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VPD Calculator

Free tool

Leaf VPD Calculator

This leaf VPD calculator uses the temperature of the leaf, not only the room, to give the VPD your plants actually feel. Enter a measured leaf temperature or an offset for LED or HPS lights.

Temperature
Growth stage
°C

%

Leaf
Your lights
Leaf compared with the air
°C

LED fills in about 2 °C (3.6 °F) cooler, HPS about 0.7 °C (1.3 °F) cooler. Measured your leaves with an infrared thermometer? Type your own.

More options
VPD unit

Leaf VPD

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Target: —

Air VPD
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Leaf vs air VPD
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Leaf temperature
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What to do

Enter your conditions.

    How the offset changes leaf VPD here

    Leaf VPD at the current air temperature and humidity for different leaf offsets
    Leaf compared with the airLeaf VPDStatus

    Next: Check whether leaves could get wet when the lights go off

    Leaf VPD vs air VPD

    Air VPD only looks at the room. Leaf VPD compares the saturated air inside the leaf, at leaf temperature, with the air around it. That is the gradient that actually moves water out of the plant.

    Under LEDs leaves usually sit 1–3 °C below air temperature. Under HPS lamps the radiant heat can push leaves level with or above the air. Measure if you can; otherwise start with the leaf 2 °C cooler than the air.

    Formula
    Leaf VPD = SVP(leaf) − SVP(air) × RH/100
SVP(T)   = 0.61078 × e^(17.27·T / (T + 237.3))

    Why use a leaf VPD calculator

    Water leaves a plant through the leaf, so the pull that matters is between the wet air inside the leaf and the room air. Inside the leaf the air is saturated at leaf temperature, and that is rarely the same as the air temperature. In research on well-watered plants, leaves were typically within 2 °C of the air under sunlight, HPS and LED.

    Two degrees sounds small, but it moves VPD a long way. At 26 °C (78.8 °F) and 60% RH, air VPD is 1.34 kPa, slightly high for veg. With the leaf 2 °C cooler, leaf VPD is 0.97 kPa, right in the middle of the veg band. Every stage target on this site is written for leaf VPD, which is why this tool and the VPD calculator both start from the leaf. The result also shows the gap between leaf and air VPD, so you can see how big it is in your own room.

    Leaf VPD formula, worked through

    Leaf VPD = SVP(leaf) − SVP(air) × RH/100. For the room above, with the leaf at 24 °C (75.2 °F): SVP(24 °C) is 2.984 kPa, SVP(26 °C) is 3.361 kPa, and 3.361 × 0.60 = 2.017 kPa. So leaf VPD is 2.984 − 2.017 = 0.97 kPa.

    The offset table under the result repeats this for five leaf temperatures. At 26 °C and 60% RH in veg it reads:

    • Same as the air: 1.34 kPa, slightly high
    • 1 °C (1.8 °F) cooler: 1.15 kPa, ideal
    • 2 °C (3.6 °F) cooler: 0.97 kPa, ideal
    • 3 °C (5.4 °F) cooler: 0.79 kPa, slightly low
    • 4 °C (7.2 °F) cooler: 0.63 kPa, slightly low

    Each degree is worth roughly 0.16–0.19 kPa here, about half the width of a stage band. That is why a guessed offset can put you in the wrong colour.

    VPD leaf temperature offset for LED vs HPS

    The LED preset sets the leaf 2 °C (3.6 °F) cooler than the air. The HPS preset sets it 0.7 °C (1.3 °F) cooler, because a study comparing light types found leaves under LEDs ran about 1.3 °C cooler than under HPS. HPS lamps give off more radiant heat, which warms the leaf. In the same 26 °C, 60% room, the HPS preset gives 1.21 kPa against 0.97 kPa under LEDs.

    A measured leaf beats either preset. Point an infrared thermometer at the top leaves with the lights on, take a few readings and average them. Enter the result under Leaf temperature, then press Use this as my leaf offset. The offset is remembered, so the VPD calculator starts from your own number next time, and its Printable VPD Chart button carries the same offset to the leaf VPD chart. Measure again when you change lights.

    Common leaf VPD mistakes

    • Measuring in the wrong place. Lower, shaded leaves run cooler than the canopy. Measure the top leaves, under the lights.
    • Reading the offset as a temperature. The offset is a difference: 2 °C cooler is 3.6 °F cooler, not 35.6 °F.
    • Forgetting the dew point. If the leaf is at or below the dew point of the air, water condenses on it and leaf VPD turns negative. The calculator warns you when the leaf is within 2 °C (3.6 °F) of the dew point. This happens most often just after lights-off, when leaves cool faster than the room dries. Check the night-time margin with the dew point calculator.
    • Changing the offset to hit a target. The offset should describe your leaves, not your goal. Change humidity or temperature instead, using the advice under the result.

    FAQ

    Questions growers ask

    What is leaf VPD?

    Leaf VPD is the vapour pressure deficit between the inside of a leaf and the air around it, in kPa. It uses the leaf's own temperature, because the air inside the leaf is saturated at that temperature. It is the pull that actually moves water out of the plant, so stage targets are written for it.

    What is the difference between leaf VPD and room VPD?

    Room, or air, VPD assumes the leaf is at air temperature; leaf VPD uses the real leaf. When leaves are cooler than the air, leaf VPD is lower. At 26 °C and 60% RH, room VPD is 1.34 kPa, while a leaf 2 °C cooler gives a leaf VPD of 0.97 kPa.

    What leaf temperature offset should I use?

    Start with the leaf 2 °C (3.6 °F) cooler than the air under LEDs, and 0.7 °C (1.3 °F) cooler under HPS. Leaves of well-watered plants are usually within 2 °C of the air. If you can, measure the top leaves with an infrared thermometer and use your own offset instead.

    Are leaves cooler under LED lights?

    Yes, by about 1.3 °C (2.3 °F) compared with HPS, according to a 2015 study of leaf temperature under different lights. HPS lamps give off more radiant heat, which warms the leaf. That is why the LED preset here is 2 °C cooler than the air and the HPS preset only 0.7 °C cooler.

    How do I measure leaf temperature?

    Use an infrared thermometer on the top leaves of the canopy while the lights are on. Take a few readings across the canopy and average them, and avoid wet or shiny leaves. Enter the average under Leaf temperature, and the calculator shows it as an offset you can reuse.

    Can leaf VPD be negative?

    Yes. It goes negative when the leaf is colder than the dew point of the air, so water condenses on it. At 26 °C and 60% RH the dew point is 17.6 °C, and a leaf at 17 °C gives −0.08 kPa. Wet leaves invite mould, so lower humidity or warm the leaves.

    Why is leaf VPD lower than air VPD?

    Leaves under lights are usually a little cooler than the air because evaporation cools them. A cooler leaf holds less vapour, so the pressure difference between leaf and air is smaller.

    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. 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
    3. Pulse Grow (2020): VPD guide for cannabis growers. pulsegrow.com/blogs/learn/vpd
    4. 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
    5. University of Arizona, AZMet: dew point from temperature and relative humidity. azmet.arizona.edu/azmet/dewpoint.html

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