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VPD Calculator

Free tool

Wet Bulb Calculator

This wet bulb calculator turns a psychrometer's dry and wet bulb readings into relative humidity, dew point and VPD. It also works the other way, giving the wet bulb for any indoor air.

I know
Temperature
°C

°C

More options

Psychrometer

Two thermometers side by side, one with a wet wick. A sling or fan model moves air past the wick fast; a plain wall model relies on the room's own air.

Growth stage (for the VPD colour)
m

Above sea level. It changes the air pressure the formula uses.

Relative humidity

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Dew point
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Air VPD
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Leaf VPD (leaf 2 °C cooler)
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Absolute humidity
—
Humidity ratio

Humidity ratio

Grams of water vapour carried by each kilogram of dry air. Unlike relative humidity, it does not change when the air warms or cools.

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Air pressure
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Check your hygrometer

Hang your digital hygrometer next to the psychrometer for 15 minutes, then read both at the same moment.

%

Use this humidity in the VPD calculator.

Next: See your VPD and what to change for your growth stage

How it works

The wet bulb is cooled by evaporation, so the gap between the two thermometers shows how dry the air is. FAO-56 turns that gap into the actual vapour pressure, using the air pressure at your altitude and a coefficient for how the wick is ventilated. Humidity, dew point and VPD follow from there.

Formulas (FAO-56, equations 7, 15, 16)
P  = 101.3 × ((293 − 0.0065·z) / 293)^5.26   kPa
e  = SVP(T_wet) − a_psy · P · (T_dry − T_wet)
RH = 100 × e / SVP(T_dry)
a_psy = 0.000662 (ventilated), 0.0008, 0.0012 (indoors)

What a wet bulb calculator tells you

A psychrometer has two thermometers. The dry bulb reads the air. The wet bulb sits in a damp wick and cools as the water evaporates. Dry air takes water from the wick quickly, so the gap between the two readings grows. Humid air takes very little, so the gap shrinks. When both read the same, the air is saturated.

That gap is all you need to work out the humidity, with no electronic sensor to drift. For a grower, that makes a psychrometer a useful second opinion on a cheap digital hygrometer. For an HVAC technician, the reverse mode gives the indoor wet bulb from a thermometer and a humidity reading.

Dry bulb and wet bulb: a worked example

Say your sling psychrometer reads 26 °C (78.8 °F) dry and 20.5 °C (68.9 °F) wet, at sea level. With the ventilated setting the calculator gives:

  • Relative humidity: 61%
  • Dew point: 17.8 °C (64.1 °F)
  • Air VPD: 1.32 kPa
  • Leaf VPD, with leaves 2 °C cooler: 0.94 kPa, inside the 0.80–1.20 kPa vegetative band

Now take the same two readings from a plain wall psychrometer with no fan, and choose Not ventilated, indoors. The answer drops to 52% humidity and a 15.3 °C (59.6 °F) dew point. Leaf VPD rises to 1.24 kPa, just above the vegetative band. A slow-moving wick cools less, so the same gap means drier air. Picking the wrong instrument type moves the result by 9 points.

Altitude matters less, but it still counts. At 1,800 m (about 5,900 ft) the air pressure is 81.8 kPa, and the same ventilated readings give 63%.

Indoor wet bulb from temperature and humidity

Choose Temperature and humidity to go the other way. At 24 °C (75.2 °F) and 50% humidity, a ventilated wet bulb would read about 17.1 °C (62.8 °F), and the dew point is 12.9 °C (55.3 °F).

That order never changes: the wet bulb is always at or above the dew point and at or below the air temperature. All three meet only at 100% humidity. If your wet bulb reads below the dew point you trust, or above the air, one of the readings is wrong.

Common psychrometer mistakes

  • Reading too soon. The wick needs time to settle. For the hygrometer check, hang both instruments side by side for 15 minutes and read them at the same moment.
  • The wrong ventilation setting. As the example shows, a fan or sling model and a still wall model give different answers from the same numbers.
  • Swapping the readings. The wet bulb can never be warmer than the dry bulb. The calculator stops you if it is.
  • A dry wick. If the wick dries out, the wet bulb creeps up to the air temperature and the humidity looks far too high.
  • A table with no stated conditions. A wet bulb converts to humidity differently with air pressure and air speed. If a printed table does not say which it was made for, trust it less.

