| Room size | Fan (rated) | Duct loss | Effective CFM | Air changes | Runtime |
|---|---|---|---|---|---|
| 8x6x8 ft | 80 CFM | 15% | 68 CFM | 2.0 | ~18 min |
| 10x8x8 ft | 110 CFM | 20% | 88 CFM | 2.5 | ~23 min |
| 6x5x8 ft | 50 CFM | 10% | 45 CFM | 2.0 | ~11 min |
1) Room volume
Volume = Length x Width x Height.
2) Effective airflow
Effective CFM = Rated CFM x (1 - DuctLoss%).
3) Runtime to achieve chosen air changes
Runtime (minutes) = (Volume_ft3 x AirChanges) / EffectiveCFM.
4) ACH check
ACH = (EffectiveCFM x 60) / Volume_ft3.
Humidity mode applies a multiplier for larger RH drops and slower drying conditions.
- Measure the room and choose your unit system.
- Enter the fan airflow and an estimated duct loss.
- Set a target air-change count for moisture purge.
- Optionally enable humidity mode and set RH values.
- Press Calculate runtime to view results above.
- Use CSV or PDF buttons to export your report.
Why runtime matters in humid utility spaces
Bathrooms near garden entries often face repeated wet loads from boots, hoses, and washdowns. Short fan bursts remove odors, but moisture persists on grout and paint. This calculator estimates how long the fan must run to replace indoor air a chosen number of times. More air changes generally mean faster drying and lower mold risk, especially after evening watering routines.
Room volume drives the baseline calculation
Ventilation time scales with volume: a larger room needs more minutes to achieve the same air-change count. Volume is length multiplied by width and height. When you switch to metric, the calculator converts cubic meters to cubic feet for a consistent airflow basis. Measuring the usable space, not closets, keeps the estimate realistic for moisture removal.
Effective airflow is rarely the rated airflow
Fan labels show rated airflow, yet ducts reduce delivered flow. Long runs, small diameter, roof caps, and multiple elbows can cut airflow by 10–30% or more. The duct loss field models that reduction so runtime is not underestimated. If you frequently see lingering condensation, increase the loss percentage and compare results to actual drying behavior.
Air changes and ACH support different operating styles
Air changes are useful for a single event, like a shower or a cleanup rinse. ACH (air changes per hour) helps plan ongoing ventilation schedules. The calculator shows minutes per hour needed to reach your target ACH, which is helpful for timer switches. For most small bathrooms, 8–12 ACH provides strong moisture control when the fan is properly sized.
Humidity mode adds a conservative safety margin
Humidity reduction is affected by temperature, wet surfaces, and air mixing, so a purely volumetric estimate can be optimistic. Humidity mode applies a multiplier based on the desired RH drop and your drying factor. Use it when the room feels clammy after plant washing, or when outdoor humidity is high. Add a few buffer minutes for comfort and material protection.
1) What air-change target should I choose?
For a typical post-shower or washdown, 1.5–3 air changes is a practical range. Start with 2 air changes, then adjust upward if mirrors stay foggy or surfaces remain damp.
2) How do I estimate duct loss without testing tools?
Use 10% for short, straight ducts, 15–25% for moderate length with bends, and 30–40% for long runs, many elbows, or restrictive exterior caps. When unsure, choose the higher value.
3) Why does the calculator show minutes per hour?
That value helps you run the fan intermittently while still meeting your target ACH. It is useful for timer switches and smart controls that cycle ventilation during humid seasons.
4) Should I run the fan during watering-related cleanup?
Yes. Run it during wet tasks and extend afterward to remove remaining moisture. This reduces lingering humidity that can affect stored fertilizer, tools, and finishes in adjacent utility areas.
5) What does the “assessment” label mean?
It reflects the estimated ACH delivered by your effective airflow. Low suggests slower moisture removal, moderate is workable with longer runtime, and good indicates strong ventilation for faster drying.
6) When should I enable humidity mode?
Enable it when you track RH and want a more cautious runtime. It is helpful in cool rooms, high outdoor humidity, or when condensation persists even after running the fan.