The vapor pressure deficit calculator above computes the difference between the water vapour the air could hold at the leaf's temperature and the vapour it actually holds. That difference, in kilopascals, is the physical pull the atmosphere exerts on water inside a plant, and it is the reason growers increasingly track VPD rather than relative humidity alone.
Arb Digital publishes this as a physics tool. It computes a published equation from figures you supply and reports what named sources say about the values that occur in practice. It sets no target for any crop, because target ranges differ by species, growth stage and production system, and belong to the crop guidance you are already following.
What This VPD Calculator Does, and How It Differs From Our Other Humidity Tools
Arb Digital already publishes several tools in this territory, and the boundaries matter because they are easy to confuse.
Our vapour pressure calculator gives the saturation vapour pressure of a pure substance against temperature, by the Antoine equation or the Clausius-Clapeyron relation. It answers a chemistry question about a liquid. VPD is different: it is a difference between two vapour pressures in a growing environment, one evaluated at leaf temperature and one being the actual vapour pressure of the air.
Our psychrometric calculator returns the full moist-air state — enthalpy, humidity ratio, wet-bulb, dew point, specific volume — for HVAC and meteorological work. It does not know anything about a leaf, and leaf temperature is exactly what turns an air property into a plant-relevant one. The relative humidity calculator, absolute humidity calculator and dew point calculator each return one air property; this page returns the air-to-leaf gradient.
In one sentence: those tools describe the air, and this one describes the difference between the air and the leaf sitting in it.
How to Use It
- Enter air temperature and relative humidity measured together, at canopy level rather than at the vent or the door.
- Set the leaf temperature. Either enter a measured value from an infrared thermometer, or use an offset from air temperature if you have not measured it.
- Read the leaf VPD in the headline panel, and the air VPD in the grid, and note how far apart they are.
- Compare the two. A cooler leaf lowers the deficit; a leaf warmer than the air raises it sharply.
- Take any target range from your own crop guidance, not from a calculator.
The Formula and How It Is Calculated
The saturation vapour pressure of water at a temperature comes from the equation given as Equation 11 in Chapter 3 of the FAO Irrigation and Drainage Paper 56, Meteorological data:
e°(T) = 0.6108 × exp[17.27T ÷ (T + 237.3)], with T in °C and the result in kPa.
Actual vapour pressure is that saturation value at air temperature scaled by relative humidity:
ea = e°(Tair) × RH ÷ 100
The same FAO chapter defines the vapour pressure deficit as the difference between the saturation and actual vapour pressure, and describes it as an accurate indicator of the evaporative capacity of the air. So air VPD = e°(Tair) − ea.
The leaf version evaluates the saturation term at the leaf's own temperature instead, because the air inside the leaf is saturated at whatever temperature the leaf is: leaf VPD = e°(Tleaf) − ea.
Worked example, which is the default loaded above. At 25 °C the saturation pressure is 0.6108 × exp(431.75 ÷ 262.3) = 3.167 kPa, matching the FAO table value of 3.168. At 60 per cent RH the actual vapour pressure is 1.900 kPa, so the air VPD is 1.267 kPa. A leaf two degrees cooler, at 23 °C, has a saturation pressure of 2.809 kPa, giving a leaf VPD of 0.909 kPa. The dew point, from the FAO inverse relation, is 16.7 °C.
Why Leaf Temperature Changes the Answer So Much
The saturation vapour pressure curve is exponential, not linear. Between 20 and 30 °C it roughly doubles. That means a leaf a couple of degrees away from air temperature shifts the deficit by a substantial fraction, and the direction of that shift is easy to get backwards.
A leaf that is transpiring freely in moving air is evaporatively cooled and typically sits below air temperature, which reduces the deficit compared with the air-only figure. Our worked example shows this: 1.267 kPa of air deficit becomes 0.909 kPa at the leaf, a reduction of nearly thirty per cent from a two-degree difference.
The reverse happens under strong radiation in still air, where a leaf can run several degrees above air temperature. Then the leaf deficit is considerably higher than the air deficit, and a grower watching only air VPD will underestimate the stress the plant is experiencing. This is precisely the case where an infrared thermometer earns its cost, because the offset is not predictable from air measurements.
Why VPD Rather Than Relative Humidity
Relative humidity is a ratio, and a ratio hides the absolute quantity. Sixty per cent relative humidity is a very different atmosphere at 15 °C than at 30 °C, because the saturation value it is a percentage of has more than doubled. Managing to a fixed relative humidity therefore means managing to a moving evaporative demand.
