Dehumidifier & ACH Room Sizer
Size dehumidifier and ACH
Dehumidifier sizing and ACH: quick guide
Use this dehumidifier & ACH room sizer to match capacity to your space. Enter length, width and height for one or more rooms; add current and target relative humidity, temperature and a dampness level. The calculator estimates a practical capacity band in pints per day or liters per day, shows an airflow target from your chosen air changes per hour (ACH), and gives a rough time to reach the target RH based on conditions. It works well for basements, crawl spaces, bathrooms, laundry rooms and garden outbuildings.
Pick a target RH between 45–55% for most living spaces. Lower than 40% often feels dry and can waste energy; above 60% risks mold and dust mites. Warmer rooms hold more water in the air, so a dehumidifier is most effective in mild to warm temperatures. In cool basements below about 60 °F (15 °C), performance drops and time to target increases; use continuous drain and allow clear airflow around the unit.
ACH complements dehumidification by moving moist air out and drier air in. For maintenance, 2–3 ACH is typical; very damp or odor-prone spaces benefit from 4–6 ACH. The tool turns your ACH selection into a fan size target in CFM or m³/h so you can choose a suitable exhaust fan or air mover. Place the dehumidifier where return and discharge are unobstructed, keep doors ajar for circulation, and fix liquid water sources first.
How capacity, time to dry, and ACH are calculated
Room volume is the sum of length × width × height across all rooms. A capacity band is estimated from volume and dampness using a per-volume water removal rate (L/day per m³) that scales from light maintenance to very wet conditions. The result is presented in liters/day and pints/day, with a 15% band for product choice. Time to reach target RH uses a simple moisture model: the calculator estimates absolute humidity from temperature and RH, multiplies the difference by room volume to get liters of water to remove, then divides by an effective liters/hour based on rated capacity and temperature. Cooler rooms reduce effective performance, so the estimate adjusts downward at lower temperatures.
ACH airflow target is volume × ACH. The tool outputs fan size in CFM and m³/h so you can compare ventilation products. These are planning numbers, not code specifications—follow manufacturer guidance and local requirements.
Dehumidifier sizer FAQs
What capacity should I pick if I’m between sizes?
Choose the next size up. Larger units run shorter cycles and often remove more water per kWh at moderate loads.
Why is drying slow in cool basements?
At low temperatures, coils remove less moisture and many units defrost frequently. Expect longer times or use a unit rated for low temps.
Do I need both dehumidification and ventilation?
Yes in many basements and bathrooms. Dehumidifiers remove moisture already indoors; ventilation controls sources and odors.
What about whole-home systems?
Use the total house volume and a moderate ACH, then compare to ducted dehumidifiers. Ensure proper return/supply ducting.
Is this a building code tool?
No. It is an educational estimator. Follow manufacturer instructions and local codes for electrical and ventilation.