Quick answer: Size a commercial dehumidifier from the water it must remove per hour, at the temperature and humidity you want to hold, not from floor area. Add up the moisture sources (outdoor air and leakage, people, processes, open water, wet materials), convert the total to litres per day, and choose equipment whose capacity at your room condition covers it with a margin. Catalogue ratings at 30 °C / 80 % RH can be two to three times higher than what the same unit removes at 22 °C / 50 % RH, so always ask for capacity at your design condition.
Quick answer
- Load (kg/h) = outdoor air + leakage + people + processes + open water + wet materials.
- Outdoor air/leakage load = air mass (kg/h) × (outdoor − indoor moisture, kg/kg).
- Select at the room condition; add about 10–20 % margin, or a second unit for redundancy.
Step 1: Set the target
Write the target as RH at a stated temperature, or as a dew point. "50 % RH at 22 °C" and "dew point 11 °C" describe the same air. Also note:
- the design outdoor condition (the most humid weather you must handle);
- peak periods (doors open, production running, occupancy);
- any recovery requirement (for example, back to target within 2 hours after a door event).
Step 2: List the moisture sources
| Source | How to estimate | Typical figures |
|---|---|---|
| Outdoor air and leakage | Air mass flow × (outdoor moisture − indoor moisture) | Often the largest load |
| People | Per person, depending on activity | About 0.05 kg/h seated to 0.2 kg/h or more for heavy work |
| Open water (tanks, pools, wet floors) | Evaporation equation; see indoor pool guide | Depends on area and water temperature |
| Wet processes (washdown, cooking, drying) | Water used minus water drained | Measure where possible |
| Wet materials and goods | Water released while drying | See water evaporation rates |
| Crops (greenhouses, grow rooms) | Irrigation minus drainage | See grow room sizing |
Related: indoor pool guide, water evaporation rates, grow room sizing.
ASHRAE Handbooks give the methods and data for these loads; NIST publishes the psychrometric algorithms used to calculate moisture content (NIST, Algorithms for psychrometric calculations).
Step 3: Calculate the outdoor air and leakage load
load (kg/h) = air volume (m³/h) × 1.2 kg/m³ × (x_outdoor − x_indoor)
where x is the moisture content in kg of water per kg of dry air.
| Condition | Moisture content |
|---|---|
| 30 °C, 75 % RH (humid summer) | about 20.2 g/kg |
| 26 °C, 55 % RH | about 11.6 g/kg |
| 24 °C, 50 % RH | about 9.3 g/kg |
| 22 °C, 50 % RH | about 8.2 g/kg |
Worked example: a production hall
A 1,000 m² hall, 6 m high (6,000 m³), held at 22 °C / 50 % RH. Design outdoor air: 30 °C / 75 % RH.
| Source | Calculation | Load |
|---|---|---|
| Leakage and doors, 0.3 air changes/h | 1,800 m³/h × 1.2 × (20.2 − 8.2) g/kg | about 25.8 kg/h |
| 20 people, light work | 20 × 0.1 kg/h | 2.0 kg/h |
| Washdown area | Measured: about 3 kg/h during shifts | 3.0 kg/h |
| Total | about 31 kg/h ≈ 740 L/day |
Selection: the equipment must remove about 31 kg/h at 22 °C / 50 % RH. At that cooler, drier condition, a refrigerant unit removes much less than its 30 °C / 80 % RH rating; the manufacturer's capacity curve gives the real figure. If, for illustration, a unit delivers 40 % of its rated capacity at 22 °C / 50 % RH, about 1,850 L/day of rated capacity is needed: two 960 L/day units, such as our RYCF-40S, plus margin. Two units also spread the dry air and give redundancy.
The bigger lesson: about 84 % of the load is leakage. Halving the air leakage (sealing, fast doors, dock seals) would save almost a whole unit and its running cost.
Smaller spaces: what size for 1,000 or 2,000 sq ft?
Floor area alone can't size a dehumidifier, but the same method gives a quick answer for a basement or small building.
