Vapor Pressure Deficit Calculator
Understand your greenhouse climate. Balance air, leaves & humidity.
Climate Inputs
Climate Summary
Based on 25 °C air temperature and 60% relative humidity.
Humidity Planning
Leaf VPD is below the same 1.1–1.5 kPa band.
What if?
+10 percentage points from current RH.
Leaf vs Air VPD Comparison
Common kPa scale, not a percentage score. Negative leaf VPD can indicate condensation.
Interactive VPD Climate Map
Hover or focus to inspect a sampled cell; click or press Enter to apply its air temperature and RH. Leaf assumptions remain as entered.
Calculation & Worked Example
FAO-56 saturation-vapor-pressure approximation over water. Below freezing, saturation over ice differs. The leaf comparison assumes saturated vapor at the leaf surface and the same ambient actual vapor pressure.
Frequently Asked Questions
What is vapor pressure deficit (VPD)?
VPD is saturation vapor pressure minus actual vapor pressure, expressed here in kPa. Air VPD uses air temperature for saturation. Leaf VPD uses leaf temperature for saturation but the same ambient actual vapor pressure.
How do I choose target VPD ranges?
The seedling, vegetative and flowering ranges are editable example bands, not universal recommendations or FAO/ASABE-endorsed targets. Use guidance specific to your crop, cultivar, light, airflow, irrigation and environment.
Should I use air VPD or leaf VPD?
Air VPD describes the ambient air. Leaf VPD estimates a leaf-to-air vapor-pressure difference. Choose the basis used by your crop guidance. An assumed leaf offset is not a measurement.
Can VPD alone predict disease or yield?
No. Leaf wetness, exposure duration, pathogens, light, airflow, water supply and plant physiology also matter. Status labels compare your inputs with the selected target, not a guarantee of crop safety or yield.
More questions (8)
What formula and units are used?
eₛ(T) = 0.6108 × exp(17.27T/(T + 237.3)), with T in °C and pressure in kPa. eₐ = eₛ(Tair) × RH/100. Air VPD = eₛ(Tair) − eₐ; leaf VPD = eₛ(Tleaf) − eₐ.
Does Fahrenheit conversion preserve the result?
Yes. Absolute temperatures convert with the 32-degree shift. Temperature offsets convert by the 9/5 scale factor only. Pressure calculations use Celsius internally.
What if leaf temperature is unknown?
Set an offset of zero to assume leaf and air temperature are equal. A −2 °C default here is an editable example, not a claim that all leaves are cooler than air.
Why can leaf VPD be negative?
A cooler leaf can have a saturation vapor pressure lower than ambient actual vapor pressure. The calculated difference becomes negative and suggests possible condensation. This does not mean negative absolute physical pressure.
What do the humidity-planning results mean?
At fixed entered temperatures, the tool solves RH = 100 × [eₛ(Tbasis) − target VPD]/eₛ(Tair). It intersects the target band with physically possible RH from 0 to 100%. Some target bands are not fully reachable at those temperatures.
Is a 10% humidity change the same as 10 percentage points?
No. Moving from 60% to 70% RH is an increase of 10 percentage points, or about 16.7% relative to the initial humidity. The what-if panel reports percentage-point changes.
Does the heatmap change my inputs?
Hover or keyboard focus only inspects. Clicking a cell or pressing Enter/Space applies that cell's air temperature and RH. Its leaf temperature follows the current offset or fixed measured setting. The marker hides when the current climate is outside the displayed map.
Is this the complete FAO-56 evapotranspiration model?
No. This uses the instantaneous saturation-pressure relationship. Daily FAO-56 reference evapotranspiration requires additional meteorological inputs and appropriate temperature/humidity averaging. Below-freezing values use the water approximation, not an ice-saturation model.
Sources & References
Food and Agriculture Organization (FAO)FAO-56 · Meteorological data and vapor-pressure equations
ASABE · Controlled-Environment EngineeringANSI/ASABE/ASHRAE EP653 · Indoor plant environment HVAC
Educational climate estimate · Example targets are not institutional standards · No endorsement
Compare instantaneous air and leaf VPD, explore an interactive temperature-humidity map and solve a user-selected target humidity range.