What is my VPD? A calculator that does not guess your leaf temperature
Air temperature and humidity in, vapour pressure deficit out, judged against the stage band. Add a measured leaf temperature and it shows the leaf number beside the air one. Every reading has its own link.
By Croplock — the build’s voice, not the founder’s. Curtis Olson, who builds it, is a retired software developer and home grower: about the author. · Last reviewed
Calculator
How to read it
- Air VPD is what a temperature-and-humidity meter can tell you: how far the air is from saturated. It is the number every VPD chart is printed in.
- Leaf VPD is the pull on the plant itself: the saturated pressure at the leaf’s own temperature, minus the water actually in the air. It only appears here if you typed in a leaf temperature you measured. When the leaf field is blank the page says so and judges the air number, because inventing a leaf temperature is worse than admitting you do not have one.
- The band line tells you the humidity range that puts this air inside the stage band. Read it before touching the humidifier: sometimes the answer is that no humidity gets there and the temperature is the lever.
Why there is no “leaf runs a few degrees cooler” box
Most online calculators offer one, and some quietly apply it by default. We used to carry that rule of thumb ourselves and took it out after reading the paper it was supposedly derived from. Nelson & Bugbee (2015, PLOS ONE) measured leaf against air under sunlight, HPS and LED: for a well-watered plant the leaf sat within about two degrees of the air under every light, and under water stress with high light and low airflow it ran hotter than the air by several degrees. The gap moves with the plant, not the lamp, and it changes sign. A fixed offset is right on the easy days and wrong in the wrong direction on the day the plant is in trouble. So the leaf field is optional and empty by default. Our own tent produced a 0.42 kPa disagreement between the two numbers one morning last August; the page on leaf temperature and VPD goes through it.
Where the bands come from
The four bands are the ones our device steers to, and they are grower-practice canon rather than experimental results. The vegetative band (0.8–1.2 kPa) is the one the field’s controlled-environment text prints in its own words, and its flowering table sits higher for a stated reason: a drier canopy gives bacteria and fungi less to work with inside a developing bud. Two honest caveats. Those tables are indexed by air temperature and humidity, so the numbers were authored in the air frame. And neither table covers a seedling or a cutting; the 0.4–0.8 kPa propagation band has no textbook behind it, and our own seedling went chlorotic after nine days held inside it, which is why we ramp out of it after the first week.
The formula, so you can check the page
Saturation vapour pressure by the Magnus–Tetens approximation, with the constants Murray (1967) gave for water over a liquid surface:
SVP(T) = 0.61078 × exp( 17.27 × T / (T + 237.3) ) kPa, T in °C Air VPD = SVP(Tair) × (1 − RH / 100) Leaf VPD = SVP(Tleaf) − SVP(Tair) × RH / 100
Results are clamped at zero: a leaf cooler than near-saturated air gives a negative difference on paper, and the plant cannot feel a negative deficit. These are the same three constants and the same clamp our device uses, and the test that ships with this page reads them out of the device’s source so the two cannot drift apart.
Sharing a reading
Everything in the form is in the page address. Type your numbers, press the copy button, and paste the link into a thread: it opens on your reading, with your stage selected, for whoever you are asking. For example, 77 °F and 60 % in veg is above the band, and the same air with a 73.4 °F leaf is inside it.
Where our device fits, stated narrowly
Croplock reads leaf-surface temperature from an infrared sensor in the canopy and steers to the leaf VPD, not a VPD worked out from the air. When it cannot get a trustworthy leaf reading it falls back to air VPD and records that it did. This page is the same arithmetic with the leaf temperature typed in by hand; it is not the device, and it makes no claim about the device beyond sharing its formula.
Sources
- Murray, F.W. (1967). On the computation of saturation vapor pressure. Journal of Applied Meteorology 6(1): 203–204 — the 0.61078 / 17.27 / 237.3 constants for water over liquid.
- Nelson, J.A. & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLOS ONE 10(10): e0138930. DOI 10.1371/journal.pone.0138930 — why no fixed leaf offset.
- Stasiak, M. & Dixon, M. (2022). Environmental control. In Zheng, Y. (ed.), Handbook of Cannabis Production in Controlled Environments, ch. 2, CRC Press — the 0.8–1.2 kPa vegetative guidance (section 2.3.2.1, Tables 2.3 and 2.4) and the flowering rationale.
- The stage bands are the ones in our device’s cannabis profile; the propagation band and the 0.2 kPa warning margin are grower-practice canon and are labelled as such there too.
More of these at Answers. The device is at croplock.com.
Also read: Does VPD need leaf temperature, or is air close enough? · Where should the probe go? · Bud rot: how humid, for how long?