Calculator Inputs
Enter your room size, ventilation target, installation details, and operating assumptions. Results appear above this form after calculation.
Example Data Table
These sample values show how different rooms can lead to very different airflow recommendations.
| Room | Dimensions | ACH | Special Conditions | Estimated Required CFM | Suggested Fan Setup |
|---|---|---|---|---|---|
| Bathroom | 8 × 6 × 8 ft | 8 | 6 ft duct, 1 bend, 10% margin | 63 | 1 × 80 CFM |
| Kitchen | 12 × 10 × 9 ft | 15 | Heavy use, filter, 2 bends | 418 | 1 × 500 CFM |
| Laundry Room | 10 × 8 × 8 ft | 10 | 8 ft duct, 2 bends | 132 | 1 × 150 CFM |
| Workshop | 20 × 15 × 10 ft | 12 | Heavy use, filter, 3 bends | 796 | 2 × 400 CFM |
Formulas Used
1) Room Volume
Volume = Length × Width × Height
2) Base Airflow
Base CFM = Room Volume (ft³) × Adjusted ACH ÷ 60
3) Adjusted ACH
Adjusted ACH = Selected ACH × Usage Factor × Occupancy Factor
4) Installation Correction
Required CFM = Base CFM × Duct Loss Factor × Margin Multiplier
5) Unit Conversion
m³/h = CFM × 1.69901
6) Power and Cost
Watts = Required CFM ÷ Fan Efficiency, then monthly cost uses hours, days, and electricity rate.
The calculator uses typical household ventilation assumptions. Final fan selection should also consider manufacturer pressure curves, actual duct routing, grille resistance, and local code rules.
How to Use This Calculator
- Choose imperial or metric units first.
- Select the room type that best matches your space.
- Enter room length, width, and ceiling height.
- Keep auto ACH or switch to custom ACH if needed.
- Adjust usage level for light, normal, or heavy ventilation demand.
- Add occupants, duct length, bends, and accessory details.
- Set your safety margin, expected efficiency, and running hours.
- Submit the form and review airflow, fan count, cost, duct size, and graph above the form.
Helpful ACH Guide
| Room Type | Typical ACH | Why It Matters |
|---|---|---|
| Bathroom | 8 | Controls moisture, odors, and fogging. |
| Kitchen | 15 | Removes heat, grease, moisture, and cooking odors. |
| Bedroom / Living Room | 6 | Supports general freshness and comfort. |
| Laundry / Basement | 8 to 10 | Helps reduce dampness and stale air. |
| Garage / Workshop | 12 | Useful where fumes, dust, or heat are higher. |
FAQs
1) What does ACH mean in fan sizing?
ACH means air changes per hour. It tells you how many times the room’s full air volume should be replaced every hour. Higher ACH means stronger ventilation and a larger required fan.
2) Is CFM the same as physical fan size?
Not exactly. CFM measures airflow capacity, while physical fan size refers to dimensions. A larger fan housing may not always move more air if the motor, blade design, or static pressure performance is weaker.
3) Why does duct length affect the required fan size?
Longer ducts create more resistance. The fan must work harder to move the same amount of air, so the calculator adds a correction factor to avoid undersizing the final selection.
4) Why do bends and dampers matter?
Every bend, damper, grille, or filter adds pressure loss. Even if room volume stays the same, installation resistance can reduce real airflow, so the selected fan should account for those losses.
5) Should I use one large fan or multiple smaller fans?
That depends on layout, noise, and available products. One fan is simpler, but multiple fans can distribute airflow more evenly and may fit better when one unit cannot meet the total required airflow alone.
6) Is a higher CFM always better?
No. Too much airflow can create noise, discomfort, excess power use, and unwanted pressure imbalance. It is better to match airflow to the room’s actual ventilation need and installation conditions.
7) Can I use this for bathrooms and kitchens?
Yes. Bathrooms and kitchens are common exhaust applications. Kitchens usually need much higher airflow because of heat, moisture, and grease, while bathrooms focus more on humidity and odor removal.
8) How accurate is the monthly running cost?
It is an estimate based on airflow, assumed fan efficiency, daily hours, and your electricity rate. Real cost may vary because actual motor power, duty cycle, and static pressure differ by model.