# VPD

> Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.

- Canonical: https://genetixbay.com/knowledge/glossary/vpd.html
- Also written: vapor pressure deficit, vapour pressure deficit, vapor-pressure deficit, VPD chart, air VPD
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Vapour pressure deficit (VPD) is the difference between the saturation vapour pressure of air at a given temperature and the actual vapour pressure, in kilopascals (kPa). It measures the drying power of the air. High VPD pulls water from leaves quickly; low VPD slows [transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md), nutrient flow and drying of surfaces. It combines temperature and humidity into one number that tracks plant water use far better than [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) alone.

## In practice

Common targets (leaf VPD):

| Stage | kPa |
|---|---|
| Clones | 0.4–0.8 |
| Veg | 0.8–1.2 |
| Flower | 1.0–1.5 |
| Late flower | 1.2–1.6 |

Too low (below about 0.4 kPa in flower) invites mould and soft growth. Too high (above about 1.6–1.8 kPa) can cause [stomata](https://genetixbay.com/knowledge/glossary/stomata.md) to close, stalling [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md), and leaves to curl.

## Units and formula

Saturation vapour pressure (Tetens, kPa): es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)), T in °C.

- Air VPD = es(air) × (1 − RH ÷ 100)
- [Leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md) = es(leaf) − es(air) × RH ÷ 100

Example: 26 °C air at 60% RH gives an air VPD of about 1.35 kPa. If the leaf is 24 °C, leaf VPD is about 1.0 kPa.

## Don't confuse with

- **Leaf VPD**: uses [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md); what the plant actually experiences.
- **Relative humidity**: a percentage that means different things at different temperatures.

## Related terms

[Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md), [Leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md), [Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), [Dew point](https://genetixbay.com/knowledge/glossary/dew-point.md), [Transpiration](https://genetixbay.com/knowledge/glossary/transpiration.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Sources

1. Allen RG, Pereira LS, Raes D, Smith M, 1998. Crop evapotranspiration: guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56), Chapter 3. FAO. https://www.fao.org/4/x0490e/x0490e07.htm
2. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/