Plant Care
- • Climate Control & VPD16
- • Disease & Pest Management14
- • Nutrient Solutions & EC34
- • Plant Health Monitoring7
VPD & Leaf Temperature Engine
True Leaf VPD calculator. Applies the real leaf temperature offset (LEDs run leaves ~2C cooler), altitude pressure correction, dissolved-oxygen risk and air-exchange context so you hit the exact kPa your stage demands.
True leaf VPD
live
leaf vpd
0.75
Leaf saturation VP
2.81
Vapour already in air
2.06
Dashed arc — the share of leaf saturation vapour pressure the air has already filled. True leaf VPD holds in the centre.
Inputs
6 parameters · live recalculation on every change
-10C
Optimal: 22 - 27C
50C
Nominal
0%
Target: 55 - 75%
100%
0.80 - 1.20 kPa
Profile: Aeroponic Root Chamber #04
Advanced Settings — Deterministic Corrections
LED Canopy: -2.0C
-5C
0.0C midpoint
5C
M
0m
0m default
5000m
Insight band · live
From saturation to true leaf VPD
Leaf saturation vapour pressure, the vapour the air already carries, and the genuine deficit the plant feels.
- Leaf saturation VP 2.81 2.81 kPa
- − Vapour already in air 2.06 0.75 kPa
- True leaf VPD 0.75 0.75 kPa
Air VPD: 1.11 kPa
Air saturation VP: 3.17 kPa
Running total after each term — the final net matches the headline reading in the result card, kept in sync live.
Deterministic model
6 inputs · 6 outputs
Runs 100% in your browser
This is a deterministic model: the same inputs always produce the same outputs, calculated entirely in your browser with nothing sent to any server. It is a planning aid — not a quote, not a dosing instruction, and not a yield forecast. Treat every line as a working estimate and validate it against your own prices and measurements.
It models one system: True Leaf VPD calculator. Applies the real leaf temperature offset (LEDs run leaves ~2C cooler), altitude pressure correction, dissolved-oxygen risk and air-exchange context so you hit the exact kPa your stage demands.
What the VPD & Leaf Temperature Engine returns
The engine computes 6 outputs from your 6 inputs. The headline number is True Leaf VPD (kPa), alongside Air VPD, Sat. Vapour Pressure (Air), Sat. Vapour Pressure (Leaf). Every output updates live as you change a parameter.
What it does not do
Outputs depend on the inputs and assumptions shown here. Confirm relevant measurements and local conditions before acting.
STEP 01
Measure air temperature, relative humidity, growth stage, altitude and leaf temperature offset.
STEP 02
Convert temperature into degrees Celsius if working in Fahrenheit.
STEP 03
Compute saturation vapour pressure of the air using the Tetens equation: SVP = 0.6108 · e^(17.27T / (T + 237.3)).
STEP 04
Compute leaf saturation vapour pressure using the leaf temperature (air temperature + leaf offset).
STEP 05
Compute actual vapour pressure: AVP = SVP(air) × RH / 100.
STEP 06
True leaf VPD = SVP(leaf) − AVP; apply the altitude pressure factor, then classify against the growth-stage target.
e_s(T_a) = 0.6108 \cdot \exp\left( \dfrac{17.27\,T_a}{T_a + 237.3} \right)
Saturation vapour pressure at air temperature — Tetens (kPa)
e_s(T_l) = 0.6108 \cdot \exp\left( \dfrac{17.27\,(T_a + \Delta T)}{(T_a + \Delta T) + 237.3} \right)
Saturation vapour pressure at leaf temperature (leaf offset applied)
e_a = e_s(T_a) \times \frac{RH}{100}
Actual vapour pressure in the air
\text{VPD}_{air} = e_s(T_a) - e_a
Air VPD — the reading most calculators stop at
\text{VPD}_{leaf} = e_s(T_l) - e_a
True leaf VPD — the number that drives stomatal behaviour
P = 101.325\left(1 - 2.25577 \times 10^{-5}\,z\right)^{5.2559}
Atmospheric pressure at altitude z (kPa) — scale SVP by P / 101.325
SVP values are scaled down by the altitude pressure factor P/101.325 — VPD falls noticeably above ~1,000 m.
LED-lit leaves typically run 1–3 °C cooler than air because direct radiative loading is low; the default −2 °C offset is conservative.
VPD only tells part of the story: pair with dissolved-oxygen risk (warm water holds less O₂) and air-exchange (CFM) in do_risk and cfm modes.
Status colors follow the grower target: clone 0.4–0.8, veg 0.8–1.2, flower 1.2–1.6 kPa.
- Jones, H.G. (2013). Plants and Microclimate. Cambridge University Press.
- NCSU CALS — Vapor Pressure Deficit (VPD) resources.
This model combines the earlier Vpd-calculator, Water-temp-do-risk, Cfm-air-exchange, Dwc-quiet-setup models.
Q1: What is the difference between air VPD and true leaf VPD? ▾
Air VPD compares saturated vapour pressure at air temperature with the actual vapour pressure in the air. True leaf VPD compares saturation at the actual leaf temperature — almost always cooler or warmer than air — with the same air vapour pressure. Because LEDs run leaves ~2 °C cooler, air VPD routinely overstates the true drying gradient your leaves feel by 0.1–0.3 kPa.
Q2: Why does the leaf temperature offset matter so much? ▾
VPD drives stomatal opening, which controls transpiration and nutrient uptake. A 2 °C offset between leaf and air temperature shifts leaf VPD outside the ideal band and quietly throttles transpiration. Accounting for the offset is the difference between "looks right on paper" and a grow that actually responds.
Q3: How does altitude affect VPD? ▾
Higher altitude means lower atmospheric pressure, so saturation vapour pressure falls and the air holds less moisture at the same temperature. At 2,000 m the correction removes roughly 20% of SVP, which lowers leaf VPD and can push you out of range — this engine applies the full barometric formula automatically.
Q4: What is the ideal leaf VPD for each growth stage? ▾
Clones root best at 0.4–0.8 kPa, vegetative growth is strong at 0.8–1.2 kPa, and flowering benefits from a firmer 1.2–1.6 kPa. Below 0.4 kPa transpiration stalls and disease risk climbs; above 1.6 kPa stomata clamp shut and salt stress compounds.