the ideal range for your plant's stage.
air VPD
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leaf VPD (recommended)
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ideal range for this stage:
What VPD is and why it matters
VPD in one sentence
VPD (Vapor Pressure Deficit) measures how hard the air "pulls" water out of the plant — it's the combination of temperature and humidity that actually matters, not either one alone. Two rooms can share the same relative humidity and still have very different VPDs if the temperature shifts, which is why staring at the hygrometer alone can mislead you.
Why the plant's stage changes the target
A freshly germinated seedling has tiny roots and thin leaves — it needs a low VPD (more humid air) so it doesn't lose water faster than it can replace it. As the plant grows and transpiration ramps up, the VPD target climbs gradually, then drops again near harvest to lower the risk of mold on dense buds.
Leaf VPD vs. air VPD
The VPD the plant actually "feels" is calculated at leaf temperature, not air temperature — and under LEDs the leaf usually runs 1 to 3°C cooler than the room, thanks to transpiration and the lack of infrared from LEDs. Ignoring that gap is the most common mistake growers make when they switch from HPS to LED and start seeing stress signs without knowing why.
How to adjust when you're outside the range
If VPD is low, the fastest fix is to lower humidity (dehumidifier, more air movement) or nudge the temperature up a bit. If it's high, do the opposite: raise humidity or lower the temperature. Small tweaks — 2 to 3 percentage points of RH or 1°C — are usually enough to land back in range.
VPD ranges by stage (reference)
Consensus values from growers and sensor makers (Pulse, Gorilla, smartfog). Cultivars and setups vary — use this as a starting point, not a hard rule.
want VPD calculated automatically, on every reading?
The GrowerHelper app calculates VPD automatically from your temperature and humidity readings, already classified by plant stage — with a heads-up on how much to adjust when you drift out of range. All offline, no signup.