What Is VPD? Vapor Pressure Deficit Explained (with Target Ranges)

Your grow tent reads 60% humidity — but is that good? The honest answer is “it depends on the temperature,” and that’s exactly the problem VPD solves. Vapor pressure deficit rolls temperature and humidity into one number that describes what the plant actually feels: how hard the air is pulling moisture out of its leaves.

What VPD actually measures

Air has a maximum amount of water vapour it can hold, and that maximum rises steeply with temperature — warm air holds much more. VPD is the difference between that maximum (at leaf temperature) and how much vapour the air is actually carrying. A big deficit means thirsty air that pulls water out of leaves quickly; a small deficit means near-saturated air where transpiration stalls and condensation and mould set in.

That’s why humidity alone misleads: 60% RH at 20 °C and 60% RH at 30 °C are completely different growing environments, because the warmer air’s “maximum” is so much higher. VPD captures the difference in a single figure.

The formula

VPD = SVP(leaf temp) − SVP(air temp) × (RH ÷ 100)

where SVP is the saturation vapour pressure — the maximum vapour pressure at a given temperature, from the Magnus equation. In practice:

1. Compute SVP at the leaf temperature (the air’s capacity right at the leaf surface).
2. Compute the actual vapour pressure = SVP(air) × RH.
3. Subtract. The result, in kPa, is the deficit.

The VPD calculator does all three steps from your temperature and humidity, and lets you set a leaf-temperature offset.

Why leaf temperature matters

Leaves aren’t the same temperature as the air. Transpiration cools them, so a healthy leaf often runs 1–3 °C below air temperature (under intense light it can be warmer). Since VPD is anchored to the leaf’s saturation pressure, that offset shifts the result — a cooler leaf means a lower VPD than the air-only figure. Serious growers measure leaf temperature with an IR thermometer; if you can’t, assuming leaves are ~2 °C cooler is a reasonable start.

Target VPD by growth stage

VPD needs vary with how much root system and leaf area the plant has to support transpiration:

Stage	VPD (kPa)	Why
Clones / seedlings	0.4 – 0.8	Few roots — keep the air gentle so they don’t dry out
Vegetative	0.8 – 1.2	Growing fast — moderate transpiration drives nutrient uptake
Flowering / fruiting	1.2 – 1.6	Higher VPD keeps humidity down and discourages bud rot

Below ~0.4 kPa the air is too humid: transpiration slows, calcium (which moves with water) lags, and mould and mildew thrive. Above ~1.6 kPa the air is too dry: the plant closes its stomata to conserve water, and growth stalls.

Dialing it in

To lower VPD (raise humidity or lower temperature): add a humidifier, or cool the space. To raise VPD (drop humidity or warm the space): add a dehumidifier or more airflow, or nudge the temperature up. Because the two levers interact, it’s easiest to pick a target VPD for your stage and adjust temperature and humidity together until the calculator lands in the zone. If you also want the actual water content of the air, the absolute humidity calculator gives it in g/m³.

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VPD here is computed from the Magnus saturation-vapour-pressure formula in your browser; nothing is uploaded. The target ranges are widely used horticultural guidelines, not hard rules — cultivar, light intensity and CO₂ all shift the ideal, so treat them as starting points and observe your plants.