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VPD Calculator

This VPD calculator turns your grow room’s temperature and humidity into leaf VPD for your growth stage. It tells you what to change, and the chart updates as you type.

Live VPD chart

Leaf VPD with the leaf 2 °C cooler than the air, coloured for your stage. Tap a cell to use it, or drag across it.

Printable VPD Chart

Too low
Slightly low
Ideal
Slightly high
Too high

Keyboard: focus the chart and use the arrow keys to move the point (Shift for bigger steps). Prefer numbers? The printable chart lists every value as a table.

Reading log

Readings you save stay in this browser on this device only. Nothing is uploaded. Export a CSV to keep a copy or move it to another device.

No readings yet. Press Save this reading under your result to start a log.

How it works

The formula, with your numbers

Saturation vapour pressure (SVP) uses the Tetens equation. Leaf VPD compares the vapour pressure inside the leaf, which is saturated at leaf temperature, with the vapour pressure of the air around it.

Status colours come from your growth stage. Within 0.2 kPa of the range counts as “slightly” low or high; further out is “too” low or high.

Formulas

SVP(T)   = 0.61078 × e^(17.27·T / (T + 237.3))  kPa, T in °C
Air VPD  = SVP(air) × (1 − RH/100)
Leaf VPD = SVP(leaf) − SVP(air) × RH/100

Your calculation

Targets

VPD targets by growth stage

Starting points for leaf VPD. Strains and crops differ, so treat these as a band to work in and watch how your plants respond.

Leaf VPD targets by growth stage
Stage	Leaf VPD	Why
Clones / Seedlings	0.5–1.0 kPa	Roots are small, so keep transpiration gentle and humidity high.
Vegetative	0.8–1.2 kPa	Steady transpiration drives nutrient uptake and fast leaf growth.
Early Flower	1.0–1.5 kPa	Lower humidity as flowers set to keep them firm and mould-free.
Late Flower	1.2–1.5 kPa	Driest air of the cycle protects dense flowers from bud rot.

What a VPD calculator tells you

Temperature and humidity on their own do not say how hard the air is pulling water out of your plants. VPD (vapour pressure deficit) puts both into one number, in kPa. Too low and plants transpire slowly while mould risk rises. Too high and the air takes water from the leaves faster than the roots can replace it.

The calculator works out leaf VPD, because plants transpire from the leaf and a leaf is rarely at air temperature. Under LEDs it starts with the leaf 2 °C (3.6 °F) cooler than the air. If you have measured your leaves, or run HPS, the leaf VPD calculator shows how much the offset changes the answer.

Alongside leaf VPD you get air VPD, dew point and absolute humidity, and you can save each reading to a log that stays in your browser. A week of readings shows a drift that one check would miss.

VPD calculator formula, worked through

Take a veg room at 26 °C (78.8 °F) and 60% relative humidity, with LED lights, so the leaf is at 24 °C (75.2 °F).

Saturation vapour pressure of the leaf, SVP(24 °C): 2.984 kPa
Saturation vapour pressure of the air, SVP(26 °C): 3.361 kPa
Vapour actually in the air: 3.361 × 0.60 = 2.017 kPa
Leaf VPD: 2.984 − 2.017 = 0.97 kPa

The veg target is 0.80–1.20 kPa, so this reads Ideal. Switch the stage to early flower (1.00–1.50 kPa) and the same room reads Slightly low; the advice then says to lower humidity to about 52% to reach the middle of the band, 1.25 kPa. Air VPD for the same room is 1.34 kPa, which is why it matters which one a target was written for.

Good VPD for veg, flowering and autoflowers

The stage targets are starting bands for leaf VPD: 0.50–1.00 kPa for clones and seedlings, 0.80–1.20 kPa in veg, 1.00–1.50 kPa in early flower and 1.20–1.50 kPa in late flower. Within 0.2 kPa outside a band counts as slightly low or high.

Running added CO₂ (about 1,200 ppm or more)? Choose Added CO₂ under More options. The target narrows to the upper half of the band, for example 1.00–1.20 kPa in veg.
Growing autoflowers? They change stage by age, not day length. The printable VPD chart has a week-by-week guide to which stage to pick.
Change one thing at a time. The advice gives either a humidity or a temperature that reaches the middle of the band. Pick one, wait for the room to settle, then check again.

