# What is the VPD of my grow room?

Computes the vapour pressure deficit (VPD) of the air from its temperature and relative humidity, with the saturation and actual vapour pressures, and the leaf VPD when you type a leaf temperature.

- Page: https://www.acalculator.org/biology/vpd-calculator
- JSON spec: https://www.acalculator.org/biology/vpd-calculator.json
- Version: c0433fb95642

## Default answer

Example with the default inputs (Air temperature 77 °F, Relative humidity 60%): At 77 °F and 60% relative humidity, the vapour pressure deficit is 1.267 kPa.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| t | Air temperature | The temperature of the air around the plants. |
| rh | Relative humidity | The relative humidity of the air, from 0 to 100%. |
| leaf | Leaf temperature | The temperature of the leaves, from an infrared thermometer. Leave empty for the air VPD only. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| vpd | Vapour pressure deficit (air) | Saturation vapour pressure − actual vapour pressure. |
| leafVpd | Leaf VPD | Saturation vapour pressure at the leaf temperature − the actual vapour pressure of the air. |
| saturation | Saturation vapour pressure | 0.6108 × exp(17.27 T ÷ (T + 237.3)) at the air temperature T in °C. |
| actual | Actual vapour pressure | The saturation vapour pressure × relative humidity ÷ 100. |

## Method

e°(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa; eₐ = e°(T_air) × RH ÷ 100; VPD = e°(T_air) − eₐ; leaf VPD = e°(T_leaf) − eₐ.

## Assumptions

- Temperatures are converted to °C before the formula. The formula is FAO-56’s Tetens form over water.
- The leaf VPD assumes the air inside the leaf is saturated at the leaf temperature.
- Temperatures run from −40 °C to 60 °C (−40 °F to 140 °F).

## Worked examples

1. t = 297.65, rh = 50% gives saturation = 3,074.649051, vpd = 1,537.324525, actual = 1,537.324525. Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03).
2. t = 288.15, rh = 80% gives saturation = 1,705.346232, vpd = 341.069246. Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03).
3. t = 298.15, rh = 60%, leaf = 296.15 gives vpd = 1,267.111087, leafVpd = 908.770992. Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03).

## FAQ

### What is VPD?

The vapour pressure deficit is how much more water vapour the air could hold at its temperature: the saturation vapour pressure minus the vapour pressure the air has now. FAO calls it an accurate indicator of how fast the air can take up water from plants and soil.

### How is VPD calculated?

First the saturation vapour pressure, e° = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa with T in °C. The air holds e° × RH ÷ 100 of that. VPD is the difference: e° × (1 − RH ÷ 100). At 25 °C and 60% that is 3.168 × 0.40 = 1.267 kPa.

### What is leaf VPD?

The deficit between the inside of the leaf and the air. The air in a leaf is taken as saturated at the leaf temperature, so leaf VPD = e°(leaf temperature) − the air’s actual vapour pressure. Leaves cooler than the air have a lower VPD.

### Why does VPD matter for plants?

VPD sets how hard the air pulls water out of leaves: a higher VPD means faster transpiration, a lower one slower. FAO uses it for this reason in crop water use. Growers change the heat and humidity to keep VPD in the range their crop and its stage need.

### Can VPD be negative?

The air VPD cannot: relative humidity is at most 100%, so the air holds at most the saturation amount. A leaf VPD can be negative when the leaf is much cooler than the air and the air is humid, which means water can condense on the leaf.

### What unit is VPD in?

Kilopascals (kPa). 1 kPa is 10 hPa, or 10 millibars, or about 0.145 psi. Grow charts give VPD in kPa.

## Sources

- FAO Irrigation and Drainage Paper 56, Crop evapotranspiration (Allen, Pereira, Raes and Smith, 1998), chapter 3, Meteorological data, Air humidity: equation 11 for the saturation vapour pressure, equation 19 for the actual vapour pressure from relative humidity, and the vapour pressure deficit as their difference; Example 3 gives e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa. https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03)