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VPD calculator

Vapour pressure deficit (VPD) is how hard the air pulls water out of a leaf, measured in kilopascals. Type in your air temperature and humidity. The calculator gives leaf VPD, air VPD and the dew point, and says which growth stage the reading suits.

Leaves under LED usually sit 1–3 °C below the air; under HPS they can run warmer. 0 gives plain air VPD.

Leaf VPD–kPa · what the plant feels
Air VPD–kPa · what a room meter shows
Dew point–°C · condensation starts here
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Chart

VPD chart by growth stage

Find your air temperature on the left and your humidity across the top. Teal is 0.4 to 0.8 kPa, where clones and seedlings live, green is 0.8 to 1.2 (mostly vegetative) and amber is 1.2 to 1.6 (mostly flower). Cannabis, tomatoes, peppers and most leafy crops share these bands; the plant does not know what it is called.

VPD chart for cannabis by growth stage: leaf VPD by air temperature and humidity
The table below as one picture. Download the chart (PNG)
  • under 0.4: too wet, growth stalls
  • 0.4 to 0.8: clones and seedlings
  • 0.8 to 1.2: mostly vegetative
  • 1.2 to 1.6: mostly flower
  • over 1.6: plants close their stomata
air temp \ RH 35% 40% 45% 50% 55% 60% 65% 70% 75% 80%
18 °C / 64 °F 1.10 0.99 0.89 0.79 0.68 0.58 0.48 0.37 0.27 0.17
19 °C / 66 °F 1.17 1.06 0.95 0.84 0.73 0.62 0.51 0.40 0.29 0.18
20 °C / 68 °F 1.25 1.13 1.01 0.89 0.78 0.66 0.54 0.43 0.31 0.19
21 °C / 70 °F 1.33 1.20 1.08 0.95 0.83 0.71 0.58 0.46 0.33 0.21
22 °C / 72 °F 1.41 1.28 1.15 1.02 0.88 0.75 0.62 0.49 0.36 0.22
23 °C / 73 °F 1.50 1.36 1.22 1.08 0.94 0.80 0.66 0.52 0.38 0.24
24 °C / 75 °F 1.60 1.45 1.30 1.15 1.00 0.85 0.70 0.56 0.41 0.26
25 °C / 77 °F 1.70 1.54 1.38 1.23 1.07 0.91 0.75 0.59 0.43 0.28
26 °C / 79 °F 1.81 1.64 1.47 1.30 1.14 0.97 0.80 0.63 0.46 0.29
27 °C / 81 °F 1.92 1.74 1.56 1.39 1.21 1.03 0.85 0.67 0.49 0.32
28 °C / 82 °F 2.04 1.85 1.66 1.47 1.28 1.09 0.90 0.72 0.53 0.34
29 °C / 84 °F 2.16 1.96 1.76 1.56 1.36 1.16 0.96 0.76 0.56 0.36
30 °C / 86 °F 2.29 2.08 1.87 1.66 1.45 1.23 1.02 0.81 0.60 0.39
31 °C / 88 °F 2.43 2.21 1.98 1.76 1.53 1.31 1.09 0.86 0.64 0.41
32 °C / 90 °F 2.58 2.34 2.10 1.87 1.63 1.39 1.15 0.91 0.68 0.44

Leaf VPD in kPa with the leaf 2 °C cooler than the air, the same estimate the calculator starts with. Air-only VPD (no leaf offset) runs about 0.2 to 0.3 kPa higher at these temperatures. The colours are a plain scale. The stage cards below give the target range for each stage, and neighbouring stages overlap.

Targets

What each stage wants

VPD is the one number that ties temperature and humidity together, which is why growers steer by it instead of by either alone. These are leaf VPD target ranges, the same stage targets the DSC brain uses; neighbouring stages overlap.

0.4 to 0.8 kPa

Clones and seedlings

Little root, thin leaf: keep the air soft.

0.7 to 1.2 kPa

Vegetative

Fast leaf growth, the plant can pull water.

1.0 to 1.4 kPa

Flower

Stretch is over, buds are setting.

1.3 to 1.6 kPa

Late flower

Drier air keeps mould off dense flowers, through the final flush.

To lower VPD (air too dry): raise humidity, or drop the temperature a degree. To raise VPD (air too wet): pull humidity down, or warm the room. Change one thing at a time and give the room twenty minutes: humidity lags temperature, and a dehumidifier adds heat while it runs. For what a humidifier and a dehumidifier have to do across a whole grow room, see humidity controllers for grow rooms.

What this page assumes

Read this before you chase a decimal

The maths

Saturation vapour pressure uses the Tetens/Magnus fit over water: es(T) = 0.6108 · exp(17.27 T / (T + 237.3)) kPa, the same constants the DSC brain uses; the bench log on the two constants says where they come from. Leaf VPD is es(leaf) − es(air) · RH/100: the saturation pressure at the leaf minus the vapour pressure actually in the air. Some calculators use es(leaf) · (1 − RH/100), which treats humidity as if it were measured at leaf temperature; that form reads high, and the gap grows with the leaf offset.

What you do not actually know

The leaf offset is a guess unless you point an infrared thermometer at a leaf: −2 °C is typical under LED, warmer under HPS, colder with strong airflow. A consumer humidity sensor is good to about ±3 %, which moves VPD by about 0.1 kPa, and a sensor by the intake reads a different room from one at the canopy. The stage bands are the ranges growers converge on, not a law: strains, pot size and light intensity all shift them.

Where this comes from

The same number, kept live and acted on

Digital Stealth Care is a grow room controller that computes leaf VPD from your canopy sensors every few seconds, marks a reading held or stale the moment it stops trusting it, and steers the fans, humidifier and dehumidifier you already own toward the band for the stage your plants are in. The calculator on this page is a copy of that maths, nothing more. It runs on a Raspberry Pi in your room, not in a cloud. You can see the same number on the Climate desk.

Get the Alpha, free
VPD climate control in DSC
How sensor trust works

0.6108

kPa: the Tetens constant e₀, shared with the brain

−2 °C

the leaf offset this page starts from

0.4 to 1.6

kPa: the range every stage lives inside