Turn your air temperature and relative humidity into the number that controls how fast your plants drink and transpire — and see exactly what humidity you need to hit your crop’s target VPD.
Fill in the form and click Calculate VPD to see your result here.
VPD describes how hard the air “pulls” water out of your plants. Three pieces go into the math.
Warmer air can hold more water. The saturation vapor pressure at your leaf temperature is the starting point — the maximum moisture the air could hold.
Relative humidity tells you how much of that capacity is already filled. Saturation pressure × RH gives the air’s actual vapor pressure.
Saturation pressure at the leaf minus the air’s vapor pressure is the deficit. Bigger deficit = faster transpiration, within your crop’s sweet spot.
Common starting ranges. Push toward the top end for mature, high-light plants and stay low for young or low-light crops.
| Crop / Stage | Target VPD (kPa) |
|---|---|
| Seedlings & clones | 0.4–0.8 |
| Leafy greens & microgreens | 0.8–1.1 |
| Culinary herbs | 0.8–1.2 |
| Fruiting crops (tomato, pepper, cucumber) | 1.0–1.5 |
| Cannabis (vegetative) | 0.8–1.1 |
| Cannabis (flowering) | 1.2–1.6 |
Vapor pressure deficit (VPD) is the difference, in kilopascals, between the moisture the air can hold and the moisture actually in it. It tells you how strongly the air pulls water out of your plants, which drives transpiration, nutrient uptake, and growth.
Work out the saturation vapor pressure at leaf temperature, then subtract the air’s actual vapor pressure (saturation at air temperature × RH). The calculator uses the standard Magnus formula: saturation VP = 0.6108 × e^(17.27 × T / (T + 237.3)).
Leafy greens run best around 0.8–1.1 kPa, fruiting crops 1.0–1.5, and flowering cannabis 1.2–1.6 kPa. Seedlings and clones prefer 0.4–0.8. See the table above for the ranges used by the calculator.
Strictly, VPD uses leaf temperature for saturation. Leaves usually run a few degrees above air temperature under bright lights, so a +1 to +2 °C offset is a good default. Many growers use air temperature alone and add the offset when they have a leaf probe.
Because RH only says how full the air is — it changes with temperature. VPD combines temperature and humidity into one number that directly predicts how fast plants transpire, so you can hold it steady even when the temperature drifts.
VPD too high (air too dry): raise humidity with a humidifier or lower temperature. VPD too low (air too wet): lower humidity with dehumidification or ventilation, or raise temperature. Make small changes and let the space stabilize before adjusting again.
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Disclaimer: Target ranges are general guidelines. Actual optimum VPD varies by cultivar, light intensity, and stage. © MistCulture.com
