VPD Calculator (Vapor Pressure Deficit)
Find the ideal temperature and humidity sweet spot for your plants.
Result
VPD targets are general guidance. Optimal ranges vary by species, genetics and CO₂ level — use this as a starting point and observe your plants.
History
What it is
Vapor Pressure Deficit (VPD) measures the drying power of the air around your plants — the gap between how much moisture the air is holding and the maximum it could hold at leaf temperature. It combines temperature and humidity into a single number (in kPa) that predicts how fast plants transpire, which directly controls nutrient uptake, stomatal behaviour and disease risk.
Who should use it
Indoor and greenhouse growers running sealed rooms or grow tents who want to fine-tune temperature and humidity together instead of chasing RH alone. It is especially useful for anyone using humidifiers, dehumidifiers or AC and wanting a single target to dial in per growth stage.
How to use it
- Enter your air temperature (the calculator shows °F or °C based on your region).
- Enter the relative humidity (%) from your hygrometer.
- Set the leaf temperature offset — leaves are usually 1–3°C cooler than the air, so a value of about −2°C (−3.6°F) is a good default. Use 0 if you measure leaf temperature directly with an IR thermometer.
- Read the VPD result in kPa and check which growth-stage zone it falls into.
- Adjust temperature or humidity until the VPD lands in the target band for your current stage.
Example calculation
Worked example
At 78°F (25.6°C) air temperature, 60% relative humidity and a −3.6°F (−2°C) leaf offset, the calculator returns a VPD of about 0.94 kPa. That sits in the 0.8–1.2 kPa band, which is ideal for vegetative growth — so this room is well balanced for veg with no change needed.
Formulas used
SVP(T) = 0.61078 · e^(17.27·T / (T + 237.3)) • VPD = SVP(T_leaf) − SVP(T_air) · RH/100
How to read your result
Lower VPD means humid, low-transpiration air (good for clones, risky for mould in flower). Higher VPD means dry, high-transpiration air (can cause stress, leaf curl and calcium issues). Aim for the lower end of each range early in a stage and the higher end as plants mature.
Recommended ranges
| Growth stage | Target VPD (kPa) |
|---|---|
| Clones / seedlings | 0.4 – 0.8 |
| Early vegetative | 0.8 – 1.0 |
| Late vegetative | 1.0 – 1.2 |
| Early flower | 1.2 – 1.4 |
| Late flower | 1.4 – 1.6 |
Common mistakes to avoid
- Using air temperature for leaf temperature. Leaves run cooler, so ignoring the offset overstates VPD.
- Targeting a single VPD for the whole grow — clones, veg and flower each want a different band.
- Chasing humidity alone. 60% RH can be perfect or far too dry depending on temperature.
- Measuring RH near a wall or vent instead of at canopy height where the plants actually live.
Frequently asked questions
What is a good VPD for plants?
Roughly 0.4–0.8 kPa for clones and seedlings, 0.8–1.2 kPa for vegetative growth, and 1.2–1.6 kPa for flowering. These are general indoor targets; species and genetics shift the ideal.
Is high or low VPD better?
Neither extreme. Very low VPD slows transpiration and invites mould and mildew; very high VPD over-stresses plants and can cause wilting and nutrient lockout. The goal is the stage-appropriate middle band.
Should I use leaf temperature or air temperature?
Leaf temperature gives the most accurate VPD. If you cannot measure it, subtract about 2°C (3.6°F) from air temperature as an estimate via the leaf offset field.
Why does VPD matter more than humidity?
Humidity alone ignores temperature. The same 60% RH produces very different transpiration at 20°C versus 30°C. VPD captures both in one number.
Does VPD change between lights-on and lights-off?
Yes. Temperature usually drops in the dark period, so for the same humidity VPD falls. Many growers allow a slightly lower night VPD and avoid letting RH spike too high.