Cornell’s hydroponic lettuce program runs the greenhouse at 75°F (24°C) during the day with relative humidity between 50 and 70%. Both ends look like “normal humidity.” The plant sees a big difference: at 70% the vapor pressure deficit (VPD) is 0.9 kPa, at 50% it is 1.5 kPa. At the dry end, the air pulls water out of the leaf almost 1.7 times harder.
Below is a VPD calculator with two modes. The first works out VPD from temperature and humidity, for both the air and the leaf, and gives you the dew point. The second runs the other way: pick a target VPD and it tells you what humidity to hold at your temperature. After the calculator you’ll find the formula, a lookup chart, where the target ranges come from, and why the dew point matters more than VPD at night.
VPD calculator: from temperature and humidity, or the reverse
- Measure temperature and humidity at plant height, in the shade, not by the door or up under the roof. A thermometer in direct sun reads higher than the air around it.
- A transpiring leaf usually runs 2-5°F (1-3°C) cooler than the air because evaporation cools it. An infrared thermometer gives you the real number. Under strong lamps a leaf can run warmer than the air; enter a negative number in that case.
- The target ranges come from a 2018 review by Shamshiri and co-authors that compiles recommendations from several sources. For your own crop or data, pick “Custom range.”
VPD formula and chart
VPD tells you how much more water vapor the air could hold at its current temperature, in kilopascals. Start with the saturation vapor pressure at air temperature, using the equation from FAO Irrigation and Drainage Paper 56 (Allen et al., 1998):
e°(T) = 0.6108 × exp(17.27 × T / (T + 237.3)), kPa, with T in °C (°C = (°F - 32) / 1.8).
Actual vapor pressure is e°(T) × RH / 100, and VPD is the gap between the two: e°(T) × (1 - RH / 100). At 77°F (25°C), e° is 3.17 kPa; at 68°F (20°C), 2.34. Warm air holds almost twice as much vapor as cool air, so the same 60% gives 0.94 kPa at 68°F (20°C) and 1.70 at 86°F (30°C).
| Temperature | RH 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|
| 61°F (16°C) | 0.91 | 0.73 | 0.55 | 0.36 | 0.18 |
| 64°F (18°C) | 1.03 | 0.83 | 0.62 | 0.41 | 0.21 |
| 68°F (20°C) | 1.17 | 0.94 | 0.70 | 0.47 | 0.23 |
| 72°F (22°C) | 1.32 | 1.06 | 0.79 | 0.53 | 0.26 |
| 75°F (24°C) | 1.49 | 1.19 | 0.90 | 0.60 | 0.30 |
| 79°F (26°C) | 1.68 | 1.34 | 1.01 | 0.67 | 0.34 |
| 82°F (28°C) | 1.89 | 1.51 | 1.13 | 0.76 | 0.38 |
| 86°F (30°C) | 2.12 | 1.70 | 1.27 | 0.85 | 0.42 |
Air VPD in kPa, calculated with FAO-56. What low and high VPD do to the plant, and how VPD ties into calcium deficiency, is covered in the wiki entry VPD: why a plant “won’t drink” at normal humidity.
VPD of the air vs. the leaf
Water evaporates from the leaf, so the more accurate number uses leaf temperature: e° at leaf temperature minus the vapor pressure of the air. A leaf that is transpiring runs cooler than the air, which lowers its VPD. At 75°F (24°C) and 60%, air VPD is 1.19 kPa. With the leaf 4°F (2°C) cooler, leaf VPD drops to 0.85. That is almost a third lower, enough to decide whether you are inside your target or not.
Once the leaf cools below the dew point, leaf VPD turns negative: vapor stops leaving the leaf and condenses on it instead. The calculator flags that as condensation. How the leaf cools itself, and why its temperature is a sign of healthy transpiration, is explained in the wiki entry plant transpiration.
VPD targets: what the sources say
There is no single number. In their review of greenhouse climate for tomatoes, Shamshiri and co-authors collected recommendations from several sources, and they don’t agree:
| VPD, kPa | What it is recommended for | Source cited in the review |
|---|---|---|
| 0.3-1.0 | Range recommended across sources | Shamshiri et al., 2018 |
| 0.5-0.8 | Optimal for most greenhouse crops; prevents smaller fruit and fungal disease | Barker, 1990 |
| 0.4-0.79 | Ideal for most greenhouse crops | OMAFRA, 2005 |
| 0.5-1.2 | Optimal; below 0.4 and above 1.37 marginal | Argus, 2009 |
| 0.3-0.5 | Rooting cuttings, to keep plants from drying out | Shamshiri et al., 2018 |
The same review cites a study of 26 crops: VPD between 0.3 and 1.0 kPa had a minor effect on growth, while values above 1.0 affected it significantly. Upper limits across sources fall between 0.8 and 1.4 kPa. That is the zone to watch.
Cornell’s lettuce set-points work out to 0.9-1.5 kPa during the day, drier than most sources in the table suggest. Their night set-point is 66°F (19°C), and at the same 50-70% that gives 0.66-1.1 kPa. According to their handbook, high humidity slows transpiration and nutrient transport to the leaves and encourages botrytis and mildew. Temperature and humidity ranges for lettuce, tomato, cucumber, and microgreens are collected in the wiki entry greenhouse temperature and humidity.
VPD at night and the dew point
At night transpiration slows down, and the exact VPD matters less than the risk of condensation. At 75°F (24°C) and 60%, the dew point is 60°F (15.8°C). If the greenhouse isn’t vented in the evening and cools to 61°F (16°C), humidity climbs close to 100% and water settles on cold glazing and leaves. That film of water is where spores of gray mold (Botrytis) and powdery mildew germinate.
The “Humidity for a target VPD” mode shows the dew point for the range you pick. If the greenhouse drops below it at night, bring humidity down before evening: vent and heat while it is still warm outside. Moving air between plants helps too, as the wiki entry ventilation and air circulation explains.
VPD in the greenhouse: how to change it
VPD depends on temperature and humidity alone, so those are the two levers you have.
- VPD too low, air too damp: vent, run a dehumidifier, raise the temperature a little. In winter, venting and heating together is usually the cheapest fix.
- VPD too high, air too dry: fog or a humidifier, shade, a lower temperature. The Shamshiri review mentions set-points that switch fogging on above 1.25 kPa and dehumidification below 0.45.
- Changed the temperature? Recalculate VPD. The same 65% gives 0.72 kPa at 64°F (18°C) and 1.18 at 79°F (26°C).
In lettuce, VPD is tied to tipburn: with weak transpiration, calcium doesn’t reach the young leaves. How that interacts with light and airflow is covered in our DLI calculator and in the wiki entry lettuce and leafy greens. Other tools are collected on the calculators page.
VPD calculator: key points
- VPD = e°(T) × (1 - RH / 100). At 75°F (24°C), 50-70% humidity gives 0.9 to 1.5 kPa.
- Sources recommend 0.3 to 1.0 kPa, 0.5-0.8 for most greenhouse crops, and 0.3-0.5 for rooting cuttings.
- A leaf 4°F (2°C) cooler than the air cuts VPD by almost a third. Measure leaf temperature if you can.
- At night, watch the dew point: at 75°F (24°C) and 60% it is 60°F (15.8°C).
