Enter temperature and humidity — the vapour pressure deficit of your grow tent is worked out instantly, with the target range for your growth stage. Free, no sign-up, with Fahrenheit conversion.
The target ranges below are air VPD — that is why the offset sits at 0. If you actually measure leaf temperature (infrared thermometer), enter the difference here: every degree Celsius takes roughly 0.15–0.20 kPa off the result. Reading Fahrenheit? Subtract 32, then multiply by 5/9 — for the leaf offset multiply by 5/9 alone, because it is a difference.
| Stage | VPD target range |
|---|---|
| Propagation / early veg | 0.4–0.8 kPa |
| Late veg / stretch | 0.8–1.2 kPa |
| Flower | 1.2–1.6 kPa |
The third column is not a separate setting — it is the result of the two before it. That is exactly why all three values sit in one row here: set temperature and humidity from two separate tables and you will almost certainly land outside your VPD target.
| Stage | Temperature (°C) | Humidity | VPD |
|---|---|---|---|
| Germination | 22–26 night 20–24 |
75–85 % | 0.4–0.8 |
| Seedling | 22–26 night 19–23 |
70–80 % | 0.4–0.8 |
| Vegetative | 22–27 night 18–22 |
55–70 % | 0.8–1.2 |
| Early flower | 20–26 night 17–21 |
48–58 % | 1.0–1.4 |
| Mid flower | 20–26 night 16–20 |
42–52 % | 1.2–1.6 |
| Late flower | 19–25 night 15–19 |
35–48 % | 1.2–1.6 |
| Flush | 18–24 night 15–19 |
35–45 % | 1.2–1.6 |
Reference values, not fixed rules — what your plant shows you is what counts.
These calculators give you reference values for your own judgement. They replace neither a measurement on site nor expert advice — all figures without guarantee.
Nearly every guide online gives you two tables: one for temperature, one for humidity. Each looks sensible on its own — and together they often describe a climate that neither table meant.
Here is an example that appears in guides word for word: a seedling at 75 °F (24 °C) and 70 % humidity. Both figures pass as correct. Put them through the formula and you get a VPD of 0.90 kPa — well above the 0.4 to 0.8 the same sources recommend for propagation. The advice contradicts itself, and nobody notices, because the two numbers are never worked out together.
The reason is physics: how much water air can hold rises exponentially with temperature. Warm air holds far more than cool air. The same 60 % humidity means a VPD of 0.94 kPa at 68 °F (20 °C) and 1.51 kPa at 82 °F (28 °C) — two completely different environments for the plant, while the hygrometer reads 60 % both times.
That gives you the way to actually work: you can set two of the three values, the third follows. In practice you fix temperature and humidity, then check where VPD lands — which is what the calculator above is for. If it sits wrong, change whichever is easier to control, and that is usually humidity.
This is why the table above puts all three values on one line. They are not a wish list but a matching set — every row has been worked out, not collected.
VPD stands for vapour pressure deficit. It describes how much more water vapour the air could still take up — in other words, how hard it pulls on the plant. It is given in kilopascals (kPa).
Temperature and humidity on their own say little, because they work together: 77 °F at 60 % feels nothing like 68 °F at 60 % to a plant. VPD folds both into a single number, which is why many growers steer their tent by it instead of by humidity alone.
A leaf is usually a little cooler than the air around it, because evaporating water carries heat away — the same effect as sweating. Strictly speaking it is the vapour pressure at the leaf surface that matters to the plant, not the one in the room.
Even so the leaf offset here sits at 0, and that is deliberate: the target ranges everyone quotes — including the ones in the table above — are air VPD figures. Combine them with a fixed 2 °C leaf deduction and you land roughly a whole zone too low, and you will run your room too humid.
If you genuinely measure leaf temperature with an infrared thermometer, enter the difference above. Every degree Celsius takes about 0.15 to 0.20 kPa off the result. Without a measurement, 0 is the more honest assumption: the gap shifts with lighting, air movement and water supply, and it is different under LED than under HPS, where the leaf can even run warmer than the air.
The target moves as the plant grows. Young plants with small root systems tolerate little pull and need a low VPD. Later it may and should rise, so the plant moves water and nutrients briskly.
A VPD that stays too low means damp, still air — the plant barely transpires, and in flower the risk of mould and botrytis climbs. A VPD that is too high dries it out faster than the roots can resupply: stomata close, CO₂ uptake collapses, growth stops.
At the top the target range ends at 1.6 kPa. Anything above that is no longer a stage but a danger zone — a common mistake is pushing deliberately to 1.8 or 2.0 in late flower. The figures in the table are reference points, not hard limits; keeping the range steady across the day matters more than hitting an exact number.
