VPD Calculator

Tools
Your tent right now
Air temperature25°C
15°C40°C
Relative humidity55%
20%95%
Leaf temp offset-2°C
-5°C+2°C

Leaf temp: 23.0°C — leaves run a couple of degrees cooler than the air. Leave this alone unless you've measured it.

Vapour pressure deficit
1.06
kPa
In the zone
Where you sit
What to do about it
What is VPD?

VPD — Vapour Pressure Deficit. It sounds like something a physicist invented to torture growers. It's not. It's just how thirsty the air is.

VPD is the difference between how much water the air could hold at your temperature and how much it actually holds. When the gap is small (humid air), your plant's transpiration stalls — she can't move water, nutrient uptake slows, and mould starts eyeing up your densest bud. When the gap is wide (dry air), she loses water faster than the roots can replace it, closes her stomata, and growth stalls the other way.

Get it in the band for your stage and she transpires properly: water and nutrients move, leaves work, nothing rots. The leaf-temp offset is there because leaves run a degree or two cooler than the air — evaporation cools them, same as it cools you.

One number, checked daily, prevents the mistake that takes the fattest cola two weeks before harvest. Ask anyone who's found the grey fuzz.

VPD, vapour pressure deficit, is just how thirsty the air is, and it decides how hard your plant drinks and breathes. Too low and she sits in a swamp; too high and she shuts the stomata and stalls. Put in your air temperature, your humidity and a leaf-temperature offset, and this gives you the VPD in kPa, whether that is in the sweet spot or heading for trouble, and what to change to fix it. It is the one number that ties your temperature and humidity together, and most room problems make more sense the moment you can see it.

Questions

What is a good VPD for veg and flower?

Roughly 0.8 to 1.2 kPa in veg — the calculator's sweet spot — climbing to about 1.2 to 1.6 kPa in late flower as the plant matures. Seedlings and fresh clones like it gentler, nearer 0.4 to 0.8; under 0.4 is a swamp and over 1.6 tips into stress. The calculator shows where your numbers land against those bands, so you are reading it off rather than guessing.

What is the leaf-temperature offset?

Leaves usually run a touch cooler than the air — often around 1 to 3 °C under LED — and VPD is really about the leaf, not the room. The offset lets you tell the calculator that gap so the kPa it returns is honest. If you have no leaf thermometer, a 2 °C offset is a fair default to start from.

My VPD is too high — what do I change?

Bring the leaf and air closer, or add moisture. Raise humidity, drop the temperature a little, or do both; the calculator moves with each change so you can see what gets you back into the band. High VPD shows as thirsty, taco-ing leaves and slow growth. Move in small steps — chase it hard and you overshoot the other way.

Where do I measure humidity?

At the canopy, where the plant actually lives, not by the floor or against the tent wall. A cheap hygrometer at bud height is enough. Feed the calculator the numbers from there and the VPD will match what your plant feels, which is the whole point of working it out at all.