Cannabis VPD Calculator

Vapour pressure deficit (VPD) is how hard the air pulls water out of your plants. Set your grow room temperature and humidity to get your VPD in kPa, whether it suits seedlings, vegetative growth or flower, and what to change if it does not. Target ranges: 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa in veg, 1.2–1.5 kPa in flower. Below 0.4 kPa the air is too humid, stomata close and mould risk climbs; above 1.5 kPa the plant loses water faster than its roots can replace it and leaf edges curl upward.

The chart below is calculated at leaf temperature, 2°C below air temperature, which is the usual assumption under LED. A chart built on air temperature alone reads high.

Cannabis VPD chart (°F)

VPD in kPa at leaf temperature, assuming the canopy runs 2°C below air temperature. Find your air temperature down the left, your relative humidity across the top, and read the cell where they meet.

Air temp °F	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%
64°F	1.18	1.08	0.98	0.88	0.77	0.67	0.57	0.47	0.37	0.27	0.16
66°F	1.27	1.16	1.05	0.94	0.83	0.72	0.61	0.51	0.40	0.29	0.18
68°F	1.36	1.25	1.13	1.01	0.89	0.78	0.66	0.54	0.43	0.31	0.19
70°F	1.46	1.34	1.21	1.09	0.96	0.84	0.71	0.59	0.46	0.33	0.21
72°F	1.57	1.43	1.30	1.16	1.03	0.90	0.76	0.63	0.49	0.36	0.23
74°F	1.68	1.54	1.39	1.25	1.11	0.96	0.82	0.68	0.53	0.39	0.25
76°F	1.80	1.64	1.49	1.34	1.18	1.03	0.88	0.72	0.57	0.42	0.26
78°F	1.92	1.76	1.60	1.43	1.27	1.10	0.94	0.78	0.61	0.45	0.29
80°F	2.06	1.88	1.71	1.53	1.36	1.18	1.01	0.83	0.66	0.48	0.31
82°F	2.20	2.01	1.82	1.64	1.45	1.27	1.08	0.89	0.71	0.52	0.33
84°F	2.35	2.15	1.95	1.75	1.55	1.35	1.15	0.96	0.76	0.56	0.36
86°F	2.51	2.29	2.08	1.87	1.66	1.45	1.23	1.02	0.81	0.60	0.39
88°F	2.68	2.45	2.22	2.00	1.77	1.54	1.32	1.09	0.87	0.64	0.41
90°F	2.85	2.61	2.37	2.13	1.89	1.65	1.41	1.17	0.93	0.69	0.45

• Under 0.4 — too humid
• 0.4–0.8 — seedling & clone
• 0.8–1.2 — vegetative
• 1.2–1.5 — flower
• Over 1.5 — too dry

Cannabis VPD chart (°C)

VPD in kPa at leaf temperature, assuming the canopy runs 2°C below air temperature. Find your air temperature down the left, your relative humidity across the top, and read the cell where they meet.

Air temp °C	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%
18°C	1.20	1.10	0.99	0.89	0.79	0.68	0.58	0.48	0.37	0.27	0.17
19°C	1.28	1.17	1.06	0.95	0.84	0.73	0.62	0.51	0.40	0.29	0.18
20°C	1.36	1.25	1.13	1.01	0.89	0.78	0.66	0.54	0.43	0.31	0.19
21°C	1.45	1.33	1.20	1.08	0.95	0.83	0.71	0.58	0.46	0.33	0.21
22°C	1.55	1.41	1.28	1.15	1.02	0.88	0.75	0.62	0.49	0.36	0.22
23°C	1.64	1.50	1.36	1.22	1.08	0.94	0.80	0.66	0.52	0.38	0.24
24°C	1.75	1.60	1.45	1.30	1.15	1.00	0.85	0.70	0.56	0.41	0.26
25°C	1.86	1.70	1.54	1.38	1.23	1.07	0.91	0.75	0.59	0.43	0.28
26°C	1.98	1.81	1.64	1.47	1.30	1.14	0.97	0.80	0.63	0.46	0.29
27°C	2.10	1.92	1.74	1.56	1.39	1.21	1.03	0.85	0.67	0.49	0.32
28°C	2.23	2.04	1.85	1.66	1.47	1.28	1.09	0.90	0.72	0.53	0.34
29°C	2.36	2.16	1.96	1.76	1.56	1.36	1.16	0.96	0.76	0.56	0.36
30°C	2.51	2.29	2.08	1.87	1.66	1.45	1.23	1.02	0.81	0.60	0.39
31°C	2.66	2.43	2.21	1.98	1.76	1.53	1.31	1.09	0.86	0.64	0.41
32°C	2.82	2.58	2.34	2.10	1.87	1.63	1.39	1.15	0.91	0.68	0.44

• Under 0.4 — too humid
• 0.4–0.8 — seedling & clone
• 0.8–1.2 — vegetative
• 1.2–1.5 — flower
• Over 1.5 — too dry

Frequently Asked Questions

What is the ideal VPD for cannabis?

Roughly 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa through vegetative growth, and 1.2–1.5 kPa in flower. Below 0.4 kPa transpiration stalls; above 1.5 kPa the plant loses water faster than the roots can replace it.

How do you calculate VPD?

Saturation vapour pressure at leaf temperature minus the actual vapour pressure of the air. This calculator uses the Tetens equation, subtracts a leaf-temperature offset (2°C under LED by default), and multiplies air saturation by relative humidity.

Should I use leaf temperature or air temperature for VPD?

Leaf temperature. A transpiring canopy under LED sits about 2°C below air temperature, so an air-temperature chart reads high. Set the offset to 0 for the air-temperature figure.

What happens if VPD is too high or too low?

Too low and stomata close, nutrient uptake slows and mould risk climbs. Too high and leaf edges curl upward as the plant loses water faster than the roots supply it, and growth stalls.

More free grow tools

• Cannabis yield calculator
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• AI plant doctor — diagnose plant problems
• The full VPD guide
• Grow tent humidity control

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