Example: a 1,000 sq ft (93 m²) basement, 2.4 m ceiling (about 223 m³), held at 20 °C / 50 % RH (7.3 g/kg), with summer outdoor air at 30 °C / 70 % RH (18.8 g/kg):
| Leakage | Moisture load from air |
|---|---|
| 0.3 air changes/h (fairly tight) | about 22 L/day |
| 0.5 air changes/h (average) | about 37 L/day |
Add internal sources such as laundry, people and damp walls or floors, and read the capacity at the basement's temperature, not at the 30 °C / 80 % RH rating. For a damp 1,000 sq ft basement this typically points to a unit rated around 50–70 L/day; for 2,000 sq ft with similar conditions, roughly double the load, or two units. Measure: if the unit runs nearly all day and never reaches 50 %, it is too small.
What if the dehumidifier is too big or too small?
| Too small | Too big |
|---|---|
| Runs constantly, never reaches the setpoint | Reaches the setpoint quickly and cycles on and off |
| Humidity drifts up in humid weeks | Higher purchase cost; more space |
| Wears out sooner | Usually harmless for a dehumidifier (unlike an oversized AC), but uneven drying if one unit serves a long space |
Step 4: Choose the technology
| Room condition | Technology |
|---|---|
| Above about 15 °C, target 40–60 % RH | Refrigerant dehumidifier |
| Below about 15 °C, or dew point below about 5 °C | Desiccant dehumidifier |
| Warm humid inlet air and very low dew point | Refrigerant pre-cooling + desiccant |
Related: desiccant dehumidifier.
Step 5: Check airflow and distribution
Capacity only helps where the dry air reaches. Check:
- the room air passes through the units several times an hour;
- units or duct outlets are spread so every zone gets dry air;
- ducted units have enough external static pressure for the duct run; see ducted dehumidifier design.
Step 6: Add margin, deliberately
- Design margin: about 10–20 % over the calculated peak load is common.
- Redundancy: two or more units, so a failure does not lose control.
- Do not double margins: if the outdoor design condition is already extreme, a large extra margin oversizes the system, which wastes capital and can make control less stable.
Step 7: Plan installation and acceptance
- Power supply and phases, drain points, service access, controls and alarms.
- Agree how you will accept the system: RH and temperature trends at stated points, condensate collected, behaviour on the most humid days.
What to send for a sizing review
- Room dimensions and use; target RH and temperature
- Outdoor design conditions or location
- Ventilation rates, door sizes and opening times, known leakage
- People, processes, open water and wet goods
- Existing HVAC
- Power supply, drainage, installation constraints
Use our commercial dehumidifier calculator guide for the inputs, or send them to us and we will calculate the load and propose models with capacity at your condition.
FAQ
Frequently asked questions
How do I calculate what size dehumidifier I need?
Add up the moisture entering the space per hour at your target condition, then choose a unit whose capacity at that condition covers the total with a margin.
Can I size a dehumidifier by square footage?
For homes, area charts give a rough start. For commercial and industrial spaces, no: leakage, doors, people and processes vary too much.
Why does my dehumidifier not reach the rated capacity?
Ratings are at warm, very humid conditions (30 °C / 80 % RH). In cooler or drier rooms, capacity is much lower.
What size dehumidifier do I need for a basement?
Calculate the moisture from leakage and sources; a damp 1,000 sq ft basement in humid summer commonly needs about 50–70 L/day of rated capacity, checked at the basement temperature.
Does dehumidifier size matter?
Yes: an undersized unit never reaches the target; an oversized one costs more but usually works, cycling more often.
How much margin should I add?
About 10–20 % on the calculated peak, or split the capacity across two or more units for redundancy.
Sources
- ASHRAE Handbook—HVAC Systems and Equipment, Dehumidification and pressure drying equipment; Desiccant dehumidification.
- NIST, Algorithms for psychrometric calculations.
- ASHRAE Handbook—Fundamentals, Psychrometrics; Nonresidential cooling and heating load calculations.
