VPD Calculator
Vapor pressure deficit from temperature and humidity, with a plant growth band.
What it does
Plants lose water through their stomata whenever the air around the leaf is not saturated. The size of that driving force is the vapor pressure deficit, usually shortened to VPD. It compares the pressure the air would have at full saturation with the pressure of the water vapor already present.
Relative humidity answers a different question. It tells you how full the air is relative to its own capacity, and that capacity climbs quickly with temperature. Air at 90% humidity on a cool morning can hold a small absolute amount of water, while the same 90% on a hot afternoon holds much more. VPD removes that ambiguity by working in kilopascals.
This tool takes air temperature and relative humidity and returns VPD in kPa, plus the saturation pressure and the actual vapor pressure behind the result. The equation is the one used in the FAO-56 irrigation guide, so it matches the numbers in greenhouse references and extension bulletins.
Growers use VPD to steer transpiration, nutrient uptake, and disease pressure. Low VPD keeps leaves wet and slows calcium movement. High VPD pulls water out of the plant faster than roots can replace it, which can close stomata and slow growth. The bands below are a starting point, not a rule for every crop.
The result describes air above a surface, not the leaf itself. A leaf running warmer than the air has a slightly different VPD. For most greenhouse decisions, air temperature and relative humidity are close enough, and they are the values most sensors report.
How to use
- Leave the unit system on Metric or switch to Imperial.
- Enter the air temperature next to the plant canopy.
- Enter the relative humidity from the same sensor and time.
- Read the vapor pressure deficit in kPa as the main result.
- Check the saturation and actual pressures to see why the deficit moved.
- Compare the band with the crop target before changing vents, heat, or fogging.
Formula
- VPD
- Vapor pressure deficit in kilopascals.
- es
- Saturation vapor pressure at air temperature, in kPa.
- ea
- Actual vapor pressure of the air, in kPa.
- T
- Air temperature in degrees Celsius for the equation.
- RH
- Relative humidity as a percent from 0 to 100.
The saturation term is equation 11 from the FAO-56 paper, in kPa. Fahrenheit values convert to Celsius before the exponential is applied, so 77 F and 25 C produce the same result. The equation assumes standard atmospheric pressure and pure water vapor, which is close enough for greenhouse and grow room work.
Worked example
Example: 25 C and 60% humidity
Metric defaults: 25 C air temperature and 60% relative humidity.
- Saturation: 0.6108 x exp(17.27 x 25 / (25 + 237.3)) = 3.168 kPa.
- Actual vapor pressure: 3.168 x 60 / 100 = 1.901 kPa.
- Deficit: 3.168 minus 1.901 = 1.267 kPa.
- Rounded: saturation 3.17 kPa, actual 1.9 kPa, VPD 1.27 kPa.
- Band: 1.27 kPa falls inside the 0.7 to 1.5 kPa moderate range.
- Tool shows 1.27 kPa, 3.17 kPa, 1.9 kPa, and Moderate.
Reference table
| Temperature | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|
| 18 C (64 F) | 1.03 | 0.83 | 0.62 | 0.41 |
| 22 C (72 F) | 1.32 | 1.06 | 0.79 | 0.53 |
| 25 C (77 F) | 1.58 | 1.27 | 0.95 | 0.63 |
| 28 C (82 F) | 1.89 | 1.51 | 1.13 | 0.76 |
| 30 C (86 F) | 2.12 | 1.70 | 1.27 | 0.85 |
Limits
- Uses air temperature, not leaf temperature. Full leaf VPD needs an infrared leaf reading.
- Assumes standard sea level pressure. At high elevation the absolute values shift slightly.
- The growth bands are general horticulture guidance and differ by species and growth stage.
- Does not model airflow, stomatal closure, or substrate moisture, all of which change real transpiration.
Common mistakes
- Reading relative humidity as VPD and expecting the same number at every temperature.
- Using a sensor sitting in direct sun, which reads its own heated housing.
- Applying one band to seedlings, leafy greens, and fruiting crops without checking the crop target.
- Forgetting to convert Fahrenheit to Celsius before the exponential term.
- Treating a single reading as the daily average when VPD swings between morning and afternoon.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is the VPD at 22 C and 50% humidity?
1.32 kPa, with saturation 2.64 kPa and actual 1.32 kPa.
What is the VPD at 30 C and 60% humidity?
1.7 kPa, with saturation 4.24 kPa and actual 2.55 kPa.
What is the VPD at 18 C and 70% humidity?
0.62 kPa, a low value that keeps leaves wet longer.
Questions
What is a good VPD for plants?
MSU Extension describes 0.7 to 1.5 kPa as a target for most controlled environment growth, with 0.3 to 0.7 kPa low and above 1.5 kPa high. Seedlings and some leafy crops prefer the lower end.
Why not just use relative humidity?
Relative humidity hides the effect of temperature. Air at 70% humidity holds very different amounts of water at 60 F and 90 F, and plants respond to the absolute gap, not the percentage.
Does 77 F give the same answer as 25 C?
Yes. The calculator converts Fahrenheit to Celsius first, so 77 F and 25 C both produce 1.27 kPa at 60% humidity.
Is this leaf VPD or air VPD?
It is air VPD. Leaf VPD uses leaf temperature, which can run a few degrees above or below air temperature under strong light.
What does a very low VPD cause?
Slow transpiration, long leaf wetness, weak calcium transport, and more favorable conditions for fungal disease.
What does a very high VPD cause?
Fast water loss, stomatal closure, wilting risk, and tip burn in sensitive crops when the root zone cannot keep up.
Sources
- FAO Irrigation and Drainage Paper 56, Chapter 3: meteorological data and vapor pressure
- Michigan State University Extension: water vapor-pressure deficit classifications
- Michigan State University Extension: why growers should watch VPD instead of relative humidity