What is VPD?
Vapor pressure deficit is how hard the air is pulling water out of the leaf: the difference between the vapor pressure inside the leaf (saturated, at leaf temperature) and the vapor pressure of the air around it (which depends on air temperature and humidity). It's the number that actually drives transpiration — and with it water and nutrient uptake — which is why growers prefer it to humidity alone. 60% RH is comfortable at 72 °F and desiccating at 90.
SVP is the saturation vapor pressure, a steep curve that roughly doubles every 10 °C (the Tetens formula: 0.611 × e17.27T/(T+237.3) kPa). At 77 °F and 60% the air holds 1.90 kPa; a leaf at air temperature is saturated at 3.17, so the deficit is 1.27 kPa. A leaf 2 °C cooler is saturated at only 2.81, and the deficit is 0.91 — a different stage band from the same two readings.
VPD targets by growth stage
The bands every grow-light company prints, and — the useful part — what they mean at 77 °F with an LED-cooled leaf: the humidity that lands you in the middle of each band, and the temperature that does it at 60% RH.
| Stage | Band | RH for the middle, at 77 °F | Temperature for the middle, at 60% RH |
|---|---|---|---|
| Clones & seedlings | 0.4–0.8 kPa | 70% | 65 °F (18.5 °C) |
| Vegetative | 0.8–1.2 kPa | 57% | 80 °F (26.5 °C) |
| Early flower | 1–1.4 kPa | 51% | 85 °F (29.5 °C) |
| Late flower | 1.2–1.6 kPa | 44% | 90 °F (32.1 °C) |
Leaf 2 °C below air. These bands are grower convention (Gorilla Grow Tent, Spider Farmer, Mars Hydro agree within a tenth); greenhouse horticulture manages most crops to 0.5–1.2 kPa, which is the veg band and a bit below it. The late-flower band is about mold pressure in dense flowers, not what the plant prefers.
How to read the chart
Find your air temperature down the side and your humidity along the top; the cell is the leaf VPD for the offset you picked, colored by band. Read along the row to see what a humidity change does at your temperature, and down the column for a temperature change at your humidity — the colored diagonal is the whole story: warmer needs wetter, cooler needs drier, to stay in the same band. A cell marked "—" is condensation: the leaf is colder than the dew point. Print it for the tent; the offset it was built with is in the footer so a chart built for LEDs isn't read under HPS.
VPD for vegetables, herbs and houseplants
The bands above are written for cannabis because that's who searches for VPD charts, but the physics doesn't know the crop. Commercial greenhouses manage tomatoes, cucumbers, peppers and most ornamentals to about 0.5–1.2 kPa — the veg band and a little below — and lettuce and other leafy greens toward the low end of it, since tender leaves with shallow roots can't supply fast transpiration. Seedlings of anything sit in the clone band until they have roots. There is no "flower band" for a tomato: the dry late-flower numbers are a mold strategy for dense, sticky flowers, and a fruiting vegetable is happier staying put around 1.0.
The mistakes that make the number meaningless
Reading the sensor at the wall. The air a foot above the canopy under the light is warmer and drier than the tent corner; a hygrometer clipped to the pole reads a different room. Chasing the second decimal. Between the leaf offset and the hygrometer, the true value is ±0.2 kPa either way; moving a dehumidifier setpoint to get from 1.05 to 1.10 is theater. Running late-flower numbers in veg. 1.5 kPa on a young plant is a plant pulling water faster than its roots deliver — the leaves cup and the edges brown, and it looks like a nutrient problem. Ignoring lights-off. The chart is for the light period; when the lights go off the air cools, RH climbs and VPD can drop below 0.4 for hours — which is when condensation and mold happen, and what the dew-point section below is for.
