Calculator
Example Data Table
| Scenario | Room (m3) | Temp | RH Current to Target | ACH | Internal (L/day) | Recommended (pints/day) |
|---|---|---|---|---|---|---|
| Small tent | 7.2 | 24C | 75% to 55% | 0.2 | 1.5 | 20 to 30 |
| Grow room | 50 | 26C | 70% to 55% | 0.5 | 4.0 | 50 to 70 |
| Greenhouse bay | 180 | 28C | 80% to 60% | 1.5 | 18.0 | 120+ |
These examples are illustrative; measured plant transpiration and outdoor humidity can change results significantly.
Formula Used
- Saturation vapor pressure (kPa): es = 0.61078 × exp(17.2694T / (T + 237.29))
- Vapor pressure: e = RH × es
- Absolute humidity (g/m3): AH = 216.7 × e(hPa) / T(K)
- Initial pull-down water (L): L = (AHcurrent − AHtarget) × Volume / 1000
- Ventilation load (L/day): ACH × Volume × 24 × max(0, AHout − AHtarget) / 1000
- Total capacity: (Internal + Vent + InitialEff) × (1 + Safety%)
How to Use This Calculator
- Measure room temperature and current RH near canopy level.
- Set a realistic target RH based on crop stage.
- Enter ventilation or leakage as ACH; use 0 if sealed.
- Estimate plant moisture per day, then add extra sources.
- Choose a pull-down period and a safety factor for swings.
- Run the calculation and select a unit at or above the result.
- Use CSV and PDF downloads to keep a sizing record.
Practical note: dehumidifier ratings depend on test conditions. If your room runs cooler than typical rating conditions, consider upsizing.
Dehumidifier Sizing Guide for Garden Spaces
Why Dehumidifier Sizing Matters
High humidity encourages mold, algae, and leaf diseases in enclosed gardens. Many growers target 50–60% RH to reduce risk while protecting transpiration. Undersized units run nonstop and still drift above setpoint. Oversized units short-cycle, waste energy, and create unstable conditions. This calculator balances pull-down needs with daily moisture generation.
Inputs That Drive Water Removal
Moisture capacity depends on air volume and the humidity drop required. A larger space stores more grams of water per cubic meter of air. Plant transpiration is often the biggest daily source. Typical values range from 0.05 to 1.00 liters per plant per day. Add extra evaporation from reservoirs, wet media, and frequent irrigation.
How Ventilation Changes the Load
Ventilation replaces indoor air with outdoor air at an ACH rate. If outside air is wetter than your target, the dehumidifier must remove that difference. For example, 0.5 ACH in a 50 m3 room moves 600 m3 per day. Even a 3 g/m3 moisture gap can add about 1.8 liters per day.
Interpreting Pints per Day Ratings
Dehumidifiers are rated in pints per day, where 1 pint is about 0.473 liters. Ratings are measured at specific test conditions and can drop in cooler rooms. Lower temperatures and lower RH reduce coil efficiency and water collection. Treat the calculated value as a minimum, then choose the next common size above it.
Practical Sizing and Control Tips
Use a safety factor to cover cycling, door openings, and weather swings. Many growers start with 15–30% margin for stable control. If you need fast pull-down after lights-off, shorten the pull-down period. Plan runtime realistically; longer runtime lowers the required liters per hour. Track results using the CSV or PDF export for future adjustments.
FAQs
1) What does “pints per day” mean?
It is the amount of water a unit can remove in 24 hours under test conditions. One US pint is about 0.473 liters. Real capacity can be lower in cooler, drier rooms.
2) Should I size for lights-on or lights-off conditions?
Size for the worst humidity period in your cycle. Many gardens spike after lights-off as air cools and RH rises. Use your highest measured RH and typical temperature for best results.
3) How do I estimate plant moisture per day?
Start with a conservative value and refine it. Small plants may be 0.05–0.20 L/day, while heavy canopy plants can exceed 0.50 L/day. Compare daily reservoir use and room RH trends.
4) What ACH should I enter if the room is sealed?
If the room is well sealed, use 0.00 to 0.10 ACH for minor leakage. If you run exhaust fans, use the measured or estimated air changes per hour based on fan flow and room volume.
5) Why does the calculator include a pull-down period?
Pull-down represents how quickly you want to reduce humidity from current to target. A shorter period increases the effective daily load. If you are already near target most days, use a longer period.
6) Is it okay to oversize the dehumidifier?
Moderate oversizing is helpful for peak events, but extreme oversizing can cause short cycling. Choose the next common size above the recommendation and use a controller with a stable differential or a variable-speed unit.
7) Will temperature changes affect the result?
Yes. Warmer air holds more moisture, and dehumidifier performance often improves at warmer conditions. If your garden runs cool, capacity may drop. Consider increasing the safety factor or selecting a larger unit.