Vapor Pressure Deficit Calculator - Temperature and Humidity to VPD

The vapor pressure deficit calculator turns a single air temperature and relative humidity reading into three pressures: the saturation vapor pressure the air could hold, the actual vapor pressure it does hold, and the deficit between them. It uses the FAO-56 Magnus-Tetens equation so the result matches the tables printed in irrigation, greenhouse, and HVAC references, and it labels the result with a plant-stage band. Use it to plan greenhouse setpoints, set indoor humidity targets, or check a classroom psychrometrics problem against a known reference value.

• • Greenhouse setpoint planning: Set a target VPD band for seedling, vegetative, or flowering rooms and read the matching temperature and humidity pair.
• • Indoor air-quality checks: Translate a temperature and relative humidity reading into a vapor pressure deficit that an HVAC controller can act on.
• • Classroom psychrometrics: Work a saturation, actual, and deficit pressure problem set against the FAO-56 reference values without rebuilding the equation by hand.
• • Mold and condensation risk: Read the VPD of a wall, attic, or supply-air stream to judge whether surfaces are close to the dew point.

Vapor pressure deficit is the difference between how much water vapor the air could hold at its current temperature and how much it actually does hold. That gap drives evaporation from a leaf and the moisture exchange between indoor air and a cold window, which is why FAO-56 is cited.

If the reason you are reading a vapor pressure deficit is human thermal comfort, the Heat Index Calculator shows the same temperature and humidity pair as a perceived temperature.

The calculator takes air temperature in degrees Celsius and relative humidity in percent, runs the FAO-56 Magnus-Tetens equation to find saturation vapor pressure, multiplies that by relative humidity to get actual vapor pressure, and subtracts the two to get the deficit.

• T (air temperature): Air temperature in degrees Celsius, driving the saturation side through the Magnus-Tetens exponent.
• RH (relative humidity): Relative humidity as a percent, between 0 and 100. The ratio RH/100 scales the saturation value to the actual vapor pressure.
• SVP (saturation vapor pressure): Maximum water vapor pressure the air can hold at temperature T, in kPa, from the FAO-56 Annex 2 equation.
• AVP (actual vapor pressure): Water vapor pressure actually present, equal to SVP times RH/100.
• VPD (vapor pressure deficit): Difference between SVP and AVP in kPa, with a secondary mbar readout. The driving force for transpiration and evaporation.

Splitting the result into SVP, AVP, and the deficit makes it clear whether the SVP is moving because the temperature changed, or whether the deficit is moving because the humidity shifted, which is the difference between a heating and a humidification problem.

For a greenhouse at 25 degrees C and 60 percent RH: SVP = 3.17 kPa, AVP = 1.90 kPa, VPD = 1.27 kPa = 12.7 mbar, plant-stage band = Flowering. The 1.27 kPa deficit sits inside the 1.0 to 1.6 kPa flowering band, matching the late-vegetative to early-flowering range.

For a cold-storage room at 10 degrees C and 50 percent RH: SVP = 1.23 kPa, AVP = 0.61 kPa, VPD = 0.61 kPa = 6.1 mbar, plant-stage band = Seedling. Cold air holds far less water vapor than warm air, so the same 50 percent humidity produces a much smaller deficit at 10 degrees C, which is why cold frames and propagation rooms read the same numbers on a different temperature scale.

According to FAO Irrigation and Drainage Paper 56, the saturation vapor pressure of air is given by SVP = 0.6108 x exp(17.27 x T / (T + 237.3)) for T in degrees Celsius, with reference values 0.61, 1.23, 2.34, 3.17, and 4.24 kPa at 0, 10, 20, 25, and 30 degrees C.

Once you have the saturation vapor pressure in kPa, the Ideal Gas Calculator handles the related textbook problem of converting between partial pressure, moles, and volume.

Four ideas sit behind the formula: a temperature-only saturation pressure, a humidity-scaled actual pressure, a deficit that drives evaporation, and a plant-stage band.

A 25 degrees C room at 60 percent RH has a 1.27 kPa deficit, while a 30 degrees C room at the same humidity has a 1.70 kPa deficit because the saturation pressure is higher. The vapor pressure deficit calculator makes the same pattern visible at a glance.

For a closer look at the partial-pressure picture behind SVP, the Gas Laws Calculator covers the ideal gas law and Dalton's law.

Two numbers go in, three pressures and a plant-stage band come out. The default values reproduce the FAO-56 reference condition.

1. 1 Enter the air temperature: Type the dry-bulb air temperature in degrees Celsius. Use 0.1 degree precision if the sensor is calibrated that fine.
2. 2 Enter the relative humidity: Type the relative humidity as a percent. Values above 100 are clamped to 100, reproducing a fully saturated air sample.
3. 3 Read the saturation vapor pressure: The SVP row reports the maximum water vapor the air could hold at the chosen temperature, in kPa.
4. 4 Read the actual vapor pressure: The AVP row reports the water vapor actually present, equal to SVP times relative humidity as a fraction.
5. 5 Read the vapor pressure deficit: The VPD row reports SVP minus AVP in kPa, with the same value in mbar underneath.
6. 6 Check the plant-stage band: The label turns the VPD into a decision: seedling, vegetative, flowering, saturated, or above range.

For a propagation room, set 22 degrees C and 75 percent RH. SVP comes out at 2.64 kPa, AVP at 1.98 kPa, VPD at 0.66 kPa, and the band reads Seedling.

Once you have a target VPD band, the CO2 Grow Room Calculator covers the carbon dioxide side.

The calculator is most useful when you want the FAO-56 reference answer in seconds and a plain-language band that tells you what to do with it.

• • FAO-56 anchored: Uses the SVP = 0.6108 x exp(17.27 x T / (T + 237.3)) form from FAO-56 Annex 2.
• • Three pressures in one place: Reports saturation, actual, and deficit side by side.
• • kPa and mbar output: Matches the unit most greenhouse and HVAC references use in print.
• • Plant-stage band included: Labels the VPD so the number becomes a decision.
• • Input clamping: Clamps out-of-range humidity entries to 0 or 100 with a small validation message.

These benefits matter most when the calculation is repeated many times a day, as in a propagation room where the operator checks the sensor every hour. The calculator returns the same answer as a printed formula in a fraction of a second.

VPD is one of three indoor-cultivation knobs, and the Daily Light Integral (DLI) Calculator covers the light side.

Two numbers drive the result, and three contextual factors decide whether the resulting VPD is comfortable, productive, or out of range.

• • The Magnus-Tetens equation is accurate within about 0.4 percent for 0 to 50 degrees C. Outside that range the error grows.
• • The plant-stage bands are a teaching aid, not a horticultural prescription. A high-value crop should be tuned against a propagation chart.
• • The calculator does not pull live sensor data. A workflow that needs logging or alerting should pair the calculation with a data acquisition layer.

These caveats matter because the FAO-56 equation is a textbook fit, not a real-world measurement. A propagation tray on a cold concrete bench reads a higher local VPD than the room average, and a leaf transpiring in still air reads a lower local VPD than the same room.

According to Wikipedia: Vapour-pressure deficit, VPD drives transpiration in plants and evaporation in HVAC systems and is the difference between the moisture in the air and the maximum the air could hold at the same temperature.

For a closer look at the heat side of the same indoor air problem, the Heat Transfer Conduction Calculator covers the conductive heat flux through a wall or glazing, the other half of an indoor climate calculation.