Start with your temperature and target VPD
Submit the form to calculate a humidity setpoint and get quick control guidance.
Inputs
Example data table
| Air temp (°C) | Leaf offset (°C) | Target VPD (kPa) | RH setpoint (%) | Dew point (°C) | Risk |
|---|---|---|---|---|---|
| 22 | 0 | 0.6 | 73.8 | 17.4 | Low |
| 25 | 0 | 0.9 | 71.1 | 19.1 | Low |
| 28 | 1 | 1.2 | 66.9 | 20.6 | Low |
| 30 | 0 | 1.4 | 62.6 | 21.7 | Medium |
| 18 | -1 | 0.6 | 70.1 | 12.7 | Low |
Examples are illustrative. Crop needs vary by species, airflow, and lighting.
Formula used
The calculator converts a target vapor pressure deficit (VPD) into a relative humidity setpoint at the leaf microclimate temperature.
- Saturation vapor pressure: es = 0.6108 × exp(17.27T / (T + 237.3)) (kPa)
- Actual vapor pressure: ea = es − VPD (kPa)
- Humidity setpoint: RH = (ea / es) × 100 (%)
- Dew point is estimated using a Magnus approximation from air temperature and RH.
- Absolute humidity: AH ≈ 216.7 × e(hPa) / T(K) (g/m³)
How to use this calculator
- Measure air temperature near the plant canopy.
- Enter a leaf offset if leaves run warmer or cooler.
- Select a crop stage preset, or choose Custom VPD.
- Set your minimum and maximum RH guardrails.
- Press Calculate, then apply the RH setpoint on controls.
Sensor placement and averaging
Track humidity where leaves exchange moisture, not at the door. Mount a probe at canopy height, shielded from direct mist and sun. If you use two sensors, average them to reduce microclimate bias. A five minute rolling average prevents short spikes from cycling equipment. Record day and night values separately because setpoints often differ with lights off, heaters running, and cooler surfaces near walls.
Choosing a target VPD for plant health
VPD links temperature and humidity to transpiration demand. Lower targets support seedlings by reducing water loss, while higher targets harden plants and limit disease. Use the preset ranges as starting points, then adjust by observing leaf posture, growth rate, and substrate moisture. Pair changes with irrigation timing. If tips burn or wilting appears, lower VPD. If mildew risk rises, raise VPD and improve air exchange.
Interpreting dew point and condensation risk
Dew point indicates when water will condense on glazing, ducts, and leaves. When air temperature is within two degrees of dew point, droplets can form and spread pathogens. Aim for a larger safety margin at night by dehumidifying, heating slightly, or venting briefly. Keep circulation fans running to reduce cold boundary layers on foliage, benches, and irrigation lines. Inspect corners after sunrise. A simple infrared scan helps find cold spots that trigger dripping.
Using RH limits to protect quality
Min and Max RH limits act as guardrails around your calculated setpoint. A lower limit helps prevent excessive transpiration, calcium issues, and slow growth. An upper limit reduces mold, botrytis, and algae on media. If the setpoint violates a limit, change VPD, temperature, or ventilation strategy. Consider shading, airflow patterns, plant density, and wet surfaces. Small changes, applied consistently, beat large daily swings.
Operational routine and documentation
After you compute a setpoint, apply it to humidifiers, dehumidifiers, and vent logic with a modest deadband, such as two percent RH. Recheck calibration monthly using a salt test or a trusted reference meter. Log adjustments with dates, crop stage, lighting schedule, and irrigation changes. Track yield and disease notes. Over time, these records reveal the best VPD band for your site, season, and cultivar.
FAQs
1) What is a humidity setpoint in a greenhouse?
It is the target relative humidity your controls aim to maintain. A stable setpoint supports predictable transpiration, limits stress, and reduces disease pressure when paired with proper temperature management and airflow.
2) Why does the calculator use VPD instead of only RH?
Relative humidity alone ignores temperature. VPD accounts for the drying power of air, which better predicts plant water loss. Converting a VPD target to RH creates a setpoint that fits current temperature conditions.
3) Should I enter a leaf temperature offset?
Yes if leaves are measurably warmer or cooler than air. Strong light, low airflow, or cold glazing can shift leaf temperature. A small offset improves setpoint accuracy for the canopy microclimate.
4) What condensation risk level should I aim for?
Low risk is preferred, especially overnight. Keep air temperature several degrees above dew point and maintain circulation. If risk shows Medium or High, reduce humidity, warm slightly, or vent briefly to protect foliage and surfaces.
5) How often should sensors be calibrated?
Check monthly in production environments. Use a salt test kit or a trusted reference meter, and clean probes regularly. Drifted sensors can cause over-humidification or unnecessary drying, both of which reduce crop performance.
6) Can I use this for indoor grow rooms?
Yes. The same temperature, VPD, and dew point relationships apply. Use room-specific airflow patterns and avoid placing sensors directly in mist streams or near exhaust vents for more representative readings.