MasterTool

VPD Calculator

Calculate vapour pressure deficit from air temperature and humidity with the Tetens formula, with a leaf-temperature offset.

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What this tool does

The VPD Calculator uses the Tetens equation for saturation vapour pressure at leaf temperature, minus the air's actual vapour pressure, to give leaf VPD in kPa — the number greenhouse and indoor growers steer temperature and humidity by.

How to use the VPD Calculator

  1. Enter canopy air temperature in °F.
  2. Enter relative humidity.
  3. Adjust the leaf offset — under lights, leaves are often about 2–4 °F cooler than air (default −3.6 °F ≈ −2 °C).
  4. Steer toward the band for your growth stage.

Formula

SVP = 0.61078 × e^(17.27T ÷ (T + 237.3)) kPa (Tetens, T in °C);  VPD = SVP(leaf) − SVP(air) × RH
SVP(leaf)
Saturation pressure at leaf temperature — the version growers actually use
RH
Relative humidity as a fraction

General bands: seedling/clone 0.4–0.8, vegetative 0.8–1.2, flowering 1.0–1.5 kPa are commonly published targets; species vary.

Worked example

At 25 °C (77 °F) and 60% RH with no leaf offset, SVP is 3.17 kPa and VPD = 3.17 × (1 − 0.60) ≈ 1.27 kPa.

Inputs

  • Air temperature (°F)77
  • Relative humidity (%)60 %
  • Leaf temperature offset (°F, leaves usually cooler)0

Result

  • Vapour pressure deficit (leaf VPD)1.27 kPa
  • Saturation vapour pressure at leaf temp3.17 kPa
  • Actual vapour pressure1.9 kPa
  • ReadingHigh — strong transpiration, watch water uptake

Results explained

Leaf VPD
The driving force for transpiration, in kPa.
SVP / AVP
Saturation pressure at the leaf and the air's actual vapour pressure.
Reading
A general interpretation band — crop-specific charts take precedence.

Frequently asked questions

Vapour pressure deficit measures how much more water vapour the air can hold before saturation, at the leaf's temperature. The same 60% RH is gentle at 70 °F but drying at 90 °F — VPD captures that, RH alone doesn't.

Commonly published grow targets: 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 for vegetative growth, 1.0–1.5 for flowering. At 25 °C and 60% RH (no leaf offset) VPD is about 1.27 kPa.

Transpiration happens at the leaf surface, and leaves under lights are usually 1–3 °C cooler than the air. Using leaf temperature (the default offset here) gives the VPD the plant actually experiences.

Raise humidity or lower temperature — both raise the air's moisture share of its (smaller) capacity. Small changes in temperature move VPD more than most people expect because SVP is exponential.

Too high: stomata close, transpiration and nutrient flow stall, leaf edges burn. Too low: plants barely transpire, calcium transport suffers and fungal disease pressure climbs.