What this calculator does
Vapour pressure deficit is the gap between how much moisture the air could hold and how much it actually holds, measured from the leaf perspective. It drives transpiration directly: a large deficit pulls water out of the plant quickly, a small one barely at all.
It is a better control variable than relative humidity because it accounts for temperature on both sides. The same 60% humidity means something quite different at 18°C than at 30°C, and leaf temperature usually differs from air temperature, which relative humidity alone cannot capture at all.
The formula
Saturation vapour pressure is calculated at both the leaf and the air temperature using the Tetens equation. The actual vapour pressure of the air is its saturation value multiplied by the relative humidity. VPD is the leaf saturation pressure minus that actual air vapour pressure, which is the gradient the leaf actually experiences.
| Term | Meaning |
|---|---|
| VPD | Vapour pressure deficit in kilopascals, the driving force for transpiration. |
| SVP | Saturation vapour pressure: the maximum moisture the air can hold at a given temperature. |
| Leaf temperature | Usually a degree or two different from air temperature, and the one that matters for the leaf side of the gradient. |
| Tetens equation | The standard empirical formula for saturation vapour pressure as a function of temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Leaf temperature (°C) | Leaf surface temperature in Celsius. Often slightly above air temperature under strong light and below it when transpiring freely. |
| Air temperature (°C) | Air temperature in Celsius. |
| Relative humidity (%) | Relative humidity as a percentage. |
When to use it
Setting greenhouse or grow room conditions
VPD targets are the standard way of specifying growing conditions, since they combine temperature and humidity into the quantity the plant actually responds to.
Diagnosing a growth problem
Very low VPD slows transpiration and with it nutrient uptake and can encourage fungal disease. Very high VPD closes stomata and stalls photosynthesis. Both show as poor growth with different causes.
Comparing conditions across temperatures
Two rooms at the same relative humidity and different temperatures have quite different VPDs, and comparing the two numbers directly shows by how much.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does humidity change VPD?
A fixed leaf and air temperature with relative humidity rising.
| Relative humidity | Vapor pressure deficit | Actual vapor pressure of air | Saturation VP at leaf temperature |
|---|---|---|---|
| 40% | 1.926 kPa | 1.058 kPa | 2.984 kPa |
| 60% | 1.398 kPa | 1.586 kPa | 2.984 kPa |
| 80% | 0.869 kPa | 2.115 kPa | 2.984 kPa |
| 95% | 0.472 kPa | 2.512 kPa | 2.984 kPa |
Questions
What VPD should I aim for?
Commonly quoted ranges are roughly 0.4 to 0.8 kPa for seedlings and clones, 0.8 to 1.2 for vegetative growth and 1.2 to 1.6 for flowering, though these are guidelines from controlled-environment horticulture rather than universal values. Crop and stage both matter.
Why use VPD instead of relative humidity?
Because relative humidity is relative to temperature, so the same figure means different things at different temperatures. VPD expresses the actual moisture gradient the leaf faces, and it incorporates leaf temperature, which relative humidity has no way of representing.
How do I measure leaf temperature?
An infrared thermometer pointed at the leaf surface is the practical method. Leaves under strong light typically run a degree or two above air temperature, and actively transpiring leaves can run below it. Assuming leaf equals air temperature is a common simplification that introduces real error.
What happens at very low VPD?
Transpiration slows, which reduces the transpiration stream carrying nutrients up from the roots, particularly calcium. Persistently low VPD also keeps leaf surfaces damp, which favours fungal disease. Too little deficit is a real problem, not simply a gentler condition.
For the plant internal water status, see the water potential calculator. For light rather than moisture, see the daily light integral calculator.