VPD Calculator (Vapor Pressure Deficit)
Work out the vapor pressure deficit at the leaf surface from air temperature, relative humidity and how much cooler your canopy runs, then check it against the target kPa zone for your growth stage.
🌱 What is Vapor Pressure Deficit?
Vapor pressure deficit, almost always shortened to VPD, is the gap between how much water vapour the air could hold if it were fully saturated and how much it is actually holding. That gap is expressed as a pressure in kilopascals, and it is the single number that describes how hard the atmosphere is pulling moisture out of a leaf. Because transpiration is driven by that pressure difference and not by humidity in isolation, VPD predicts plant water demand, nutrient uptake and stomatal behaviour far more reliably than a relative humidity reading on its own.
Indoor growers use VPD to set temperature and humidity targets for each growth stage, since propagation, vegetative growth and flowering each want a different deficit. Greenhouse managers use it to decide when to vent, when to run a fogger and when a screen is doing more harm than good. Vertical farms and controlled environment agriculture facilities log it continuously, because a room that drifts to a very low deficit is a room quietly incubating powdery mildew and botrytis long before any visible symptom appears.
The distinction that matters most, and the one most often skipped, is between air VPD and leaf VPD. Air VPD uses air temperature on both sides of the subtraction and describes only the air. Leaf VPD uses the temperature of the leaf itself for the saturation term, because water evaporating from a stomate leaves at leaf temperature. An actively transpiring canopy usually runs one to three degrees cooler than the surrounding air, and that offset can move the reading by 0.3 kPa or more, which is easily the difference between the vegetative band and the propagation band. A second common misconception is that more deficit means more growth. Past roughly 1.6 kPa most species defend themselves by closing stomata, which shuts down carbon dioxide uptake, so photosynthesis falls even as water loss rises.
This calculator takes air temperature, relative humidity and the offset between leaf and air temperature, then reports leaf VPD alongside air VPD so the effect of that offset is visible rather than hidden. It also shows the saturation vapour pressure at the leaf, the actual vapour pressure of the air, and the dew point, which is a separate practical safety check because any surface colder than the dew point will condense regardless of how healthy the VPD number looks. The chart traces leaf VPD across the entire humidity range at your current temperature so you can see exactly how much humidity has to move to reach a target. If you need the underlying vapour pressure of water at a given temperature rather than the deficit, the vapor pressure of water calculator covers that directly.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Vegetative Room at 25 °C
Air 25 °C, RH 60 %, canopy running 2 °C cooler than the air
Example 2 — Propagation Dome for Clones
Air 24 °C, RH 80 %, unrooted cuttings barely transpiring so only 1 °C cooler
Example 3 — Flowering Room Running Warm and Dry
Air 27 °C, RH 50 %, strong transpiration holding the canopy 2 °C below air
Example 4 — Stressed Canopy With Closed Stomata
Air 30 °C, RH 35 %, water stressed plants no longer cooling themselves, so zero offset
❓ Frequently Asked Questions
🔗 Related Calculators
What is vapor pressure deficit (VPD)?
Vapor pressure deficit is the difference between the amount of moisture the air could hold when saturated and the amount it currently holds, expressed in kilopascals. It measures the drying power of the air, which is what drives transpiration, so it predicts plant water demand far better than relative humidity alone.
What is the formula for VPD?
Leaf VPD equals the saturation vapour pressure at leaf temperature minus the actual vapour pressure of the air. Actual vapour pressure is the saturation vapour pressure at air temperature multiplied by relative humidity divided by 100. Saturation vapour pressure comes from the Tetens equation, 0.61078 times e raised to (17.27T divided by (T + 237.3)).
What is the difference between leaf VPD and air VPD?
Air VPD uses air temperature for both terms, so it describes the air alone. Leaf VPD uses leaf temperature for the saturation term, because the water evaporating from a leaf leaves at leaf temperature, not air temperature. Since transpiring canopies run cooler than the air, leaf VPD is usually lower than air VPD.
What is a good VPD for plants?
Roughly 0.4 to 0.8 kPa for seedlings, clones and propagation, 0.8 to 1.2 kPa for vegetative growth, and 1.2 to 1.6 kPa for flowering and fruiting. Below 0.4 kPa transpiration stalls and disease risk rises. Above 1.6 kPa many plants close stomata and growth slows despite the higher water loss.
How much cooler is a leaf than the surrounding air?
A healthy, well watered, actively transpiring canopy typically sits 1 to 3 degrees Celsius below air temperature, because evaporation carries heat away. Under very intense lighting, poor airflow, or when the plant is water stressed and has closed its stomata, leaves can instead run warmer than the air.
Why is VPD better than relative humidity?
Relative humidity is relative to temperature, so 60 percent humidity means very different things at 18 °C and at 30 °C. VPD collapses temperature and humidity into a single absolute number in kPa that directly reflects how hard the air is pulling water out of the leaf, which is what the plant responds to.
What units is VPD measured in?
Kilopascals (kPa) are standard in horticulture, and typical growing values fall between 0.4 and 1.6 kPa. Some references use hectopascals or millibars, where 1 kPa equals 10 hPa, and older agronomy literature sometimes uses millimetres of mercury, where 1 kPa is about 7.5 mmHg.
How do I lower VPD in a grow room?
Raise humidity, lower temperature, or both. Adding humidity is the direct route with a humidifier or by increasing evaporative surface. Lowering air temperature reduces saturation vapour pressure sharply, since it rises exponentially with temperature, so a two degree drop often moves VPD more than a five percent humidity change.
How do I raise VPD if it is too low?
Dehumidify, increase air temperature, or improve airflow across the canopy. Increasing airflow does not change the bulk room VPD but it thins the boundary layer of saturated air sitting on each leaf, which restores effective transpiration even when the room reading looks marginal.
What is the dew point and why does this calculator show it?
Dew point is the temperature at which the current air would become saturated and start condensing. It is a practical safety check: any surface colder than the dew point, such as a window, a wall or a chilled duct, will grow condensation, which is a direct route to mould regardless of how good your VPD reading is.
Does this calculator work outdoors and in greenhouses?
Yes. The physics is identical, only the leaf temperature offset differs. Greenhouse and field canopies in direct sun often run warmer than air rather than cooler, so use a positive offset there, and measure with an infrared thermometer rather than assuming the indoor negative offset applies.
Is VPD the same as vapor pressure?
No. Vapor pressure is the partial pressure that water vapour exerts in the air, and saturation vapour pressure is the maximum it can reach at a given temperature. VPD is the gap between those two. For pure vapour pressure over a liquid at a given temperature, use the vapor pressure calculator or the vapor pressure of water calculator instead.