Lights-off humidity and other common mistakes

Most VPD tools stop at lights-on conditions. The trouble often starts after the lights go off: the same water stays in the air while the room cools, so humidity climbs. In the room above, cooling to 21 °C (69.8 °F) at night lifts humidity to about 81%. The dew point is 17.6 °C (63.7 °F), so leaves at 21 °C sit only 3.4 °C (6.1 °F) above it: a moderate condensation risk. Open the Lights-off check to see this for your room, and run the dehumidifier or exhaust fan into the dark period if it warns you. If the dehumidifier cannot keep up, the dehumidifier size calculator works out the litres or pints a day you need. The dew point calculator does the same check for a cold wall or window.

Trusting one cheap hygrometer. At 26 °C, a humidity reading that is 5% out moves VPD by about 0.17 kPa, nearly half a stage band. Check it against a second sensor, or with the wet bulb calculator.
Comparing air VPD with leaf VPD targets. Every target here is leaf VPD.
Leaving the LED offset on under HPS. Leaves run warmer under HPS, so choose HPS under Your lights.

FAQ

Questions growers ask

What is VPD?

VPD, vapour pressure deficit, is how far the air is from being saturated with water vapour, measured in kPa. Leaf VPD compares the saturated air inside a leaf with the room air around it. The bigger the gap, the faster the plant transpires, which makes it a better guide than humidity alone.

How do I calculate VPD?

Leaf VPD = SVP(leaf) − SVP(air) × RH/100, where SVP(T) = 0.61078 × e^(17.27·T/(T+237.3)) kPa. At 26 °C air, 60% RH and a leaf at 24 °C, that is 2.984 − 3.361 × 0.60 = 0.97 kPa. The calculator shows this sum with your own numbers under the result.

Why is my VPD so high?

Usually the air is too warm, too dry, or both. At 28 °C (82.4 °F) and 50% RH, leaf VPD is about 1.47 kPa under LEDs and 1.74 kPa under HPS, where leaves run warmer. Both are above the 1.20 kPa top of the veg band. A hygrometer that reads low can also be the cause.

How do I lower VPD without raising humidity?

Lower the air temperature. Leaf VPD falls as the room cools, even when humidity stays the same. At 28 °C (82.4 °F) and 55% RH under LEDs, leaf VPD is 1.28 kPa; cooling the room to 24 °C (75.2 °F) brings it to 1.0 kPa. The advice under your result gives the exact temperature for your stage.

What is the difference between VPD and relative humidity?

Relative humidity alone does not give you VPD, because the same humidity means a different VPD at each temperature. At 60% RH, air VPD is 0.94 kPa at 20 °C (68 °F) but 1.70 kPa at 30 °C (86 °F). VPD includes the temperature, so it follows what the plant feels more closely.

Does VPD matter when lights are off?

Yes, mainly as a condensation check. The same water stays in the air as the room cools, so humidity climbs. Air at 26 °C and 60% RH cooled to 21 °C reaches about 81% RH, with leaves only 3.4 °C above the dew point. The Lights-off check on this page works it out for your room.

How do I convert VPD to kPa?

Divide millibars by 10, or divide psi by 0.145, to get kPa. One kPa equals 10 mbar or 0.145 psi, so 12 mbar and 0.174 psi are both 1.2 kPa. The calculator and the printable VPD chart can show any of the three units, so you rarely need to convert by hand.

Should I use leaf VPD or air VPD?

Leaf VPD. Plants transpire from the leaf, and a leaf is rarely at air temperature: measured leaves usually sit within about 2 °C (3.6 °F) of it. Leaf VPD is what the stage targets on this site are written for. Air VPD is shown alongside for reference.

Sources

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

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
Murray, F. W. (1967). On the computation of saturation vapor pressure. Journal of Applied Meteorology 6(1), 203–204. doi.org/10.1175/1520-0450(1967)006%3C0203:OTCOSV%3E2.0.CO;2
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
Pulse Grow (2020): VPD guide for cannabis growers. pulsegrow.com/blogs/learn/vpd
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
University of Arizona, AZMet: dew point from temperature and relative humidity. azmet.arizona.edu/azmet/dewpoint.html
American Meteorological Society, Glossary of Meteorology: gas constant (water vapour, about 461 J kg⁻¹ K⁻¹). glossary.ametsoc.org/wiki/Gas_constant
Hydrobuilder Learning Center. Vapor pressure deficit (VPD): “adjust toward the higher end of each range under elevated CO2 (roughly 1,200+ ppm)”. learn.hydrobuilder.com/vapor-pressure-deficit-vpd/

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