VPD collapses temperature and humidity into a single number that is proportional to the driving force for evaporation, which is why it correlates better with transpiration than either input does alone. It is also directly comparable between different climates and different times of day.
The University of Arizona Cooperative Extension publication What the Air Is Telling You: Understanding Vapor Pressure Deficit describes VPD as the difference between the moisture the air can hold when fully saturated and the moisture actually present, and reports the seasonal range measured in Yuma agriculture: roughly 0.91 to 1.18 kPa in winter, 1.33 to 2.50 kPa through the spring transition, and 3.41 to 3.78 kPa in summer. Those are observed field values for one desert location, not targets, and they give a useful sense of the real spread the number covers.
Where the Number Stops Being Useful
VPD describes the atmospheric demand. It says nothing about whether the plant can meet that demand, which depends on root function, soil or substrate water, stomatal behaviour and the plant's own hydraulic capacity. Two plants in identical air can be under entirely different water stress.
Measurement location is the other practical limit. Air inside a dense canopy is often warmer and considerably more humid than air above it, so a sensor at head height and a leaf in the middle of the crop are describing different environments. If you are computing a leaf VPD, the humidity measurement should come from as close to that leaf as you can practically place it.
Finally, the equation itself is an empirical fit. The FAO form is accurate across the ordinary agricultural temperature range but it is a correlation, not a first-principles derivation, and small differences exist between the various published saturation equations. Those differences are well below the uncertainty in a typical greenhouse humidity sensor, so they rarely matter in practice — but it is worth knowing that two VPD calculators can disagree in the third decimal place for entirely legitimate reasons.
Arb Digital publishes hundreds of free, no-signup tools covering plant science, physics, biology, finance and everyday maths — no accounts, no stored data.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Treating air VPD and leaf VPD as the same number. A two-degree leaf offset moves the deficit by tens of per cent.
- Assuming leaves are always cooler than the air. Under strong radiation in still air they can be several degrees warmer, which raises the deficit rather than lowering it.
- Measuring humidity at the vent or the door rather than at canopy level, where the plants actually are.
- Managing to a fixed relative humidity, which means a moving evaporative demand as the temperature changes through the day.
- Reading VPD as a plant water status. It measures atmospheric demand, not whether the plant can supply it.
Related Free Tools From Arb Digital
For a single air property, use the relative humidity calculator, the absolute humidity calculator or the dew point calculator. For the complete moist-air state in HVAC terms, the psychrometric calculator, and for the saturation pressure of a pure liquid the vapour pressure calculator. In a controlled growing environment the grow room CO2 calculator covers enrichment arithmetic, and the water potential calculator handles the plant-water side. Browse the full free online tools hub for more.
Frequently Asked Questions
It is the difference between the saturation vapour pressure and the actual vapour pressure of the air, expressed in kilopascals. The FAO describes it as an accurate indicator of the evaporative capacity of the air.
Air VPD evaluates the saturation term at air temperature. Leaf VPD evaluates it at the leaf's own temperature, because the air spaces inside a leaf are saturated at whatever temperature that leaf is. The two differ whenever the leaf is not exactly at air temperature.
Find the saturation vapour pressure with the FAO equation, 0.6108 times the exponential of 17.27T divided by T plus 237.3. Multiply it by relative humidity over one hundred to get actual vapour pressure, then subtract that from the saturation value at leaf temperature.
Relative humidity is a ratio of a value that itself changes with temperature, so the same percentage means different evaporative demand at different temperatures. VPD combines both into one number proportional to the driving force for evaporation.
Both happen. A freely transpiring leaf in moving air is usually cooler through evaporative cooling. A leaf under strong radiation in still air can be several degrees warmer. An infrared thermometer is the only reliable way to know which applies.
No. That tool gives the saturation vapour pressure of a pure liquid against temperature using the Antoine equation or Clausius-Clapeyron. This one computes a deficit between two vapour pressures in a growing environment, with leaf temperature included.
This page does not set a target. Published ranges differ by crop, growth stage and production system, so take the figure from the crop guidance or extension publication you are already working to, and use this tool to compute where you currently are.
This calculator computes a published physical relationship for general and educational use. It does not recommend an environment for any crop and does not assess plant health. Target ranges and growing decisions should come from crop-specific guidance, your local extension service or an agronomist.