A VPD that is too high means the air pulls more water out of the plant than the roots deliver. There are two levers, and the faster one is almost always humidity.
Raise humidity — a humidifier in the tent works most directly. Cheaper options are wet towels or an open tray of water in front of the circulation fan; in a small tent that is often worth ten to fifteen percentage points. Grouping plants closer together uses their own transpiration: a dense canopy holds moisture far better than a single plant on a large footprint.
Throttle the exhaust — if the extraction fan runs too hard, it pulls humid air out faster than the plants create it. A fan controller, or a unit that switches on humidity, fixes that for good. Do not simply switch it off, or heat builds up and air exchange stops.
Lower the temperature — every degree down lowers VPD noticeably, because cooler air holds less water. With LED you can dim the fixture or raise it; that costs light, so it is the second choice — how much of the daily total you lose is what the PPFD to DLI calculator works out. In flower the order is: humidify first, touch temperature second.
A VPD that is too low is the more dangerous case. The plant barely transpires and therefore moves few nutrients — and in dense buds the risk of mould and botrytis rises. This one is worth acting on quickly.
Dehumidify and extract — a dehumidifier is the most reliable answer, especially in late flower when the plants themselves give off a lot of water. Increase the exhaust alongside it: humid air has to leave, dry air has to follow. Humidity usually climbs at night as the room cools — a great many problems start exactly there.
Move the air — still air right at the leaf forms a humid boundary layer, leaving the plant sitting in its own haze. Circulation fans that stir the canopy gently solve it without touching the climate at all. Do not aim them at the plants; aim above and below the canopy.
Raise the temperature — warmer air takes up more water, so VPD rises without a dehumidifier. This only works while you stay under the plant's upper limit. And defoliation helps, because less leaf mass gives off less water and air moves through the canopy more freely.
First the saturation vapour pressure at leaf temperature is worked out (Magnus formula: 0.6108 × e^(17.27 × T / (T + 237.3)), with T in °C). From that the actual vapour pressure of the air is subtracted — the saturation vapour pressure at air temperature multiplied by the relative humidity. The result is VPD in kPa. The formula only works in Celsius — convert a Fahrenheit reading first (see below).
Convert before you enter: subtract 32, then multiply by 5/9. So 77 °F becomes 25 °C, and 82 °F becomes 27.8 °C. For the leaf offset multiply by 5/9 alone and skip the 32 — it is a temperature difference, not a temperature, so 3.6 °F cooler is 2 °C cooler. The result stays in kPa either way: that is the unit VPD is defined in worldwide, in Fahrenheit countries too.
During stretch, meaning the first weeks of flower, the target sits around 1.0 to 1.4 kPa, and after that 1.2 to 1.6 kPa. The reason for the higher figures compared with veg is not growth but mould protection: humidity in flower has to stay below roughly 55 percent, and the VPD follows from that. Above 1.6 kPa the danger zone begins — the widespread advice to push to 1.8 or 2.0 towards the end cannot be supported and drives the plant into drought stress.
The table above gives all three values together for each of the seven stages: temperature under lights and at night, humidity, and the VPD that results. Roughly: from 75 °F (24 °C) and 80 percent at germination down to 70 °F (21 °C) and 40 percent at flush. Take the values of one row together — they are matched to each other.
No — not independently of one another. Of the three values temperature, humidity and VPD you can set two, and the third follows. The same 60 percent humidity means a VPD of 0.94 kPa at 68 °F (20 °C) and 1.51 kPa at 82 °F (28 °C). That is why two separate tables, one for temperature and one for humidity, regularly produce a climate neither of them intended.
A dehumidifier is the most effective step, followed by stronger extraction. Both lower humidity and therefore raise VPD. If you increase extraction, keep an eye on what it does to temperature: cooler air carries less water, so VPD drops again. In flower humidity belongs below roughly 55 percent, in late flower below 50 — above that bud rot (botrytis) becomes a real risk.
It states how much cooler the leaf is than the surrounding air. The calculator starts at 0, because the usual target ranges — including the ones in the table — apply to air VPD. If you measure leaf temperature with an infrared thermometer, enter the difference: every degree Celsius takes about 0.15 to 0.20 kPa off the result.
Below roughly 0.4 kPa the air is very humid and the plant barely transpires — a mould risk in flower. Above roughly 1.6 kPa it loses more water than the roots deliver; stomata close, CO₂ uptake falls and growth slows. From 2.0 kPa transpiration largely breaks down. In between, the growth stage decides.
Yes. This calculator uses exactly the same formula as the VPD calculator in the CRIS app, including leaf offset and zone boundaries. In the app you can also save the values with every journal entry and follow how they develop across the whole grow.
VPD, DLI, pH, EC, running costs and more — in the CRIS app, right next to your grow journal, fully offline and without an account.