What to do with the result

Use the hygrometer check first. If your sensor reads a few points off, correct its readings or calibrate it. Then take the humidity to the VPD calculator to see what to change, or to the leaf VPD calculator if you measure leaf temperature. Compare it with the targets on the VPD chart. For condensation on walls or leaves at lights-off, the dew point calculator shows how close each surface is. If the air is too damp, size a unit with the dehumidifier size calculator.

FAQ

Questions growers ask

How do you calculate wet bulb?

With the FAO-56 psychrometer equation: vapour pressure equals the saturation pressure at the wet bulb, minus a coefficient times the air pressure times the gap between the dry and wet bulb. Dividing that by the saturation pressure at the dry bulb gives the humidity. To find the wet bulb from humidity, the calculator solves the same equation backwards.

What is the wet bulb rule?

The rule behind this calculator is that the wet bulb always sits between the dew point and the air temperature. The smaller the gap between wet and dry bulb, the more humid the air; when they match, it is at 100%. Weather forecasting and heat-safety rules use the term too, and this page does not cover those.

How much is 95% humidity?

95% means the air holds 95% of the water it could hold at that temperature. At 25 °C (77 °F) that leaves an air VPD of only 0.16 kPa, and a leaf 2 °C cooler sits below the dew point, so water condenses on it. In a grow room that is high mould risk, which starts at 85%.

Is wet bulb the same as dew point?

No. The wet bulb is always at or above the dew point, and at or below the air temperature. At 24 °C and 50% humidity the ventilated wet bulb is about 17.1 °C, while the dew point is 12.9 °C. The two only meet at 100% humidity, when the air, wet bulb and dew point all read the same.

How do I check my hygrometer?

Hang it beside a psychrometer for 15 minutes, read both at the same moment, and type the hygrometer's figure into the check box. If the psychrometer gives 61% and your sensor shows 65%, it reads 4 points high. Subtract 4 from its readings, or calibrate it if it has a setting for that.

What is the indoor wet bulb temperature?

It is the temperature a wet, ventilated wick would reach in the room's air, and you can work it out from a thermometer and a hygrometer. Choose Temperature and humidity and enter both: at 26 °C (78.8 °F) and 60% the ventilated wet bulb is about 20.4 °C (68.7 °F).

Why do my wet and dry bulb read the same?

Equal readings mean 100% humidity: no water can evaporate from the wick, so it does not cool. If the room is clearly not that damp, the wick has probably dried out, lost contact with its water, or has no air moving past it. Wet the wick, fan or swing it, and read again.

What is a wet-bulb reading?

The temperature of a thermometer wrapped in a wet wick with air moving past it. Water evaporating from the wick cools it, and the drier the air, the more it cools. Comparing it with an ordinary (dry-bulb) thermometer gives the humidity, with no electronic sensor needed.

Sources

The formulas and target ranges on this page come from these references.

  1. Allen, Pereira, Raes & Smith (1998). Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, chapter 3, equations 11 and 19. www.fao.org/4/x0490e/x0490e07.htm
  2. American Meteorological Society, Glossary of Meteorology: gas constant (water vapour, about 461 J kg⁻¹ K⁻¹). glossary.ametsoc.org/wiki/Gas_constant
  3. American Meteorological Society, Glossary of Meteorology: mixing ratio, w = εe / (p − e) with ε ≈ 0.622. glossary.ametsoc.org/wiki/Mixing_ratio
  4. Corredor-Perilla, Kwon & Park (2025). Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis. Frontiers in Plant Science (leaf VPD equation 3, and the cannabis VPD ranges it reports from earlier work). doi.org/10.3389/fpls.2025.1678142
  5. Pulse Grow (2020): VPD guide for cannabis growers. pulsegrow.com/blogs/learn/vpd
  6. Nelson & Bugbee (2015). Analysis of environmental effects on leaf temperature under sunlight, HPS, metal halide and LED. PLoS ONE 10(10), e0138930. doi.org/10.1371/journal.pone.0138930

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