Greenhouse VPD calculator
Short answer
Vapor pressure deficit (VPD) is the difference between how much water vapor the air could hold at saturation and how much it holds now, in kilopascals. Enter temperature, relative humidity and an optional leaf temperature offset to get air VPD, leaf VPD and the growth stage it suits.
Key takeaways
- VPD = saturation vapor pressure x (1 - RH/100) for air VPD; leaf VPD uses the saturation pressure at leaf temperature instead.
- At 77 °F and 70 percent humidity air VPD is about 0.95 kPa; a leaf 2 °F cooler sees about 0.75 kPa.
- Common starting targets: roughly 0.4 to 0.8 kPa for propagation, 0.8 to 1.2 kPa for vegetative growth, 1.0 to 1.5 kPa for flowering.
- Misting and fogging lower VPD two ways at once: they add vapor and they cool the air.
How VPD is calculated
- Saturation vapor pressure (es). Tetens equation: es = 0.6108 x exp(17.27 x T / (T + 237.3)), with T in °C and es in kPa.
- Actual vapor pressure (ea). ea = es x RH / 100.
- Air VPD. es minus ea.
- Leaf VPD. Saturation vapor pressure at leaf temperature minus ea. The inside of a leaf is effectively saturated at leaf temperature, so this is the gradient that drives transpiration.
| Temperature | es (kPa) |
|---|---|
| 50 °F (10 °C) | 1.23 |
| 59 °F (15 °C) | 1.71 |
| 68 °F (20 °C) | 2.34 |
| 77 °F (25 °C) | 3.17 |
| 86 °F (30 °C) | 4.24 |
| 95 °F (35 °C) | 5.62 |
The table shows why summer greenhouses dry plants so quickly: saturation vapor pressure roughly doubles between 68 and 86 °F, so the same relative humidity leaves a much bigger deficit.
Target ranges by growth stage
| Stage | Leaf VPD | Why |
|---|---|---|
| Unrooted cuttings, germination | 0.4 to 0.8 kPa | No roots to replace lost water; keep transpiration low. |
| Vegetative growth | 0.8 to 1.2 kPa | Steady transpiration pulls nutrients up without stress. |
| Flowering, mature plants | 1.0 to 1.5 kPa | Drier air limits disease in dense canopies. |
| Stress zone | above about 1.6 kPa | Stomata begin to close; growth and cooling slow. |
Treat these as starting points. Crop guides from growers' associations and extension services give species-specific ranges, and a crop that looks healthy outranks any chart. The humidity control and VPD guide covers how to hit the targets.
Worked example: a hot afternoon under fog
A greenhouse at 86 °F and 50 percent humidity has an air VPD of about 2.1 kPa, well into the stress zone. A high-pressure fog system that evaporates enough water to cool the air to about 81 °F raises the humidity to roughly 65 percent (the wet-bulb temperature stays the same, about 72 °F). VPD falls to about 1.3 kPa. Cooling a little further to 79 °F gives about 70 to 72 percent humidity and roughly 1.0 kPa, inside the vegetative-stage range. A humidifier that adds vapor without cooling would have to push humidity to about 76 percent at 86 °F for the same 1.0 kPa, and the plants would still sit at 86 °F. See fog vs pad-and-fan cooling for which system suits your house.
Getting inputs you can trust
- Measure at canopy height in shade, with an aspirated or shielded sensor. A sensor in direct sun can read several degrees high.
- Cheap humidity sensors drift by 5 percent or more, which shifts VPD by about 0.15 kPa at 77 °F. Check them against a salt test or a calibrated reference once a season.
- For leaf temperature, an infrared thermometer aimed at several sunlit and shaded leaves gives a realistic offset.
- Controllers that act on VPD rather than humidity alone are covered in controllers, timers and humidistats, and mist bench timing in propagation mist systems.
Frequently asked questions
Why use VPD instead of relative humidity?
Relative humidity means different things at different temperatures. Seventy percent humidity at 68 °F is a VPD of about 0.7 kPa, while 70 percent at 86 °F is about 1.3 kPa, nearly double the drying pull on leaves. VPD describes what the plant actually experiences.
Should leaf temperature be higher or lower than air temperature?
Actively transpiring leaves in a ventilated greenhouse usually run a little cooler than the air, often 1 to 4 °F. Leaves under strong lighting or in still air can run warmer. An infrared thermometer pointed at a leaf gives the best offset to enter.
What VPD is too low?
Below roughly 0.4 kPa transpiration slows, nutrient uptake suffers, and water films persist on leaves, which favors fungal diseases such as botrytis. Short periods during propagation are normal, but sustained low VPD in established crops calls for ventilation or reduced misting.
How much does fogging lower VPD?
It depends on how dry the air is. At 86 °F and 50 percent humidity (about 2.1 kPa), evaporating fog that cools the air to about 81 °F raises humidity to roughly 65 percent and brings VPD to about 1.3 kPa, a large change from water alone.
Sources and further reading
- Tetens, O. (1930) saturation vapor pressure approximation, as given in FAO Irrigation and Drainage Paper 56 (Allen et al., 1998)
- University extension greenhouse management publications on humidity and VPD targets
Figures on this page are cross-checked against the shared planning values in our research methodology. Where manufacturers publish different numbers, your equipment's data sheet takes precedence.
Spotted an error or an outdated figure? Tell us and see how we handle corrections.