VPD calculator: leaf vapour pressure deficit from temperature and humidity

Type 26 °C and 60% RH into a wall chart and you get an air vapour pressure deficit (VPD) of 1.34 kPa, comfortably inside most flowering targets. Point an infrared thermometer at the canopy, though, and under LED lighting the leaves usually read 1–3 °C cooler than the air. Redo the sum at leaf temperature and the true deficit drops to 0.96 kPa, a full corridor lower. The calculator below runs both numbers side by side so the gap is visible instead of hidden inside one figure.

Enter a reading: temperature, humidity and leaf offset

Air temperature and relative humidity (RH) come from any hygrometer. The leaf-to-air offset is the harder number to get right: most growers start at −2 °C under LED and correct it once they have actually measured a leaf. The tool reports leaf VPD next to air VPD (the same reading with the offset forced to zero), so you can see exactly what the offset is costing you.

Calculator

Air temperature°CRelative humidity%Leaf-to-air temperature offset°C

Leaf VPD0.97kPa

Air VPD (offset = 0)1.34kPa

Formula [1]

SVP(T) = 0.61078 × exp(17.27 × T / (T + 237.3)) kPa (Tetens, per Murray 1967). VPD_leaf = SVP(T_air + leaf_offset) − SVP(T_air) × RH/100. VPD_air = SVP(T_air) × (1 − RH/100)

The formula behind the two numbers

Both outputs come from the Tetens saturation vapour pressure equation, in the form Murray checked against more exact reference formulas ^[1]:

SVP(T) = 0.61078 × exp(17.27 × T / (T + 237.3))      kPa, T in °C
VPD_air  = SVP(T_air) × (1 − RH/100)
VPD_leaf = SVP(T_air + leaf_offset) − SVP(T_air) × RH/100

Murray found this form of the equation accurate to under 0.1% against the more exact Goff–Gratch reference for any temperature from −5 °C upward ^[1], which comfortably covers every grow room you will ever measure. Buck's 1981 refinement adds a small enhancement-factor correction, around 0.4–0.5% at sea-level pressure and fairly flat across the room-temperature range, that matters more for meteorological work at low pressure (high altitude) than for a sealed tent at ground level ^[2]; at ordinary indoor conditions it moves the answer by less than a typical hygrometer can even resolve, which is why the simpler Tetens form is what the calculator above uses.

Work it by hand: 26 °C, 60% RH, a −2 °C offset

Saturation pressure at air temperature. SVP(26 °C) = 0.61078 × exp(17.27 × 26 / 263.3) = 3.36 kPa. That is the most vapour pressure the air could hold if it were fully saturated.
Air VPD. The air already holds 60% of that: 3.36 × 0.60 = 2.02 kPa. Air VPD = 3.36 − 2.02 = 1.34 kPa.
Saturation pressure at leaf temperature. With a −2 °C offset the leaf sits at 24 °C. SVP(24 °C) = 2.98 kPa.
Leaf VPD. 2.98 − (3.36 × 0.60) = 2.98 − 2.02 = 0.96 kPa.

Type those same three numbers into the calculator above and you should see 0.96 kPa and 1.34 kPa. If your own numbers land elsewhere, check that you typed the measured RH, not a target, and that the offset carries its sign (−2, not 2).

Why a 2 °C offset is worth 0.4 kPa

That 0.4 kPa gap is large enough to flip which stage corridor a reading falls into. As air VPD, 1.34 kPa reads as a late-flower number. As leaf VPD, the same room at the same instant reads 0.96 kPa, a vegetative number. A controller or a logbook set from an air-temperature chart is dehumidifying, or adding humidity, for a deficit the leaves never actually experience. Fixture type sets the direction of the error: LEDs put less infrared onto the canopy than HPS, so LED-lit leaves usually run cooler than the air, while HPS-lit leaves can run level with it or slightly warmer.

Before you compare a reading against a target, it helps to know roughly where the targets sit.

Stage	Leaf VPD target (kPa)
Propagation	0.4–0.8
Vegetative	0.8–1.2
Early flower	1.0–1.5
Late flower	1.2–1.6

What the evidence saysWhere these corridors come from

These bands are grower and equipment-manufacturer convention, repeated across controller software and grow guides, not a peer-reviewed optimum measured on cannabis. Grossiord and co-authors' review of VPD physiology gives the underlying mechanism: stomatal conductance falls as VPD rises, a pattern seen across species, though how sharply it falls varies a great deal from one species to the next ^[3]. Transpiration itself is not simply the mirror image of that: in most species it keeps rising with VPD, past the point where stomata are already closing, and only turns down once closure becomes severe enough to outpace the rising deficit — which is closer to what "too dry" means for a plant than a straightforward fall in water loss. Nobody has yet published a cannabis dose-response trial that pins these exact bands down to two decimal places, so treat them as a place to start, not a number to chase precisely. Elevated CO₂ shifts the picture further, since stomata can partly close at a given VPD without cutting carbon uptake; see CO₂, VPD and stomata if you are running enrichment.

Fig. 1The marked point is the worked example. The shading is leaf VPD, not air VPD, so the same room reads differently here than on an air-only chart.Horus

Fig. 2The same 0.96 kPa reading placed on a single scale: inside the vegetative target, one band below where the air-only number would land.Horus

Two ways to get the offset wrong

The most common input mistake is leaving leaf offset at 0, which quietly assumes the leaf is exactly as warm as the surrounding air. Under LED that is rarely true. The second mistake is typing in the RH you are aiming for rather than the RH your hygrometer is actually showing: the calculator can only tell you what a real reading means, it cannot tell you whether your target is sensible. If you are also tuning light intensity, the DLI and PPFD calculator covers the other half of that decision.

When to stop trusting the default and measure

A −2 °C offset is a starting assumption, not a measurement. Checking a handful of mid-canopy leaves with an infrared thermometer once a week is usually enough to confirm or correct it; see measuring leaf temperature against air temperature for how to do that without picking up heat from the fixture by mistake. Where the sensor points matters more than any of the arithmetic above: a probe reading the tent wall, the pot rim or a leaf directly under a hot fixture is not reading a representative leaf, and that placement error dwarfs the formula's own 0.1% accuracy.

Once you trust the leaf VPD number, it feeds directly into the next decision: dehumidification should match the transpiration load implied by that target, not the room's volume. See the dehumidifier sizing calculator for that step. None of this changes what you may legally grow or where: check the law section for the rules that apply to you before you act on any of it.