VPD calculator
Enter the air temperature and relative humidity in your tent, pick the growth stage, and get the vapor pressure deficit in kPa with a check against that stage's target range. Free, no signup.
How to use it
- Measure air temperature and relative humidity at canopy height, not by the floor or right under the lamp.
- Type the temperature (switch between °C and °F) and the humidity in %.
- Choose the stage: seedling/clone, vegetative or flowering.
- Read the VPD and the verdict: in range, too low (raise temperature or lower humidity) or too high (raise humidity or lower temperature).
How the number is calculated
Saturation vapor pressure (Tetens): SVP = 0.6108 × e^(17.27 × T / (T + 237.3)) kPa, with T in °C. Air VPD = SVP × (1 − RH / 100).
Stage targets used by the calculator: seedling/clone 0.6–0.9 kPa, vegetative 0.8–1.1 kPa, flowering 1.2–1.5 kPa. These are common grower ranges, not hard limits — a plant a little outside them is not in danger.
This calculator gives air VPD. The leaf is usually a little cooler than the air under LED light, so leaf VPD is somewhat lower than the number shown here.
Quick VPD table (air VPD, kPa)
| Air temperature | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|
| 20 °C | 1.40 | 1.17 | 0.94 | 0.70 |
| 22 °C | 1.59 | 1.32 | 1.06 | 0.79 |
| 24 °C | 1.79 | 1.49 | 1.19 | 0.90 |
| 26 °C | 2.02 | 1.68 | 1.34 | 1.01 |
| 28 °C | 2.27 | 1.89 | 1.51 | 1.13 |
| 30 °C | 2.55 | 2.12 | 1.70 | 1.27 |
In the GrowScope app
The app uses the same saturation formula but works with leaf VPD: if you log a leaf temperature it uses it, otherwise it estimates the leaf from the light type (LED 2 °C below air, HPS equal to air) and labels the number as an estimate. Readings are kept apart for lights-on and lights-off, and the night line warns about high humidity in flower.
Questions
What is a good VPD for flowering?
This calculator uses 1.2–1.5 kPa for flowering. Many growers run the lower part of that range early in flower and the upper part in the last weeks, when drier air lowers the risk of bud rot.
What VPD for seedlings and clones?
0.6–0.9 kPa. Young plants with few roots lose water easily, so humid air (low VPD) keeps them from drying out.
Is VPD more important than humidity?
VPD combines temperature and humidity into one number that describes how hard the air pulls water from the leaf, so the same 60% humidity means something different at 20 °C and at 28 °C. Humidity on its own still matters for mould: high humidity at night in flower is a risk even if daytime VPD looks fine.
Why is my leaf VPD different from air VPD?
Leaf VPD uses the temperature of the leaf, which is usually 1–3 °C below the air under LED light. A cooler leaf means lower leaf VPD for the same room.
Can I put this calculator on my website?
Yes. The widget is free to embed with an iframe; the code is in the guide linked below.