What VPD actually measures

Vapour Pressure Deficit is the gap between how much moisture the air is holding and how much it could hold at that temperature, measured in kilopascals (kPa).

That gap is what pulls water out of your plants. A leaf transpires into the air around it, and the size of the deficit sets how fast that happens. Transpiration is not a side effect — it is the mechanism that moves water and dissolved nutrients up from the roots. Stall it and feeding stalls with it. Push it too hard and the plant closes its stomata to protect itself, which stops both water loss and the gas exchange it needs to grow.

This is why relative humidity alone misleads you. 60% humidity at 65 °F and 60% at 85 °F are completely different environments for a plant, because warm air holds far more moisture. The humidity reading is identical; the deficit is nearly double. VPD collapses temperature and humidity into the single number that describes what the plant is experiencing.

How this calculator works

Saturation vapour pressure is calculated with the Tetens equation as published in FAO Irrigation & Drainage Paper 56, the standard reference for crop water requirements:

e°(T) = 0.6108 × exp[ (17.27 × T) / (T + 237.3) ]
T in °C, result in kPa.

Actual vapour pressure = e°(Tair) × RH ÷ 100
Leaf VPD = e°(Tleaf) − actual vapour pressure

Note that leaf VPD uses saturation pressure at leaf temperature and subtracts the actual vapour pressure of the surrounding air. Calculators that use air temperature for both are computing air VPD, which is a different and less useful number. This tool shows both.

The leaf temperature assumption

Leaf VPD needs a leaf temperature, and most growers do not measure one. A transpiring leaf runs cooler than the air around it — evaporation carries heat away — though the gap varies with airflow, light intensity and how freely the plant is transpiring.

If you do not enter a leaf temperature, this tool assumes leaves are 2 °F (about 1.1 °C) below air temperature. That is a working assumption, not a measurement, and the result panel says so. An infrared thermometer pointed at a healthy leaf removes the guess, and under strong lighting the real gap can differ enough to matter.

Target ranges

The healthy band shown on the gauge — 0.45 to 1.15 kPa — comes from e-GRO, the university extension collaborative. Below 0.45 kPa they report increased disease infection potential and very soft leaf tissue. At or above 1.15 kPa they report rapid wilting, hard leaf tissue, deformed leaves and marginal edge burn.

Stage	Suggested target	Reasoning
Propagation	0.45 – 0.75 kPa	Cuttings and seedlings have little or no root system and cannot replace water quickly.
Growth	0.75 – 1.00 kPa	Established roots support steady transpiration and vigorous leaf development.
Fruiting	0.90 – 1.15 kPa	Higher transpiration moves calcium and other nutrients toward developing fruit and flowers.

These stage targets are general guidance, not a published standard. The 0.45–1.15 kPa envelope is sourced; how you divide it by stage varies by crop, cultivar and light intensity. Treat the bands as a starting point and let the plants correct you.

Common mistakes

• Chasing a number instead of watching the plants. VPD is a guide to the environment, not a diagnosis. Drooping, curling or crisping leaves tell you more than a decimal.
• Using air temperature as leaf temperature. It inflates your calculated VPD. A tool that never asks about leaf temperature is quietly doing this.
• Measuring in the wrong place. A sensor on the floor or against a wall is not reading what the canopy experiences. Put it at canopy height, out of direct light.
• Fixing humidity while ignoring temperature. They move VPD together. Dropping temperature raises humidity and lowers VPD at the same time.
• Overcorrecting. Change one variable, let the environment settle, then measure again. Chasing the number in real time produces swings, and swings stress plants more than a steady reading slightly outside the ideal band.

Questions

What is a good VPD?

For most greenhouse crops, 0.45 to 1.15 kPa. Within that, aim lower for seedlings and cuttings and higher for established, fruiting plants. Outside it in either direction you start trading growth for either disease risk or water stress.

Is VPD better than relative humidity?

For deciding what to change, yes. Humidity alone does not describe what a plant experiences, because its meaning shifts with temperature. VPD accounts for both. You still need a humidity reading — it is an input to the calculation.

Why is leaf temperature lower than air temperature?

Evaporation cools. As water evaporates from the leaf surface it carries heat away, the same reason sweating cools you. A plant that has stopped transpiring loses that cooling, and leaf temperature rises toward air temperature or above it under strong light.

Do I need an infrared thermometer?

Not to get started. The 2 °F assumption is reasonable for a healthy, well-ventilated plant. It becomes worth measuring when you are running intense lighting, when airflow is poor, or when your calculated VPD looks fine but the plants disagree.

My VPD is perfect but my plants look unhappy. Why?

VPD only describes the air. Root zone problems, nutrient issues, pH drift, light stress, pests and disease all produce symptoms that a good VPD reading will not prevent or explain. Rule environment in, then look elsewhere.

How to read these numbers. The VPD figure is calculated from your inputs using a published formula — it is arithmetic, and it is exact. The healthy range is cited to horticultural extension research. The stage-by-stage targets and the suggested actions are general guidance that varies by crop and setup. Nothing here replaces watching your own plants.

Sources. Saturation vapour pressure: Allen et al., Crop Evapotranspiration, FAO Irrigation and Drainage Paper 56, equation 11. Healthy VPD range and the symptoms above and below it: e-GRO Alert 7.24, Vapor Pressure Deficit? Why Should I Track That? Last reviewed 27 August 2026.