VPD Chart: What Vapour Pressure Deficit Means for Your Plants

VPD - vapour pressure deficit - joins temperature and humidity into one number that tells you how hard the air is pulling water out of your plants. Growers use it because "60% humidity" means something different at 20°C than at 28°C. This guide explains VPD in plain terms and gives you a chart to read it off.

What VPD means

Warm air can hold more water than cool air. VPD is the gap between how much water the air could hold at its temperature and how much it holds now, measured in kilopascals (kPa).

Low VPD (humid air): plants lose water slowly. Fine for cuttings, but leaves stay damp and mould is a risk later on.
High VPD (dry air): plants lose water fast. Some is good - it pulls feed up through the plant - but too much and they close up and stall.

VPD chart

Find your room temperature down the side and the relative humidity across the top; the number is the VPD in kPa.

VPD in kPa, worked out from air temperature and relative humidity with the standard (Tetens) formula, taking leaf temperature as equal to air temperature.
Temp	40% RH	45% RH	50% RH	55% RH	60% RH	65% RH	70% RH	75% RH	80% RH
18°C	1.24	1.14	1.03	0.93	0.83	0.72	0.62	0.52	0.41
20°C	1.40	1.29	1.17	1.05	0.94	0.82	0.70	0.58	0.47
22°C	1.59	1.45	1.32	1.19	1.06	0.93	0.79	0.66	0.53
24°C	1.79	1.64	1.49	1.34	1.19	1.04	0.90	0.75	0.60
26°C	2.02	1.85	1.68	1.51	1.34	1.18	1.01	0.84	0.67
28°C	2.27	2.08	1.89	1.70	1.51	1.32	1.13	0.94	0.76
30°C	2.55	2.33	2.12	1.91	1.70	1.49	1.27	1.06	0.85

Leaves under LED lights often run a degree or two cooler than the air. A cooler leaf means a slightly lower VPD than the chart shows, so if you can measure leaf temperature, a controller that works out leaf VPD will be more exact.

What to aim for

There is no single right VPD, but these are the ranges growers commonly work to:

Commonly used VPD ranges (a rule of thumb, not a maker's figure).
Stage	Typical VPD	What that looks like
Cuttings and seedlings	0.4-0.8 kPa	Warm and humid: a closed propagator
Vegetative growth	0.8-1.2 kPa	Moderate humidity
Flowering	1.0-1.5 kPa	Drier air, which also helps keep mould away

For example, at 24°C a VPD of about 1.0 kPa means roughly 65% relative humidity; at 26°C, about 70%.

How to measure and control VPD

Measure: a digital hygrometer or the ThermoPro TP358 shows temperature and humidity to read against the chart. See all hygrometers.
Automate: the G.A.S Intelligent Humidity Controller adjusts humidity according to VPD; the AC Infinity AI+ Controller monitors VPD and runs fans, Cloudforge humidifiers and the Thermoforge VPD heater.
VPD too low (too humid): more extraction, a dehumidifier, or a little more heat.
VPD too high (too dry): a humidifier, less extraction, or bring the temperature down.

Getting the extraction right first makes all of this easier: see how to size a fan and carbon filter.

Quick answers

Is VPD better than just watching humidity?

It's more useful, because it takes temperature into account. The same humidity can be fine at one temperature and too damp or too dry at another.

What is a good VPD for flowering?

Growers commonly aim for about 1.0-1.5 kPa, which keeps the air dry enough to cut the risk of mould.

More Growers' Guides

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