VPD Calculator for Indoor Farming
Calculate Vapor Pressure Deficit from air temperature, humidity, and leaf offset. Compare your VPD against crop-specific targets by growth stage, with instant recommendations to optimize transpiration and yield.
What Is VPD?
Vapor Pressure Deficit (VPD) measures the difference between how much moisture the air currently holds and how much it could hold at saturation. In indoor farming, VPD is the primary driver of transpiration — the process by which plants pull water and nutrients through their roots, transport them through the stem, and release water vapor through stomatal pores in their leaves. A VPD that is too low (below 0.4 kPa) means the air is nearly saturated, transpiration stalls, and disease risk increases. A VPD that is too high (above 1.6 kPa) forces plants to close stomata to conserve water, halting photosynthesis and nutrient uptake. The optimal range for most indoor crops is 0.8–1.2 kPa during active growth.
How to Use This Calculator
- 1 Set your room’s air temperature and relative humidity. The leaf offset accounts for evaporative cooling — most leaves run 1–4°F below air temperature.
- 2 Choose your crop and growth stage. Each combination has a different optimal VPD range that the calculator targets.
- 3 Review the VPD result and zone classification. The heatmap shows how VPD changes across temperature and humidity combinations.
- 4 Follow the targeted recommendations to raise or lower VPD based on your current conditions.
VPD Targets by Crop & Growth Stage (kPa)
| Crop | Stage | Low | High |
|---|---|---|---|
| Butterhead | Propagation | 0.4 | 0.8 |
| Butterhead | Grow-out | 0.8 | 1.2 |
| Butterhead | Pre-Harvest | 1.0 | 1.3 |
| Romaine | Propagation | 0.4 | 0.8 |
| Romaine | Grow-out | 0.8 | 1.2 |
| Romaine | Pre-Harvest | 1.0 | 1.3 |
| Basil | Propagation | 0.4 | 0.8 |
| Basil | Vegetative | 0.9 | 1.3 |
| Basil | Harvest | 1.0 | 1.4 |
| Arugula | Propagation | 0.4 | 0.8 |
| Arugula | Grow-out | 0.8 | 1.1 |
| Arugula | Harvest | 0.9 | 1.2 |
| Kale | Propagation | 0.4 | 0.8 |
| Kale | Grow-out | 0.8 | 1.2 |
| Kale | Harvest | 1.0 | 1.3 |
| Microgreens | Germination | 0.2 | 0.5 |
| Microgreens | Greening | 0.5 | 0.9 |
| Microgreens | Harvest | 0.6 | 1.0 |
| Strawberry | Establish | 0.5 | 0.9 |
| Strawberry | Flower | 0.9 | 1.3 |
| Strawberry | Fruit | 1.0 | 1.5 |
| Tomato | Transplant | 0.4 | 0.8 |
| Tomato | Vegetative | 0.9 | 1.3 |
| Tomato | Flower | 1.0 | 1.5 |
| Tomato | Fruit | 1.2 | 1.6 |
| Cucumber | Transplant | 0.4 | 0.8 |
| Cucumber | Vegetative | 0.8 | 1.2 |
| Cucumber | Fruit | 1.0 | 1.5 |
| Pepper | Transplant | 0.4 | 0.8 |
| Pepper | Vegetative | 0.9 | 1.3 |
| Pepper | Flower | 1.0 | 1.5 |
| Pepper | Fruit | 1.2 | 1.6 |
Frequently Asked Questions
What is a good VPD for lettuce?
Most lettuce varieties (butterhead, romaine, leaf) do best at 0.8–1.2 kPa during grow-out. During propagation, keep VPD lower at 0.4–0.8 kPa to reduce stress on young seedlings. Drop slightly before harvest to keep leaves turgid.
How does leaf temperature offset affect VPD?
Leaves are typically 1–4°F cooler than air due to evaporative cooling from transpiration. This offset lowers the saturation vapor pressure at the leaf surface, which changes the effective VPD. A larger offset means a lower leaf VPD. Use an IR thermometer to measure your actual leaf offset for best accuracy.
What happens when VPD is too low?
When VPD drops below 0.4 kPa, the air is nearly saturated. Transpiration slows dramatically, which reduces nutrient transport through the plant. Condensation can form on leaf surfaces, creating ideal conditions for fungal diseases like botrytis and powdery mildew. Dehumidify, increase airflow, or raise temperature to increase VPD.
What happens when VPD is too high?
Above 1.6 kPa, plants close stomata to prevent excessive water loss. This halts both transpiration and photosynthesis, limiting growth and potentially causing wilting, tip burn, or leaf edge necrosis. Add humidity, lower temperature, or reduce light intensity to bring VPD down.
How is VPD calculated?
VPD is the difference between the saturation vapor pressure (SVP) at a given temperature and the actual vapor pressure (AVP) of the air. This calculator uses the Tetens equation: SVP = 0.6108 × exp((17.27 × T) / (T + 237.3)), where T is temperature in Celsius. Leaf VPD uses the leaf surface temperature instead of air temperature for the SVP calculation.
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