Why leaf temperature changes everything
The leaf is where the water is, so its temperature — not the air's — sets the inside of the equation. A transpiring leaf under LEDs runs a couple of degrees cooler than the air; under HPS the radiant heat pushes it warmer. Every printed chart quietly assumes the leaf is at air temperature, and most that offer an "offset" then apply the shortcut SVP(leaf) × (1 − RH) — which moves both sides of the equation to the leaf and gets a different wrong answer:
| Leaf vs air | Leaf VPD (this calculator) | Chart shortcut, SVP(leaf) × (1 − RH) | Shortcut error |
|---|---|---|---|
| -3 °C | 0.74 kPa | 1.06 kPa | +0.31 |
| -2 °C | 0.91 kPa | 1.12 kPa | +0.21 |
| -1 °C | 1.08 kPa | 1.19 kPa | +0.11 |
| 0 °C | 1.27 kPa | 1.27 kPa | — |
| +1 °C | 1.46 kPa | 1.34 kPa | -0.12 |
| +2 °C | 1.66 kPa | 1.43 kPa | -0.24 |
77 °F air, 60% RH. The two agree only at 0 offset. Under LEDs (−2 °C) the shortcut reads 1.12 where the leaf sees 0.91.
A 2 °C leaf offset is worth 0.36 kPa at these readings — about half a stage band — and the offset depends on the light, the airflow and how hard the plant is transpiring, none of which the sensor knows. The honest fix is a $20 infrared thermometer pointed at a fan leaf in the top of the canopy, which turns the guess into a measurement. Without it, treat the calculator's verdict as "in the band" or "well outside it", and don't chase the second decimal.
What to change when VPD is off
Humidity is the cheap lever and temperature the expensive one, and they don't move VPD equally. Near 77 °F, one point of RH moves VPD about 0.032 kPa; one degree Celsius of air temperature, with the leaf tracking it, about 0.06 — so a 10-point humidity change is worth roughly a 5 °C temperature change. Too low (air too humid): dehumidify, or exhaust more, before you touch the heat. Too high (air too dry): a humidifier, or drop the temperature a few degrees, which also helps the leaf. The calculator prints both targets for your readings.
How accurate is your reading?
A consumer hygrometer is ±3% RH on a good day (many are ±5); a cheap infrared thermometer is ±1 °C on a leaf. In kPa, at 77 °F and 60%:
| Reading | Typical error | Moves VPD by |
|---|---|---|
| Humidity | ±3% RH | ±0.10 kPa |
| Air temperature (leaf tracks it) | ±1 °C | ±0.06 kPa |
| Leaf temperature | ±1 °C | ±0.17 kPa |
| Leaf temperature, not measured | ±2 °C guess | ±0.35 kPa |
Finite-difference sensitivities from the engine at these readings. Add them: an unmeasured leaf and a cheap hygrometer together put the true VPD anywhere within about ±0.4 kPa of the displayed number. Hang the hygrometer at canopy height and calibrate it once with the salt test.
Dew point: the mold number
The other thing the same two readings give you. At 77 °F and 60% the dew point is 62 °F (16.7 °C): any surface cooler than that — a leaf under LEDs, the inside of a dense flower at lights-off, the tent wall — gets wet. That is what the late-flower band is actually guarding against, and why the calculator flags a negative VPD as condensation rather than "very low".
| RH at 77 °F | Dew point | LED leaf margin above dew point |
|---|---|---|
| 50% | 57 °F | 16.5 °F |
| 60% | 62 °F | 11.3 °F |
| 70% | 66 °F | 6.9 °F |
| 80% | 70 °F | 3.0 °F |
| 90% | 74 °F | -0.4 °F |
Leaf at 2 °C below air. When lights go off the air cools toward the dew point while the humidity climbs — the first hour of dark is when a flowering tent needs its dehumidifier most.
Sources: Allen, Pereira, Raes & Smith, FAO Irrigation and Drainage Paper 56 (1998), eq. 11 (Tetens saturation vapor pressure) and the psychrometric definitions; Magnus dew-point form; grower stage bands as published by Gorilla Grow Tent, Spider Farmer and Mars Hydro; greenhouse VPD management ranges from Argus Control Systems, Understanding and Using VPD.