Enter your targets
Example data table
| Plant | Stage | Day VPD (kPa) | Day RH (%) | Night VPD (kPa) | Night RH (%) | Leaf offset (°C) | Day setpoint (°C) | Night setpoint (°C) |
|---|---|---|---|---|---|---|---|---|
| Tomato | Vegetative | 1.00 | 65 | 0.80 | 70 | 1.0 | 22.28 | 21.14 |
Formula used
This calculator derives an air temperature setpoint from a target VPD and relative humidity. First it estimates the saturation vapor pressure (SVP) at leaf temperature:
VPD is then modeled as the fraction of SVP not filled by moisture:
The calculator solves for Tleaf by inverting the SVP equation, then converts to air temperature using your leaf-air offset:
How to use this calculator
- Pick a plant group and growth stage to load sensible starting targets.
- Set day and night VPD targets in kPa, then set humidity goals.
- Enter a leaf-air offset to reflect light, airflow, and sensor placement.
- Tune deadbands and hysteresis to match your controller behavior.
- Set safety minimum and maximum temperatures to prevent extremes.
- Press calculate, review warnings, then export CSV or PDF.
VPD-driven setpoints improve transpiration control
Vapor pressure deficit links temperature and humidity to plant water loss. When VPD is too low, leaves stay wet longer and nutrient uptake slows. When VPD is too high, stomata close and growth stalls. This calculator targets a chosen VPD in kPa and solves the air setpoint that supports it. For seedlings, many growers start near 0.4–0.8 kPa; fruiting crops often run 0.9–1.3 kPa.
Day and night targets should differ on purpose
During lights-on periods, higher VPD usually supports stronger transpiration and calcium movement. At night, a slightly lower VPD can reduce stress, but humidity often rises. Enter separate day and night targets so the controller can avoid sudden swings and keep the canopy stable across the full 24-hour cycle. If day humidity is 60–75%, night may climb above 80%, so tightening ventilation or heating slightly can maintain balance.
Leaf–air offset makes the result practical
Canopy leaves often run warmer than ambient air under strong light, or cooler when evaporation is intense. A small offset bridges this gap by converting a leaf-based VPD target into an air setpoint. If you use an infrared leaf sensor, the offset can be near zero and the setpoint becomes more direct. Offsets of 0.5–2.0°C are common in greenhouses; strong fans can cut the difference by improving boundary-layer mixing.
Deadbands and hysteresis protect equipment
Controllers that switch too frequently wear relays, stress compressors, and create unstable climates. Heating and cooling deadbands define how far temperature can drift before action starts. Hysteresis separates ON and OFF thresholds to prevent short cycling. The exported thresholds can be copied into most thermostats and greenhouse controllers. Record results and review them weekly.
Dew point margin reduces condensation risk
Night condensation promotes fungal pressure and reduces pollination success. The calculator estimates dew point and reports the margin between air temperature and dew point. Aim for 2°C margin. Improve airflow if it narrows. Safety limits also cap extreme setpoints, helping protect tender crops in variable weather.
FAQs
1) What is a good VPD range for leafy greens?
Many growers target about 0.6–0.9 kPa in active growth, with humidity commonly 65–80%. Start conservative for seedlings, then raise VPD slightly as roots and airflow improve.
2) Why do my setpoints change when humidity changes?
VPD depends on both temperature and relative humidity. If humidity increases, the same temperature produces lower VPD, so the calculator raises the temperature setpoint to reach your target VPD.
3) How should I choose the leaf–air offset?
Use 0.5–2.0°C as a starting point when leaves run warmer under lights. If you measure leaf temperature directly, reduce the offset toward zero. Strong airflow usually reduces the offset.
4) What do heating and cooling thresholds mean?
They are practical ON and OFF temperatures for your controller. Deadband sets how far from setpoint action begins, and hysteresis separates ON from OFF so equipment does not short cycle.
5) How can I reduce condensation at night?
Increase air mixing, avoid overwatering late, and prevent humidity spikes after lights off. A larger dew point margin is safer; if the margin is small, a slight night temperature lift often helps.
6) Is this calculator a replacement for crop-specific guides?
No. It converts your chosen targets into actionable setpoints. Use crop guides, local experience, and sensor validation to pick targets, then fine-tune with weekly observation and logs.