Vapour pressure deficit (VPD) measures how hard the air in your grow room is pulling moisture out of your plants’ leaves, and it is a more reliable climate target than relative humidity on its own. VPD combines air temperature, relative humidity and leaf temperature into a single figure, expressed in kilopascals (kPa), that predicts transpiration rate and nutrient movement through the plant. This guide covers the saturated vapour pressure (SVP) formula, a step-by-step worked example, a VPD chart, and target ranges by growth stage.
📏 What is vapour pressure deficit?
VPD is the difference between saturated vapour pressure (SVP) — the maximum amount of water vapour the air could hold at a given temperature — and actual vapour pressure (AVP), the amount of water vapour actually present. When VPD is low, the air is close to saturated, transpiration slows, and moisture can sit on leaf surfaces, raising the risk of mould and botrytis. When VPD is high, the air is very dry and draws water from the leaf faster than the roots can resupply it, which forces stomata to close and stresses the plant.
Relative humidity alone does not capture this relationship, because the same RH reading represents a different drying force at different temperatures. VPD accounts for temperature and humidity together, which is why researchers and commercial growers use it to set climate targets rather than RH alone.
💧 The VPD formula explained
VPD is calculated as:
VPD = SVP − AVP, where AVP = (RH ÷ 100) × SVP
Saturated vapour pressure is derived from air temperature using the Tetens equation, a standard meteorological formula:
SVP (kPa) = 0.6108 × e^(17.27 × T ÷ (T + 237.3))
where T is temperature in degrees Celsius and e is Euler’s number (≈2.71828).
Saturated vapour pressure (SVP) table
The table below lists SVP values calculated with the Tetens equation for common grow room temperatures.
| Temperature (°C) | SVP (kPa) | SVP (Pa) |
|---|---|---|
| 14 | 1.60 | 1599 |
| 16 | 1.82 | 1818 |
| 18 | 2.06 | 2064 |
| 20 | 2.34 | 2338 |
| 22 | 2.64 | 2644 |
| 24 | 2.98 | 2984 |
| 26 | 3.36 | 3361 |
| 28 | 3.78 | 3780 |
| 30 | 4.24 | 4243 |
| 32 | 4.75 | 4755 |
| 34 | 5.32 | 5319 |
Air VPD vs leaf VPD
The formula above uses air temperature for both SVP and AVP, which gives an approximate “air VPD”. True VPD at the leaf uses leaf temperature for the SVP side of the equation instead, because a transpiring leaf typically runs 1–3°C cooler than the surrounding air. Without an infrared thermometer, a common approximation is to assume the leaf is 2°C cooler than air temperature and calculate SVP from that adjusted figure. Air VPD is a reasonable working estimate for most hobby grow rooms; leaf VPD is more accurate under intense lighting, where canopy temperature can diverge further from ambient air.
📐 Worked example: calculating VPD step by step
Take a grow room at 25°C and 65% relative humidity.
- Find SVP at 25°C from the table (or the Tetens equation): SVP = 3.17 kPa.
- Calculate AVP: AVP = (65 ÷ 100) × 3.17 = 2.06 kPa.
- Calculate VPD: VPD = 3.17 − 2.06 = 1.11 kPa.
A VPD of 1.11 kPa sits in the range commonly targeted for vegetative growth through to early flowering, so this room would not need adjustment for a plant at that stage.
💡 A-Grade Tip: Pair a digital hygrometer with an infrared thermometer to compare air VPD against leaf VPD under your actual lights — canopy temperature under high-output LEDs or HID fixtures often runs warmer than the 2°C-cooler assumption, which changes your real target humidity.
🌱 Target VPD ranges by growth stage
The ranges below are widely cited across horticultural and cannabis-cultivation literature as general starting points. They are guidelines, not fixed rules — species, cultivar and canopy density all shift the ideal figure, so treat these as a tuning range rather than an exact target.
| Growth stage | Target VPD | Status | Notes |
|---|---|---|---|
| Seedling / clone | 0.4–0.8 kPa | Low VPD | Gentle transpiration while roots establish |
| Vegetative | 0.8–1.2 kPa | Moderate VPD | Supports active growth and nutrient uptake |
| Early flower | 1.0–1.2 kPa | Moderate–high VPD | Balances stretch with stable humidity |
| Late flower | 1.2–1.6 kPa | High VPD | Lower humidity load reduces botrytis and bud rot risk |
⚙️ Reading a VPD chart
A VPD chart cross-references air temperature and relative humidity so you can read the resulting VPD without doing the maths each time. Find your temperature along the top, follow the row for your relative humidity, and read the VPD value in kPa where they meet.
| RH \ Temp | 20°C | 24°C | 28°C | 32°C |
|---|---|---|---|---|
| 40% | 1.40 | 1.79 | 2.27 | 2.85 |
| 50% | 1.17 | 1.49 | 1.89 | 2.38 |
| 60% | 0.94 | 1.19 | 1.51 | 1.90 |
| 70% | 0.70 | 0.90 | 1.13 | 1.43 |
| 80% | 0.47 | 0.60 | 0.76 | 0.95 |
All values are air VPD (kPa), calculated with the Tetens equation. If your reading sits above your stage target, raise humidity or lower temperature; if it sits below target, lower humidity or raise temperature slightly.
💡 A-Grade Tip: A humidity controller wired to an inline fan or humidifier will hold VPD inside your target band automatically, which is more consistent than manually adjusting a standalone hygrometer reading throughout the day.
💨 VPD In A Nutshell
VPD gives you a single, physiologically meaningful number instead of juggling temperature and humidity separately. Calculate it with the SVP formula and the worked example above, check your reading against the VPD chart and stage targets, and adjust temperature or humidity to bring your room back into range. Browse A-Grade Hydroponics’ range of hygrometers and humidity controllers to start tracking VPD in your own grow room.
