VPD Calculator
Enter your air temperature and relative humidity to get the vapour pressure deficit (VPD) in kPa, with a leaf temperature option and a guide to common ranges.
How the calculation works
First the saturation vapour pressure (SVP) at a temperature T in °C, using the Tetens equation:
SVP = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa
Then VPD is the SVP at the leaf minus the actual vapour pressure of the air, which is the air SVP times the relative humidity:
VPD = SVP(leaf) − SVP(air) × RH ÷ 100
Example: at 25 °C, SVP is 3.17 kPa. With 60% humidity the air actually holds 3.17 × 0.6 = 1.90 kPa, so VPD = 3.17 − 1.90 = 1.27 kPa. If the leaf is 2 °C cooler (23 °C, SVP 2.81 kPa) the leaf VPD is 2.81 − 1.90 = 0.91 kPa.
Higher VPD pulls water out of plants faster, so a warmer reservoir and sturdy airflow matter more. Light is the other half of the picture, so check it with the DLI calculator.
Ranges quoted are general guidance. Sensors differ in accuracy, so check yours against a reference when you can.
Frequently asked questions
What is VPD?
Vapour pressure deficit is the difference between how much moisture the air could hold at a given temperature and how much it holds now. It is a single number that combines temperature and humidity and drives how fast plants lose water through their leaves.
What VPD range is good?
Commonly cited ranges are roughly 0.4 to 0.8 kPa for seedlings and cuttings, 0.8 to 1.2 kPa for leafy growth and 1.0 to 1.5 kPa for fruiting plants. These are rules of thumb. Your crop and growing conditions may call for different values.
What is the leaf temperature offset?
Leaves are often a degree or two cooler than the air because water evaporating from them cools them. If you measure leaf temperature with an infrared thermometer, enter how many degrees below air temperature it is. Leave it at 0 for the simple air-only VPD.
Is the answer in kPa or mbar?
The tool shows kPa, the unit most grow guides use. 1 kPa equals 10 mbar.