Yes. 1.2 kPa sits right on the line between late veg and early flower, which is one of the most productive ranges for most plants. Stomata stay open, transpiration is strong, and CO2 uptake is high.

VPD Calculator.

Q: What is the ideal VPD for growing?

Ideal VPD shifts with growth stage. Use 0.4 to 0.8 kPa for clones, 0.6 to 0.9 kPa for seedlings, 0.8 to 1.2 kPa in veg, and 1.2 to 1.5 kPa in flower, drifting toward 1.6 kPa in late flower.

Q: What VPD causes mold?

Holding VPD below about 0.8 kPa in flower raises mold risk. Plants transpire slowly, moisture lingers in the canopy, and powdery mildew or bud rot can take hold, especially at lights-off when humidity climbs.

Q: How do I measure VPD?

Measure air temperature and relative humidity at canopy height with a hygrometer, then enter both into a VPD calculator. For leaf VPD, also read leaf temperature with an infrared thermometer, or subtract 2 to 3 degrees Fahrenheit under LED.

Q: How do I lower VPD?

To lower VPD when the air is too dry, add a humidifier, drop temperature 1 to 2 degrees, slow the exhaust fan so humid air lingers, or wet the floor as an emergency fix. Humidity is usually the faster and gentler lever than temperature.

Set your temperature and humidity. Watch the canopy react, and land every stage in its sweet spot.

Instant, live results Air & leaf-temp accurate Built by growers

Live canopyTranspiration: steady

Leaf feel: happy Stomata: open Mold risk: low

1.31kPa

Vegetative sweet spot

Air temperature78°F

Relative humidity60%

Leaf temp offsetleaves run cooler under LED

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What is VPD and why growers obsess over it

Vapor pressure deficit is the gap between how much moisture the air could hold when saturated and how much it actually holds right now, measured in kilopascals (kPa). That gap is the force pulling water through your plants, up from the roots and out through the stomata on every leaf.

Why VPD beats humidity alone

Two rooms at the same 60% humidity can sit in completely different VPD zones simply because one runs warmer. Warm dry air pulls hard. Cool damp air barely pulls at all. VPD folds temperature and humidity into the single number the plant actually feels.

Leaf VPD calculator: leaf vs air temperature

Transpiration happens at the leaf surface, which usually runs 2 to 3°F cooler than the room under LED lighting. Setting the leaf offset computes VPD where it counts, and the true value will land noticeably lower than the air-only number.

How to calculate VPD

The tool uses the Magnus equation to find saturation vapor pressure, then subtracts what the air already holds:

SVP(T) = 0.6108 × e^(17.27T / (T + 237.3))

VPD = SVP(T_leaf) − SVP(T_air) × RH / 100

Both readings update live from those two lines, with no lookup-table rounding. Temperature goes in as degrees Celsius, and the result comes out in kilopascals.

How to dial in your VPD, step by step

A complete field routine, from a bare hygrometer reading to a locked-in target you hold day and night. About five minutes the first time, seconds after that.

1

Measure temperature and humidity

Read air temperature and relative humidity at canopy height, where the plant lives, not at the floor or up by the light. A cheap hygrometer is fine. Enter both into the calculator above.

Place the sensor in the canopy shade, away from direct airflow, for a reading that matches the leaves.
2

Correct for leaf temperature

Transpiration happens at the leaf surface, and under LED a leaf runs 2 to 3°F cooler than the surrounding air. Tap the leaf offset to −2 or −3°F, or measure a shaded fan leaf with an infrared thermometer. Leaf VPD is the number the plant actually feels, and it lands lower than the air-only value.
3

Compare against your stage target

Match the reading to the band for the plant’s current stage. Tap a preset to drop the target straight onto the chart:

Clones0.4–0.8 kPa Veg0.8–1.2 kPa Flower1.2–1.5 kPa Late flower1.4–1.6 kPa
4

Adjust humidity first, then temperature, and hold it

To move VPD, reach for humidity before heat, it is the faster and gentler lever. A humidifier or dehumidifier on a controller will hold the band far better than chasing the number by hand. Watch it at lights-off too, since night is where most mold losses begin. Then share the reading or save the chart for your grow log.

Interactive VPD chart

Every combination of temperature and humidity, colored by zone with contour lines at the stage boundaries. Drag anywhere on the map and the calculator follows. Want the classic printable grid? Open the full VPD chart tool.

Computed at air temperatureContours at 0.4, 0.8, 1.2 and 1.6 kPa

Target VPD by growth stage

Tap any stage to load it into the calculator and see it on the chart. Ranges assume leaf VPD, so apply the leaf offset for the most accurate read.

Clones

0.4 to 0.8 kPa

try 75°F and 80% RH

No roots yet, so keep the air humid and let cuttings lose almost no water while they strike.

Seedling

0.6 to 0.9 kPa

try 77°F and 70% RH

Gentle transpiration while the first true roots and leaves establish.

Vegetative

0.8 to 1.2 kPa

try 78°F and 65% RH

A stronger pull moves water and nutrients fast for structural growth.

Flower

1.2 to 1.5 kPa

try 78°F and 55% RH

Drier air bulks buds and keeps mold out of a thickening canopy.

