Why relative humidity is the wrong unit
Relative humidity is a ratio: the water vapour actually present divided by the most the air could hold at that temperature. The denominator changes with temperature, and it changes steeply, so the same physical quantity of water in the air reads as different humidity numbers as the room warms and cools. Air at 25 degrees Celsius and 60 percent contains the same water as air at 30 degrees and about 45 percent. Nothing was added or removed; only the reference moved.
Vapour pressure deficit removes the moving reference. It is the difference between how much vapour pressure the air could support at a given temperature and how much it actually has, expressed in kilopascals. That difference is the physical driving force pulling water out of a wet surface, which is exactly what a leaf is. Twice the deficit, roughly twice the pull.
The formula, and the coefficients used here
Saturation vapour pressure is calculated here with the Magnus form, using the widely quoted Tetens coefficients for water above freezing:
es(T) = 0.61078 × exp(17.27 × T ÷ (T + 237.3)), with T in degrees Celsius and the result in kilopascals.
At 25 degrees this gives 3.168 kPa. Other coefficient sets exist and give answers differing in the third decimal place, which is far below the accuracy of any sensor you are likely to be holding, but it is worth stating which set is in use since published tables vary slightly. The actual vapour pressure is that saturation figure multiplied by the relative humidity, and it is the number that stays put when temperature changes.
Air VPD is then saturation at air temperature minus actual vapour pressure. Leaf VPD is saturation at leaf temperature minus that same actual vapour pressure. The air term is identical in both; only the saturation term moves.
Why the leaf temperature is the point
The air inside a leaf, in the spaces behind the stomata, is effectively saturated at whatever temperature the leaf is. Water moves out of the leaf down the gradient between that saturated interior and the drier air outside. So the deficit that actually drives transpiration is set by leaf temperature on one side and air vapour pressure on the other, and the air temperature only enters through its effect on how much vapour the air is carrying.
A transpiring leaf with air moving over it is cooler than the surrounding air, because evaporation removes heat. Offsets of one to three degrees Celsius are ordinary. That cooler leaf has a lower saturation pressure, so the gradient is smaller than the air figure suggests. Work the numbers at 25 degrees and 60 percent humidity: air VPD is 1.27 kPa, and with a leaf sitting 2 degrees cooler the leaf VPD is 0.91 kPa. Nearly thirty percent lower. A controller working on air humidity alone cannot see that gap.
The offset can also run the other way. A leaf close to a fixture, in still air, absorbing radiant heat and unable to shed it, can sit above air temperature. Then leaf VPD is higher than air VPD and the plant is under more evaporative demand than the room readings suggest. Both cases are why an infrared thermometer aimed at an actual leaf is worth more than any amount of arithmetic, and why this calculator takes a measured value if you have one.
Dew point, condensation and the hard limit
Dew point is the temperature at which the air currently in the room would be saturated. Any surface below it collects water: glass, walls, the cold side of an enclosure, and leaves. The calculation is a rearrangement of the same Magnus formula and is included here because it sets a boundary that no target can be pushed past. When leaf temperature falls to the dew point, free water forms on the foliage, and that is the condition that fungal problems need. A VPD number chased into condensation has stopped being useful.
Absolute humidity, in grams of water per cubic metre, is shown for the same reason: it is what you are actually adding or removing when you run a humidifier or a dehumidifier, and unlike relative humidity it does not change when the room temperature does. If you are trying to work out how much water a piece of equipment has to shift, it is the honest unit.
What this page will not tell you
It will not suggest a target. Appropriate VPD depends on the species, the cultivar, the stage of growth, the light intensity and how readily the root zone can supply water to replace what leaves. A value that suits a mature plant under strong light and a well-supplied root zone can dehydrate a cutting with no roots, and different species have entirely different tolerances. If you have a target from a reference for your crop or from your own records, enter it and the calculator will tell you what humidity or temperature would reach it.
It also assumes a single leaf temperature, and a canopy has many. Leaves at the top under the fixture and leaves in the shade below are at different temperatures and therefore at different VPDs at the same moment. Measure at the part of the canopy you care about most.
Related pages
Transpiration rate depends on light as much as on VPD, and the light side is handled by the grow light DLI calculator. The water that transpiration removes has to leave the space, which is the grow tent ventilation calculator, and the concentration effect it has on a reservoir is the reservoir top-up and drift calculator. For sizing equipment to move humidity in a whole room, see the dehumidifier sizing calculator and the humidifier sizing calculator, both of which work in the pints or gallons per day that this page expresses as grams per cubic metre.
Questions people ask
What VPD should I be running?
Not a question with a single answer, and this page deliberately does not offer one. The appropriate range depends on species, cultivar, growth stage, light intensity and how well the roots can supply water. A rooted plant under strong light can handle a demand that would desiccate an unrooted cutting, and different species evolved under very different atmospheres. What is genuinely universal is the mechanism: higher deficit means faster water loss from the leaf, which is helpful up to the point where the roots cannot keep up, and unhelpful past it. Take a target from a reference for your crop or from your own records, enter it, and the calculator will show what humidity or temperature reaches it.
Why does leaf VPD differ so much from air VPD?
Because the saturation pressure inside the leaf is set by the leaf temperature, not the air temperature, and saturation pressure changes steeply with temperature. At 25 degrees Celsius and 60 percent humidity the air VPD is 1.27 kPa. Drop the leaf two degrees below the air, which is ordinary for a transpiring leaf with airflow over it, and leaf VPD falls to 0.91 kPa. That is a difference of nearly thirty percent produced by two degrees. It is the single largest source of error in humidity control, because almost every controller and every published chart works on air readings alone.
How do I measure leaf temperature?
An infrared thermometer aimed at the upper surface of a healthy leaf, from close range so that the spot it reads is entirely leaf rather than partly the surface behind it. Beam divergence matters: most handheld units read a spot that widens with distance, so a reading taken from across the room is an average of leaf, pot and floor. Take several readings across the canopy, because leaves at the top under the fixture and leaves in shade differ substantially. If you have no infrared thermometer, an offset of one to three degrees below air is the usual range for a transpiring leaf in moving air, and zero offset is the one value that is almost certainly wrong.
Which saturation vapour pressure formula does this use?
The Magnus form with the Tetens coefficients: es equals 0.61078 times the exponential of 17.27 T divided by T plus 237.3, with T in Celsius and the result in kilopascals. At 25 degrees that yields 3.168 kPa. Several other coefficient sets are in circulation, including the Buck equations, and they differ from this one in the third decimal place over the temperature range anyone grows in. That difference is far smaller than the error in any affordable humidity sensor, so the choice does not matter practically, but it is worth stating which is in use because published VPD charts occasionally disagree slightly for exactly this reason.
Can I hit a VPD target by changing temperature instead of humidity?
Yes, and often more easily, which is why the calculator shows both routes. Raising air temperature at constant relative humidity raises both saturation pressures, and because the leaf follows the air with an offset, the deficit widens. Cooling narrows it. The catch is that temperature affects far more than VPD: growth rate, respiration, the root zone, the dissolved oxygen in a reservoir and the capability of your ventilation are all in play. Humidity is generally the more surgical lever because it moves fewer things at once. Where temperature is set by what your ventilation can achieve rather than by choice, humidity is the only lever you have.