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Enter your values, then click Calculate result.How this calculator helps
This VPD calculator compares actual water-vapor pressure with saturation pressure at a supplied temperature. Air VPD uses the air temperature; leaf VPD uses a separately measured leaf temperature. The RH calculator mode works in the other direction: it estimates relative humidity from air temperature and dew point. These are related physical quantities, so one canonical tool covers them with clear modes rather than creating competing pages. The result describes your measurements and the stated equation. It does not select a humidity target for a crop, prescribe ventilation settings or assess whether a room is healthy.
How to use it
- 1
Choose Air VPD, Leaf VPD, or RH from dew point before interpreting the second field.
- 2
Enter measured temperatures in Celsius. The approximation is restricted to 0–50 °C liquid-water conditions.
- 3
For leaf mode, supply a separate leaf temperature; for RH mode, enter a dew point no higher than air temperature.
- 4
Click Calculate result. Editing measurements keeps the previous submitted output until the next click.
Formula and methodology
es(T) = 0.6108 × exp(17.27T/(T+237.3)) kPa. Actual pressure = es(air) × RH/100 or es(dew point). VPD = es(reference temperature) − actual pressure. RH = 100 × actual/es(air).
The calculator applies the displayed arithmetic to the values entered on this device. It does not silently load a local tax rate, currency conversion or commercial assumption.
Worked calculation example
At 25 °C and 60% RH, saturation pressure is about 3.1678 kPa, actual pressure is 1.9007 kPa, and air VPD is 1.2671 kPa. A 24 °C leaf has a smaller deficit under the same air conditions.
How to interpret your result
Liquid-water approximation over 0–50 °C; simultaneous user measurements only. No crop target, ice formula or equipment sizing. Inputs are processed locally; no weather feed is loaded.
For different inputs or formulas, use BTU Calculator; Epoxy Resin Volume Calculator; Soil and Loam Calculator.
Related questions this calculator covers
- vpd calculator
- rh calculator
Scenario comparison
| Scenario | What it shows |
|---|---|
| 25 °C and 60% RH: air VPD about 1.2671 kPa. | |
| Equal air and leaf temperatures: the two deficits agree. | |
| Dew point equals air temperature: calculated RH is 100%. |
Common mistakes to avoid
- Entering Fahrenheit as Celsius.
- Confusing dew-point temperature with a humidity percentage.
- Treating a physical deficit as a universal plant target.
Check the equal-temperature case, confirm es(air) minus actual pressure equals air VPD, and compare RH from a known dew point with an independent meteorological reference using the same equation.
Authoritative reference. FAO Irrigation and Drainage Paper 56, meteorological data: saturation and actual vapor-pressure relationships.What can affect the result?
Three modes, one vapor-pressure balance
Relative humidity is a percentage of saturation pressure at the current air temperature, while VPD is a pressure difference. They cannot be substituted as if their units were identical. At a fixed air temperature, increasing RH lowers air VPD. At a fixed RH percentage, raising air temperature changes both saturation and actual vapor pressure. Leaf mode retains the surrounding air actual pressure but changes the saturation reference to the leaf temperature. This makes a measured leaf temperature useful without pretending that leaves always equal the air sensor.
Use measurements from the same place and time
Pair air temperature and humidity from a consistent observation, preferably close to the area you are evaluating. Combining yesterday’s humidity with today’s temperature produces an arithmetic answer for a synthetic state rather than the measured environment. A leaf sensor or infrared reading has its own placement and calibration limits. Label the crop location, observation time, instrument and measurement basis in your records. The calculator cannot correct a sensor offset, locate a poorly positioned probe or average fluctuating conditions automatically.
Interpret negative leaf deficits honestly
Air VPD is non-negative for the accepted RH range. Leaf VPD can be negative when the supplied leaf temperature has a saturation pressure below the air actual vapor pressure. The tool preserves that sign instead of silently changing the answer to zero. Such inputs indicate a condensation-related condition in this simplified balance, rather than a negative evaporative demand to be used as a control setting. Check the temperature and humidity observations before deciding what the result means for a particular plant, surface or room.
Celsius, pressure units and formula scope
All temperature fields take degrees Celsius, not Fahrenheit. Convert Fahrenheit by subtracting 32 and multiplying by five ninths before entry. Pressure outputs are kilopascals, so multiplying by one thousand gives pascals. The displayed decimal places are calculation precision, not instrument accuracy. This page uses the published FAO saturation-pressure approximation over liquid water and excludes subzero ice conditions. It does not calculate wet-bulb temperature, enthalpy, HVAC equipment capacity, evapotranspiration or the full Penman–Monteith model.
Compare scenarios without creating a universal target
Run separate measured cases to see how the deficit changes, and keep the mode and units with each saved result. Change one observation at a time if you want to understand the equation’s sensitivity. A warmer leaf and cooler leaf can have different deficits under the same surrounding air. Species, growth stage, radiation, air movement and water availability are outside these inputs. A crop-specific target requires an appropriate horticultural reference; the calculator deliberately supplies neither a universal optimal band nor automatic heater, fan or irrigation instructions.
Privacy and browser processing
Values entered on this page are processed in the current browser session. SolvePilot does not require an account and does not receive the values entered into the calculator. Refreshing or closing the page clears the working values unless the browser itself restores a previous session. Avoid entering identifying or account information because the calculation needs summary values only.
Accuracy and verification
Accuracy depends first on input quality. Confirm definitions, scales, dates and source information before entering a value. Keep an independent record of any result used for planning because this page does not create an official statement or retain a calculation history.
Limits of this estimate
Liquid-water approximation over 0–50 °C; simultaneous user measurements only. No crop target, ice formula or equipment sizing. Inputs are processed locally; no weather feed is loaded.
Sources and review information
Frequently asked questions
What does RH calculator mean here?＋
RH means relative humidity of air. This page estimates it from air temperature and dew point; it does not handle Rh blood groups or a medical test. Choose the dew-point mode so the second input is interpreted as Celsius rather than percent.
Can dew point exceed air temperature?＋
Not in the unsupersaturated liquid-water state supported here. The input check rejects that combination. Recheck the measurement times, units and calibration instead of accepting a percentage above one hundred as ordinary humidity.
Is leaf VPD the same as air VPD?＋
Only when the leaf and air temperatures are equal in this model. Actual surrounding vapor pressure is shared, but the saturation reference changes. A leaf temperature is therefore a separate measured input, not a fixed offset assumed by the calculator.
Can I use Fahrenheit or subzero measurements?＋
Convert Fahrenheit to Celsius first. Inputs below zero Celsius are outside this page’s declared liquid-water range. An ice-saturation calculation uses a different treatment and is not silently approximated by this tool.
Does the result recommend a humidity setting?＋
No. The outputs describe a pressure balance using the entered observations. They are not crop-specific recommendations, a mould assessment, an indoor-health determination or a control-system design. Use the appropriate specialist methodology for those decisions.