Inputs
Example Data Table
| Scenario | Room (L×W×H) | Base Target (CFM) | Recommended (CFM) | Duct Suggestion |
|---|---|---|---|---|
| Half Bath (Toilet + Sink) | 6×5×8 ft | 50 | 60 | 4 in |
| Standard Bath (Shower + Toilet) | 8×6×8 ft | 75 | 90 | 5 in |
| Large Bath (Tub + Shower + Toilet) | 12×10×9 ft | 144 | 190 | 6 in |
Formula Used
- Area (sq ft) = Length × Width
- Volume (cu ft) = Area × Height
- Area method airflow (CFM) = Area × (CFM per sq ft)
- Air changes method airflow (CFM) = Volume × ACH ÷ 60
- Fixtures method airflow (CFM) = Σ( Fixture count × Fixture rate )
- Base target airflow = max( Minimum requirement, enabled method results )
The duct adjustment is estimated from equivalent length, diameter, and material. Final recommendation = Base target × (1 + duct penalty), rounded up to the nearest 10 CFM.
How to Use This Calculator
- Enter room length, width, and ceiling height.
- Select intermittent or continuous fan operation.
- Enable the sizing methods you want to compare.
- Add fixture counts, and adjust fixture rates if needed.
- Enter duct diameter, length, elbows, and termination type.
- Press Calculate Exhaust Size to view results.
- Use the export buttons to save CSV or PDF.
Airflow targets for comfort and odor control
A practical bathroom exhaust target starts with a minimum airflow floor, then scales with room use. Intermittent fans commonly start at 50 CFM, while continuous operation can start near 20 CFM. Higher targets reduce mirror fogging and lingering odors, especially after showers. If the bathroom has no operable window, consider selecting above the minimum to improve drying reliability.
Area and air-change sizing for predictable turnover
Area-based sizing is quick: multiply room area by a planning factor such as 1.0 CFM per square foot. Air-change sizing uses volume to reach a turnover goal: CFM = Volume × ACH ÷ 60. Typical design ranges are 6–10 ACH, with higher values for frequent bathing, higher ceilings, or limited drying time.
Fixture-driven demand when moisture generation is high
Fixture counts help represent moisture events that do not track area alone. A shower can justify 50 CFM on its own, while a toilet often adds 20–30 CFM for odor capture. Steam showers and large tubs can push the dominant method above area sizing, improving resilience. When two moisture fixtures are used together, the fixture method helps prevent persistent damp surfaces.
Duct routing effects and why margin matters
Delivered airflow drops as static losses rise. Long runs, multiple elbows, roof terminations, and flexible ducting increase resistance and can reduce real CFM. This calculator estimates an equivalent length and applies a capped adjustment factor, then rounds to the next 10 CFM. Keeping ducts short, smooth, and properly sized reduces losses and noise. Crushed flex or restrictive caps reduce airflow.
Selection checks: sound, energy, and verification
After sizing, confirm the selected fan can meet the target at the expected resistance. Quieter units support longer run times, which improves moisture control. Use a timer or humidity sensor to extend operation after bathing. Provide make-up air through an undercut or transfer path so the fan can move air efficiently. During handover, verify airflow at the grille and inspect backdraft dampers for free movement.
FAQs
Use both, then choose the highest CFM. Area works well for typical rooms, while ACH responds to tall ceilings and large volumes. Picking the maximum provides a safer moisture and odor control target.
Longer or more restrictive ducts add resistance and reduce delivered airflow. The calculator converts fittings to equivalent length and applies a margin so the fan can maintain the target CFM at the grille.
Start around 8 ACH for a standard bathroom. Use 6 ACH for light use and fast drying, and 10 ACH for frequent showers, higher humidity, or limited natural ventilation.
Treat fixture rates as event-driven demand. A shower adds moisture quickly, so higher rates improve recovery. Enter your project criteria and compare fixture results against area and ACH to select the most conservative airflow.
Larger ducts usually reduce resistance, but routing and sealing still matter. Keep transitions smooth, avoid sharp bends, and match the fan outlet where possible. Poorly installed or leaky ducts can waste capacity.
Run the fan during bathing and for 15–30 minutes afterward using a timer or humidity control. Confirm the cap is clear, the duct is supported, and dampers move freely to prevent backflow.