Late flower

1.4 to 1.6 kPa

try 75°F and 48% RH

A higher deficit through ripening protects dense, resinous flowers.

How to raise or lower your VPD

Two levers move the number: temperature and humidity. Reach for the cheapest correction first, and remember that humidity is usually faster and gentler than heat.

How to raise VPD

When the air is too wet and the number is too low

Run a dehumidifier. The fastest fix in a sealed tent.
Increase exhaust fan speed to pull humid air out and drier air in.
Raise temperature 1 to 2 degrees. Warmer air holds more moisture, which drops relative humidity.
Space out plants and defoliate. A packed canopy transpires into its own microclimate.
Water at lights-on, not before lights-off, so surface moisture clears.

How to lower VPD

When the air is too dry and the number is too high

Add a humidifier, sized to the room and on a controller if you can.
Lower temperature 1 to 2 degrees. Cooler air closes the deficit quickly.
Slow the exhaust fan so humid transpired air lingers a little longer.
Wet the floor or hang damp towels. The zero-budget emergency fix.
Dim or raise the lights. Less radiant heat on the leaves lowers leaf VPD directly.

VPD problems and symptoms

1What you see	2Likely VPD problem	3First move
Leaf edges curl up (tacoing), crispy tips	Too high, heat plus dry air	Lower temp, add humidity
Droopy leaves that feel soft, slow growth	Too low, the plant cannot transpire	Dehumidify, add airflow
Powdery mildew spots, damp smell	Held below 0.8 kPa too long	Raise VPD, prune canopy
Tip burn or blossom end rot	Low VPD stalling calcium transport	Raise VPD in veg, check feed
Fast growth that suddenly stalls in heat	Spiking above 1.6 kPa mid-day	Smooth the day and night swing

VPD questions growers ask

What is the ideal VPD for growing?

It shifts with the stage: 0.4 to 0.8 kPa for clones, 0.6 to 0.9 for seedlings, 0.8 to 1.2 in veg, and 1.2 to 1.5 kPa in flower, drifting up to 1.6 in late flower. Use the stage buttons above to load each target onto the chart.

Is 1.2 VPD good?

Yes. 1.2 kPa sits on the line between late veg and early flower, one of the most productive ranges for most plants. Stomata stay open, transpiration is strong, and CO2 uptake is high.

What VPD causes mold?

Holding below about 0.8 kPa in flower. Plants transpire slowly, moisture lingers in the canopy, and powdery mildew or bud rot take hold. The mold-risk pill in the live canopy panel flags this as you adjust.

What happens if VPD is too high?

Above roughly 1.6 kPa plants lose water faster than the roots can replace it. Stomata close to protect the plant, photosynthesis stalls, and leaves taco or crisp at the edges.

What should VPD be at night with the lights off?

Night VPD naturally drops because plants transpire far less in the dark. The priority in flower is keeping humidity under about 60% so condensation never forms on buds. Many growers run dehumidification that ramps up at lights-off rather than down, since night is where most mold losses begin.

Do CO2-enriched rooms use different VPD targets?

Slightly higher, yes. With CO2 at 1,200 to 1,500 ppm and strong light, plants can handle warmer temperatures and a higher VPD than the standard chart suggests. Treat the chart as a starting point and raise the ceiling in a sealed CO2 room.

Should I use leaf temperature or air temperature for VPD?

Leaf temperature is more accurate because transpiration happens at the leaf surface. Under LED, leaves typically run 2 to 3°F cooler than the air. Measure a shaded fan leaf with an infrared thermometer, or apply the leaf offset in this calculator.

Is this a leaf VPD calculator?

Yes. Turn on the leaf temperature offset and the tool returns leaf VPD, the vapor pressure difference between the leaf surface and the air, which is what actually drives transpiration. Under LED light leaves usually sit 2 to 3°F cooler than the air, so leaf VPD reads lower than air VPD at the same room settings.

How do I measure VPD?

Measure air temperature and humidity at canopy height with a hygrometer, then enter both here. For leaf VPD, also read leaf temperature with an infrared thermometer, or subtract 2 to 3°F under LED using the leaf offset.

How do I lower VPD?

When the air is too dry and VPD is too high, add a humidifier, drop temperature 1 to 2 degrees, slow the exhaust fan so humid air lingers, or wet the floor as an emergency fix. Humidity is usually the faster, gentler lever than temperature.

How do I raise VPD?

When the air is too wet and VPD is too low, run a dehumidifier, increase exhaust fan speed, raise temperature 1 to 2 degrees so the air holds more moisture, and defoliate a packed canopy so it stops transpiring into its own microclimate.

Does VPD affect nutrient uptake and blossom end rot?

Yes. Transpiration pulls water and dissolved nutrients up through the plant, including calcium. When VPD runs too low, calcium moves poorly, which shows up as tip burn in cannabis and blossom end rot in tomatoes and peppers.

Is VPD different for autoflowers?

The target ranges are identical. Autoflowers just move through stages faster, so match the range to the plant's current stage rather than the calendar.

More grow-room climate tools

VPD chart

The printable temperature-by-humidity grid, color-coded by grow stage for your tent wall.

Dew point calculator

Find the temperature where condensation forms and get your grow-room mold risk instantly.

Dew point chart

Read the dew point for any temperature and humidity, and watch a cold pane bead with condensation.