VPD Reference Table (kPa)
Calculated with leaf temp = air temp − 2°C.■ Seedling/Veg optimal ·■ Flowering ·■ Stress zone
| RH \ Temp | 18°C | 20°C | 22°C | 24°C | 26°C | 28°C | 30°C |
|---|---|---|---|---|---|---|---|
| 85% | 0.06 | 0.08 | 0.09 | 0.11 | 0.13 | 0.15 | 0.17 |
| 80% | 0.17 | 0.19 | 0.22 | 0.26 | 0.29 | 0.34 | 0.39 |
| 75% | 0.27 | 0.31 | 0.36 | 0.41 | 0.46 | 0.53 | 0.60 |
| 70% | 0.37 | 0.43 | 0.49 | 0.56 | 0.63 | 0.72 | 0.81 |
| 65% | 0.48 | 0.54 | 0.62 | 0.70 | 0.80 | 0.90 | 1.02 |
| 60% | 0.58 | 0.66 | 0.75 | 0.85 | 0.97 | 1.09 | 1.23 |
| 55% | 0.68 | 0.78 | 0.88 | 1.00 | 1.14 | 1.28 | 1.45 |
| 50% | 0.79 | 0.89 | 1.02 | 1.15 | 1.30 | 1.47 | 1.66 |
| 45% | 0.89 | 1.01 | 1.15 | 1.30 | 1.47 | 1.66 | 1.87 |
Zone Reference
Frequently Asked Questions
What is a VPD chart used for?
A VPD chart shows the vapor pressure deficit at any combination of temperature and relative humidity. Growers use it to quickly verify whether their current environment is in the optimal range for their growth stage without doing manual calculations.
How do I read a VPD chart?
Find your temperature on the horizontal axis and your relative humidity on the vertical axis. The value where they intersect is your VPD in kPa. Cross-reference with the zone color: green for optimal growth zones, yellow for acceptable but monitor closely, red for stress zones.
Why does leaf temperature matter on a VPD chart?
Most VPD charts show air temperature, but VPD is actually calculated using leaf temperature. Leaves are typically 1–3°C cooler than the surrounding air. Our interactive calculator uses a configurable leaf offset (-2°C by default), giving more accurate results than static charts.
What color zones are on a VPD chart?
Common VPD charts use: blue (0–0.4 kPa) for under-transpiration risk, green (0.4–1.2 kPa) for seedling and vegetative optimal range, yellow (1.0–1.6 kPa) for flowering range, and red (above 1.6 kPa) for stress zone where plants lose water faster than they can replace it.