Example exhaust fan sizing scenarios
The table below shows example rooms with typical air change rates and the resulting airflow and fan size suggestions. Use these values as a quick reference guide.
| Room type | Dimensions (L × W × H) | ACH | Airflow (CFM) | Airflow (m³/h) | Recommended fan size |
|---|---|---|---|---|---|
| Small bathroom | 2.0 m × 1.8 m × 2.4 m | 10 | 80 | 136 | 100 mm |
| Family kitchen | 4.0 m × 3.0 m × 2.7 m | 15 | 260 | 441 | 150 mm |
| Laundry room | 3.0 m × 2.5 m × 2.4 m | 12 | 180 | 306 | 125 mm |
| Workshop / garage bay | 6.0 m × 4.0 m × 2.7 m | 8 | 460 | 781 | 200 mm |
Formulas used in this calculator
This exhaust fan sizing tool is based on the standard volumetric airflow relationship between room volume, required air changes per hour, and the resulting flow rate expressed in cubic feet per minute (CFM).
- Room volume (metric): Volume (m³) = Length (m) × Width (m) × Height (m).
- Room volume (imperial): 1 m³ ≈ 35.3147 ft³.
- Airflow requirement: CFM = (Room volume in ft³ × ACH) ÷ 60.
- Metric airflow: m³/h ≈ CFM × 1.699.
- Duct correction factor: Loss factor = 1 + (0.03 × duct length in meters) + (0.05 × number of bends).
- Design safety factor: Safety factor = 1 + Safety margin (%) ÷ 100.
- Per-fan airflow: Per-fan CFM = Total adjusted CFM ÷ Number of fans.
- Energy use: kWh per day = (Fan wattage × hours × fans) ÷ 1000.
The final adjusted airflow is then used to select a suitable fan diameter by comparing the per-fan CFM against a table of typical capacities for residential axial or mixed-flow models.
How to use this exhaust fan size calculator
- Measure the internal length, width, and height of the room in metres or feet.
- Select the correct dimension unit and enter the three values in the form.
- Choose the room type (kitchen, bathroom, laundry, workshop, or bedroom) to load a typical air change rate.
- Adjust the ACH value if your local guidelines or design preference require higher or lower ventilation rates.
- Enter the duct length from the fan to the outside outlet, count the number of bends, and optionally add a safety margin to cover unknown losses.
- Specify how many fans you intend to install and their individual wattage, plus the typical number of hours they will run each day.
- Provide your local energy price per kWh if you want estimates of running cost over a day, month, and year.
- Click the calculate button to compute room volume, base airflow, adjusted airflow, per-fan requirements, recommended fan size, and optional energy usage values.
- Review the calculated CFM and m³/h values, and check whether available models in your region match or exceed the per-fan airflow and noise requirements.
- Use the CSV or PDF export buttons to save the results or example data with your project documentation or design notes.
Always verify fan performance using manufacturer datasheets, particularly for long ducts, high resistance grilles, very quiet installations, or projects that must meet a specific building standard.
Typical air change rate ranges
Different rooms require different ventilation levels. Bathrooms and kitchens usually sit at the top of the range because of moisture and odours, while bedrooms and living rooms often use lower air changes per hour.
| Room type | Recommended ACH range | Notes |
|---|---|---|
| Bathroom | 8 – 15 | Higher end for showers without windows or natural ventilation. |
| Kitchen | 12 – 20 | Use higher values for heavy cooking or open-plan layouts. |
| Laundry / Utility room | 10 – 15 | Moisture from dryers and washing machines justifies robust extraction. |
| Workshop / Garage | 6 – 12 | Consider pollutant sources such as solvents, fuels, or dust. |
| Bedroom / Living room | 4 – 8 | Lower ranges can still maintain comfort in well-insulated homes. |
Balancing exhaust capacity and noise
Oversizing an exhaust fan improves extraction but can introduce unwanted noise and drafts. Aim for a model that meets the required airflow with a noise rating suitable for bedrooms or bathrooms, especially in small apartments or quiet homes.
Humidity, condensation, and mould control
Adequate exhaust ventilation limits condensation on mirrors, windows, and cold surfaces. This reduces the risk of mould behind cabinets and in ceiling corners. Persistent dampness is often a sign that airflow or run time is insufficient for the room size.
Coordinating exhaust with room lighting
Many projects coordinate fan sizing with artificial lighting levels so bathrooms and bedrooms feel comfortable and usable. You can size illumination for sleeping spaces using the Bedroom lighting calculator alongside this exhaust fan sizing tool.
Combining ventilation with energy-efficient upgrades
When renovating, it is common to upgrade both lighting and extraction equipment. Switching to efficient fixtures lowers running cost and heat output. Use our Metal halide to LED savings calculator to compare lighting energy savings alongside fan energy estimates.
Related everyday life calculators
Moisture from aquariums, hot tubs, and indoor plants can significantly affect how much ventilation a room needs. For tank volumes and footprints, try the Aquarium size calculator and combine its results with this tool when planning hobby rooms.
Frequently asked questions (FAQs)
How accurate are the exhaust fan size results?
Results are engineering-style estimates based on room volume, air change rates, and simplified duct loss factors. Always compare with manufacturer performance data, local codes, and practical experience before final fan selection or ordering equipment.
What air changes per hour should I use for a bathroom?
Small bathrooms commonly use 8–10 air changes per hour, while larger rooms or spaces with long ducts may benefit from 12–15. If condensation or odours persist, increase either airflow or extraction run time.
Can I use two smaller exhaust fans instead of one large fan?
Yes. Enter the number of fans in the calculator to split total airflow between them. Ensure each fan can deliver the required per-fan CFM and check that combined noise levels remain acceptable.
How much does duct length affect fan performance?
Longer ducts, flexible ducting, and sharp bends all add resistance, reducing delivered airflow. The calculator increases required airflow with a simple loss factor, but actual performance should always be checked against detailed manufacturer pressure–flow charts.
Does this calculator work for commercial kitchens or industrial spaces?
The tool is aimed at residential and light everyday applications. Commercial kitchens, laboratories, or industrial workshops typically need specialist design using local regulations, grease or fume extraction rules, and detailed mechanical ventilation calculations.
Why is my bathroom still steamy after installing a new fan?
Possible causes include insufficient airflow, short run time, blocked ducts, backdraft dampers stuck closed, or leaks around duct joints. Check fan sizing with this calculator, then inspect duct routing, terminations, and any obstructions carefully.
How can I reduce noise from an exhaust fan?
Choose a quieter model, use smooth rigid ducting, avoid tight bends near the fan, and decouple mounting points from thin plasterboard where possible. Running the fan at slightly reduced speed can also lower noise in some installations.