Quick answer: A commercial dehumidifier calculator adds up the water entering a space per hour and converts it to the capacity you need at your room condition. The core inputs are: room volume and target (temperature and RH), outdoor design condition, air leakage and ventilation rate, people and activity, open water and wet materials, and process moisture. The key formula is air load (kg/h) = airflow (m³/h) × 1.2 × (outdoor − indoor moisture content, kg/kg). Convert with L/day = kg/h × 24 and pints/day = L/day × 2.11. The most common mistake is comparing the result with a catalogue rating at 30 °C / 80 % RH instead of capacity at the room condition.
Quick answer
- Air load = m³/h × 1.2 × Δx (kg/kg).
- L/day = kg/h × 24; pints/day ≈ L/day × 2.11.
- Select at room conditions, not rating conditions.
Inputs
| Input | Units | Notes |
|---|---|---|
| Room size | m² and height (m³) | For leakage as air changes per hour |
| Target | °C and % RH (or dew point) | Converted to moisture content |
| Outdoor design condition | °C and % RH | Most humid weather to handle |
| Leakage | air changes per hour (ACH) | Tight 0.1–0.3; average 0.3–0.5; leaky 0.5–1+ (rough guide) |
| Ventilation | m³/h | Mechanical outdoor air |
| Doors | size, openings per hour, duration | Large dock doors can dominate |
| People | number, activity | About 0.05–0.2 kg/h each |
| Open water | m², water temperature | Evaporation equation |
| Wet materials or processes | kg/h released | Measure or estimate |
Formulas
Moisture content: x (g/kg) = 622 × e ÷ (p − e), with e = RH ÷ 100 × saturation pressure at the air temperature, p ≈ 101.3 kPa.
Air load (leakage + ventilation): kg/h = (ACH × volume + ventilation m³/h) × 1.2 × (x_out − x_in) ÷ 1,000.
People: kg/h = number × per-person rate.
Open water: kg/h ≈ 0.144 × area × (p_water − p_air) × activity factor (see pool dehumidification).
Total = sum of all loads, then add 10–20 % margin.
Worked worksheet (illustrative)
| Line | Input | Result |
|---|---|---|
| Room | 800 m², 5 m high → 4,000 m³ | — |
| Target | 22 °C, 50 % RH → 8.2 g/kg | — |
| Outdoor | 30 °C, 75 % RH → 20.2 g/kg | Δx = 12.0 g/kg |
| Leakage | 0.3 ACH → 1,200 m³/h | 1,200 × 1.2 × 0.012 = 17.3 kg/h |
| Ventilation | 500 m³/h | 500 × 1.2 × 0.012 = 7.2 kg/h |
| People | 15 × 0.1 kg/h | 1.5 kg/h |
| Process | measured | 2.0 kg/h |
| Total | 28 kg/h ≈ 672 L/day | |
| + 15 % margin | ≈ 770 L/day at 22 °C / 50 % RH |
Then ask suppliers for units whose capacity at 22 °C / 50 % RH covers about 770 L/day; at the 30 °C / 80 % RH rating, that is usually a much larger number. For the full method, see how to size a commercial dehumidifier.
Unit conversions
| From | To | Multiply by |
|---|---|---|
| kg/h | L/day | 24 |
| L/day | pints/day (US) | 2.11 |
| g/kg | grains/lb | 7 |
| m³/h | CFM | 0.589 |
Calculator mistakes to avoid
- Sizing from floor area alone.
- Using the outdoor average instead of the humid design condition.
- Forgetting door openings and ventilation.
- Comparing the result with ratings at 30 °C / 80 % RH.
- Adding margin on every line and again at the end.
- Ignoring how dry air reaches the whole space.
Application notes
- Greenhouses: night transpiration dominates; see greenhouse moisture load calculation.
- Grow rooms: irrigation minus runoff; see grow room sizing.
- Pools: evaporation equation; see pool guide.
- Water damage: water to evaporate from materials; see after a water leak.
Send your inputs to us and we will calculate the load and propose industrial dehumidifiers with capacity at your condition.
FAQ
Frequently asked questions
How do I calculate dehumidifier size for a commercial space?
Add the moisture from leakage, ventilation, people, water surfaces and processes in kg/h, convert to L/day, add a margin, and select at the room condition.
How many pints is 1 litre?
About 2.11 US pints.
What air change rate should I assume for leakage?
About 0.1–0.3 ACH for tight buildings, 0.3–0.5 for average, and higher for leaky buildings or large door activity; measure where possible.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics; Ventilation and infiltration.
- ASHRAE Handbook—HVAC Systems and Equipment, Dehumidification equipment.
