VPD Calculator.
VPD Calculator
Air and leaf vapor pressure deficit with per-stage target bands. The single most important environmental number for indoor cultivation.
What is VPD?
Vapor Pressure Deficit (VPD) is the difference between the amount of water vapor your air currently holds and the maximum it could hold at its temperature. It is, more usefully, a measure of how thirsty the air is — how aggressively the air will pull moisture out of plant leaves and the substrate.
Plants don't experience temperature and humidity as separate variables. They experience the rate at which they lose water through their stomata, which is what governs how much water they pull from their roots, which is what governs how much nutrient solution moves through them. VPD captures all of that in one number.
This is why VPD is the single most important environmental number in indoor cultivation. Two rooms with identical temperature and humidity readings can have different VPD if the leaf temperature differs. Two recipes that hit the same humidity setpoint at different temperatures produce different VPDs and different transpiration rates and different growth.
Air VPD vs leaf VPD
The calculator above produces both numbers. They are not the same thing.
Air VPD uses air temperature. It is the deficit in the air above the canopy. It is easy to measure — you only need a temperature sensor and a humidity sensor — but it is an approximation of what the plant actually feels.
Leaf VPD uses leaf surface temperature. Leaves transpire, which cools them — typically 1–4°F (0.5–2°C) below air temperature when the plant is healthy and stomata are open. Leaf VPD is the deficit between the leaf surface and the surrounding air, which is exactly what governs the plant's transpiration rate. It is the more accurate metric.
To measure leaf temperature, point an inexpensive infrared thermometer (around $20–30) at a few representative leaves. The reading is the leaf surface temperature. Plug that into the calculator above and you have leaf VPD.
If you don't have an IR thermometer, air VPD is a reasonable approximation. Most growers operate on air VPD without serious problems. But if your plants are showing transpiration-related issues (excessive stomata closure, calcium deficiency despite adequate Ca in solution, slowed growth), leaf VPD often reveals what air VPD missed.
Target VPD by stage
Different stages of plant development want different VPDs:
| Stage | Target VPD (kPa) | Reasoning |
|---|---|---|
| Cuttings / clones | 0.4 – 0.8 | Roots not established; high transpiration would dehydrate cuttings before they can take up water |
| Seedlings | 0.6 – 0.9 | Root systems still establishing; gentle transpiration |
| Vegetative | 0.8 – 1.2 | Active biomass accumulation; healthy transpiration drives nutrient uptake |
| Pre-flower / fruit set | 1.0 – 1.3 | Hormonal transition; moderate stress acceptable |
| Mid-flower | 1.0 – 1.5 | Reproductive structures developing; higher VPD encourages compact growth |
| Late flower / ripening | 1.3 – 1.6 | Reduces botrytis risk; encourages secondary metabolite accumulation |
| Drying | 1.4 – 1.8 | Pulls moisture from harvested material at controlled rate |
These ranges are general. Specific cultivars want different values within them. Cannabis sativas tolerate higher VPD than indicas. Lettuce wants lower VPD than tomato. The Cultivar Browser (linked below) carries cultivar-specific overrides where the community has contributed them.
How VPD interacts with everything else
VPD is downstream of temperature and humidity, and upstream of:
- Transpiration rate — higher VPD means faster moisture loss from leaves
- Nutrient uptake — water moving through the plant carries dissolved nutrients (mass flow); slow transpiration means slow nutrient delivery
- Calcium and boron uptake — these elements move primarily through transpiration; low VPD shows up as Ca/B deficiency even when solution levels are correct
- Stomatal behavior — extreme VPD (too high) closes stomata, halting photosynthesis
- Disease pressure — high humidity / low VPD encourages botrytis and powdery mildew; very low VPD over time invites mold
- Heat dissipation from leaves — transpiration cools the canopy; impaired transpiration means leaves run hotter than air
The single most common environmental mistake in indoor cultivation is running humidity too high. The room "feels comfortable" to humans at 65% RH, but at 78°F that's only 0.65 kPa VPD — too low for productive vegetative growth and dangerously low for late flower. Lowering humidity to 50% raises VPD to 1.05 kPa, which is in the target band for veg.
The math, briefly
The calculator uses the Tetens equation for saturation vapor pressure:
SVP(T) [kPa] = 0.6108 × exp(17.27 × T / (T + 237.3)) where T is in °C
Then:
Air VPD = SVP(T_air) × (1 − RH / 100) Leaf VPD = SVP(T_leaf) − SVP(T_air) × (RH / 100)
The Tetens approximation is accurate to about 0.1% over normal cultivation temperatures (0–50°C). For research-grade precision, the ASHRAE standard formulation is more accurate but the difference is negligible for grower-grade decisions.
VPD and dew point
Dew point is the temperature at which air, cooled to that point, would be 100% saturated and start condensing. The calculator displays your current dew point because:
- Surfaces colder than the dew point will condense water — important when designing canopy airflow and avoiding cold spots that get wet
- Night setpoint should not drop near dew point — otherwise leaves wet from condensation invite disease
- Dehumidifier capacity is best understood in dew-point terms, not RH percentage
References
The Tetens equation is from O. Tetens (1930), "Über einige meteorologische Begriffe", Zeitschrift für Geophysik. It is also published in standard atmospheric and agricultural physics texts. The ASHRAE Handbook (Fundamentals chapter on psychrometrics) and FAO Irrigation and Drainage Paper 56 (Allen et al., 1998) are authoritative sources for evapotranspiration and VPD math in agricultural contexts.
Per-stage VPD targets are synthesized from peer-reviewed cultivation literature and grower community practice. The Cannabis Encyclopedia (Cervantes), Indoor Cultivation Handbook (Caplan, et al.), and published University of Tennessee, UC Davis, and Wageningen UR controlled-environment agriculture research are useful reference sources.
This widget is part of the Open Agriculture Technology library. Free for any grower to use; embeddable on any site under CC BY 4.0 attribution.