Cannabis VPD Calculator

Phase-specific VPD targets with leaf temperature correction. See exactly where you stand and what to adjust.

Vapor Pressure Deficit is the single best number for dialing in your grow room environment. It captures the relationship between temperature, humidity, and leaf surface conditions in one reading that tells you whether your plants can actually transpire. Most VPD charts ignore leaf temperature, which throws your target off by 0.1 to 0.3 kPa. This calculator corrects for it.

Set your growth phase, enter your air temperature and humidity, and adjust the leaf temperature offset for your lighting setup. The calculator shows your current VPD against phase-specific targets and tells you exactly what to adjust. For a deeper look at the science, read the full cannabis VPD guide.

Your Environment

Clones
Vegetative
Early Flower
Late Flower
Air Temperature 78°F
60°F 95°F
Relative Humidity 55%
20% 90%
Leaf Temp Offset -3°F
-10°F +2°F

Leaf surface is typically 2-5°F cooler than air under grow lights. Use 0 if measuring leaf temp directly.

Leaf Temperature 75°F

Your VPD

1.22kPa
OPTIMAL
0.0 0.4 0.8 1.2 1.6 2.0+

Phase Targets

Phase Target VPD Ideal RH*

*Ideal RH calculated from your current air temp and leaf offset to hit the midpoint of each target range.

What is VPD?

Vapor Pressure Deficit (VPD) measures the drying power of the air around your plants. It is the difference between the amount of moisture the air currently holds and the maximum it could hold at that temperature.

VPD matters because it controls transpiration. Transpiration is the engine that pulls water and nutrients from roots to leaves. Too little VPD (humid air) and plants cannot transpire, slowing growth and inviting mold. Too much VPD (dry air) and plants lose water faster than roots supply it, stomata close, and photosynthesis stops.

Why Leaf Temperature Matters

Most VPD charts use air temperature, but transpiration happens at the leaf surface. Under HPS or LED grow lights, leaf temperature typically runs 2 to 5°F below air temperature. This difference changes your real VPD by 0.1 to 0.3 kPa. Ignoring it means your adjustments are based on the wrong number.

VPD Across Growth Phases

VPD and Yield

Environment is one of the five dimensions that determines batch quality. VPD that stays in the right range for each phase means consistent transpiration, steady nutrient delivery, and plants that spend energy building flowers instead of fighting stress. Growers who track VPD alongside their other environmental data can spot the patterns that separate a good run from a great one.

VPD Is One Piece of the Puzzle

VPD is a number. Growgoyle watches the whole canopy — every photo, every reading, every note — and ends each run with a plan for the next one. Start with the batch you're already growing; finishing-phase plants are where a missed call costs the most.

Growgoyle doesn't replace your sensors. It analyzes your data and tells you what it means.

Try AI Plant Analysis Free

Frequently Asked Questions

What is VPD in cannabis cultivation?
VPD (Vapor Pressure Deficit) measures the drying power of the air. It is the difference between the moisture the air holds and the maximum it could hold at a given temperature. In cannabis cultivation, maintaining proper VPD is critical for healthy transpiration, nutrient uptake, and preventing mold or drought stress.
What is the ideal VPD for flowering cannabis?
Early flower (weeks 1-4) targets 1.0 to 1.4 kPa. Late flower (weeks 5 through harvest) targets 1.2 to 1.6 kPa. Higher VPD in late flower drives resin production and reduces mold risk as buds become dense.
Why does leaf temperature matter for VPD?
Transpiration happens at the leaf surface, not in the surrounding air. Leaf temperature is typically 2 to 5°F below air temperature under grow lights. Using air temperature alone overestimates your actual VPD by 0.1 to 0.3 kPa, which can lead to incorrect environmental adjustments.
Should I adjust temperature or humidity to fix VPD?
If VPD is too low (too humid), the most energy-efficient fix is usually lowering humidity with dehumidification. If VPD is too high (too dry), raising humidity is generally easier than lowering temperature. In sealed rooms with CO2 supplementation, higher temperatures (up to 85°F) with moderate humidity often achieve ideal VPD ranges.
How often should I check VPD in a commercial grow room?
VPD changes throughout the day as lights cycle, HVAC runs, and plants transpire. Continuous sensor monitoring is ideal. At minimum, check VPD at lights-on, mid-day (peak transpiration), and before lights-off. Those three points are where VPD swings the most.
What happens if VPD is too low?
Low VPD (below 0.4 kPa) means the air is nearly saturated. Plants cannot transpire efficiently, slowing nutrient uptake and growth. Standing moisture on leaf surfaces creates ideal conditions for powdery mildew, botrytis, and other fungal pathogens. This is especially dangerous in dense canopies during late flower.
What happens if VPD is too high?
High VPD (above 1.6 kPa) means the air is very dry. Plants transpire faster than roots can supply water, causing stomata to close. This shuts down photosynthesis, slows growth, and can cause leaf curling, tip burn, and wilting. Persistent high VPD leads to smaller yields.

These tools exist because I needed them. I'm Eric, commercial grower and software engineer in Michigan. I built Growgoyle to run my own facility and these calculators are just a piece of it. If you're running a grow and want to talk shop, text me.

Text me: 616-221-9856  ·  info@growgoyle.ai

- Eric