Grow Calculator
VPD Calculator
Vapor Pressure Deficit is the single best number for dialing in your tent's climate. Enter your temperature and humidity to see what your plants actually feel.
1.16 kPa
In range
Target for vegetative: 0.8-1.2 kPa. This is the sweet spot for the selected stage. Hold it steady day and night.
How this VPD calculator works
VPD measures the difference between how much moisture the air is holding and how much it could hold at the leaf's temperature. It is the force pulling water — and the calcium and magnesium dissolved in it — up through your plant.
We compute saturation vapour pressure with the Tetens equation at both leaf temperature and air temperature, then subtract the air's actual vapour pressure. Because water evaporates off the leaf and not out of the air, leaf temperature is what sets the ceiling — that is why the leaf offset field matters.
Target VPD by stage
- Seedlings and clones: 0.4-0.8 kPa — they have almost no roots to replace lost water
- Vegetative: 0.8-1.2 kPa — maximum growth rate without stress
- Early flower: 1.0-1.4 kPa — push transpiration as stretch finishes
- Late flower: 1.2-1.6 kPa — dry air protects dense buds from rot
Fix VPD by changing the input you have the most control over. Too low usually means a dehumidifier or more exhaust. Too high usually means a humidifier or slightly lower temps.
Common questions
- What VPD should cannabis be grown at?
- Roughly 0.4-0.8 kPa for seedlings and clones, 0.8-1.2 kPa in vegetative growth, 1.0-1.4 kPa in early flower, and 1.2-1.6 kPa in late flower. Higher VPD late in flower keeps humidity down and reduces bud rot risk.
- Is VPD better than just watching humidity?
- Yes. Relative humidity alone is meaningless without temperature. 60% RH at 68F and 60% RH at 82F create completely different transpiration pressure on the plant. VPD combines both into one number that reflects what the leaf actually experiences.
- Why does leaf temperature matter for VPD?
- Water evaporates from the leaf surface, not from the air, so the leaf's temperature sets the saturation point. Under LEDs leaves usually run about 2-4F below air temperature. Under HPS they can run above air temperature. Measure with an infrared thermometer for the accurate number.
- What happens if VPD is too low?
- Transpiration slows down, which slows calcium and magnesium uptake because those nutrients move with the water stream. You get soft growth, slower development, and much higher mold and powdery mildew pressure.
- What happens if VPD is too high?
- The plant closes its stomata to stop losing water. Photosynthesis drops, growth stalls, leaves taco or curl upward, and leaf edges can crisp. Growers often mistake this for a nutrient problem.