Fan CFM Calculator – Plan Airflow Before You Choose a Fan
Proper airflow is essential for comfort, indoor air quality and equipment performance. Too little fan capacity can leave rooms stuffy or humid, while too much airflow can create noise, drafts and wasted energy. The Fan CFM Calculator on MyTimeCalculator gathers several practical HVAC airflow tools into one place so you can build realistic estimates before you look at fan catalogs or request quotes.
Instead of working through multiple formulas by hand, you can use this calculator to estimate room ventilation (ACH-based), size a fan with an efficiency and safety margin, approximate HVAC airflow from BTU ratings, compute duct airflow from velocity and size, convert between CFM, m³/h and L/s, and size exhaust fans for bathrooms, kitchens and general rooms.
Overview of the Six Fan & HVAC Airflow Tools
The calculator is organized into six tabs that reflect common design and troubleshooting questions:
- Room CFM (ACH): Required airflow from room dimensions and air changes per hour.
- Fan Sizing: Adjust design CFM for fan efficiency and oversizing margin.
- HVAC CFM from BTU: Approximate supply airflow for cooling systems from BTU/h.
- Duct Airflow: Airflow from duct velocity and cross-sectional area.
- CFM Conversions: Quick conversions between CFM, m³/h and L/s.
- Exhaust Fan Sizing: Ventilation fan estimates for bathrooms, kitchens and general rooms.
1. Room CFM from Air Changes per Hour (ACH)
Air changes per hour (ACH) describes how many times the total air volume in a room is replaced each hour. For a room with volume V (in cubic feet) and target ACH, the required fan airflow is:
CFM = (V × ACH) ÷ 60
In this tab you enter:
- Room length, width and height (in feet or meters)
- Target air changes per hour
- Decimal precision
The calculator converts metric dimensions to cubic feet if needed, computes room volume and then applies the ACH formula to find the required CFM. It also converts the result to m³/h and rounds to a convenient fan size.
2. Fan Sizing with Efficiency and Safety Margin
The CFM listed on a fan nameplate or in a catalog is usually a rating under specific test conditions. Real installations can deliver less airflow because of duct losses, filters, fittings and static pressure.
In this tab you provide:
- Design airflow requirement in CFM (for example, from the Room CFM tab)
- Estimated fan efficiency percentage
- Safety or oversizing margin
- Static pressure for reference
The calculator scales the design CFM by your efficiency and safety inputs to suggest a rated fan CFM. The final value is rounded to a convenient size for catalog comparison. Manufacturer fan curves at your actual static pressure are still required for final selection.
3. HVAC CFM from BTU for Cooling Systems
For cooling applications, a common rule of thumb is roughly 400 CFM per ton of cooling capacity (with one ton equal to 12,000 BTU/h). This tab lets you turn a system’s BTU rating into a target airflow.
You enter:
- System capacity in BTU per hour
- CFM per ton (default 400, but adjustable)
- Decimal precision
The tool converts BTU/h to tons, multiplies by your CFM-per-ton value and returns an estimated supply airflow target. This is useful as a starting point when checking system performance or planning ductwork, but detailed HVAC design should always be verified against standards and equipment data.
4. Duct Airflow from Velocity and Cross-Sectional Area
Duct airflow is the product of air velocity and cross-sectional area. In imperial units:
CFM = Velocity (fpm) × Area (ft²)
In this tab you choose a duct shape and provide:
- Air velocity in feet per minute (fpm)
- Round duct diameter, or rectangular duct width and height (in inches)
- Decimal precision
The calculator converts inches to feet, computes the area (π×d²/4 for round ducts or width×height for rectangular ducts), and multiplies by velocity to estimate airflow in CFM. This is handy for quick checks of duct sizing and velocity.
5. CFM, m³/h and L/s Conversions
Different standards and equipment data sheets use different airflow units. The conversion tab provides three-way conversion between:
- CFM – cubic feet per minute
- m³/h – cubic meters per hour
- L/s – liters per second
You enter a value and choose its unit; the calculator converts it using standard factors (for example, 1 CFM ≈ 1.699 m³/h and 1 CFM ≈ 0.4719 L/s) and displays all three units.
6. Exhaust Fan Sizing for Bathrooms, Kitchens and General Rooms
Exhaust fans remove stale, humid or contaminated air from spaces such as bathrooms and kitchens. Many design guides and codes specify minimum air changes per hour or minimum CFM values for these rooms.
In this tab you select a room type and enter:
- Room length, width and height (feet or meters)
- Target air changes per hour (with a suggested starting value per room type)
- Decimal precision
The calculator computes the room volume, applies the ACH formula and rounds to a convenient exhaust fan size. It also provides the equivalent flow rate in m³/h. You can adapt the ACH value to align with local codes or design guidelines.
Using the Fan CFM Calculator Responsibly
- Use ACH and exhaust tabs for early planning and sanity checks, not as the only design reference.
- Always compare fan selections with manufacturer performance data at your expected static pressure.
- Consult local building codes and HVAC design standards for minimum ventilation requirements.
- In critical applications (commercial kitchens, labs, healthcare), work with qualified professionals.
This calculator is most powerful when used as an exploratory tool: you can see how changes in room size, ACH, system BTU or duct velocity affect the required airflow and then use that insight in your design discussions.
Related HVAC & Comfort Tools from MyTimeCalculator
Continue planning your comfort and building performance with these tools:
Fan CFM & HVAC Airflow FAQs
Frequently Asked Questions Fan CFM and Ventilation
Clarify how to interpret airflow numbers before you choose a fan or adjust your HVAC system.
CFM stands for cubic feet per minute and measures how much air a fan moves. It is a central sizing parameter for ventilation, exhaust and HVAC systems because it determines how quickly air in a room can be replaced, cooled or filtered.
Not always. A fan that is too large can create drafts, noise and excessive energy use. The goal is to meet airflow requirements from design standards or guidelines without greatly overshooting them. That’s why this calculator focuses on matching CFM to room volume, ACH targets and system capacity.
Fan ratings are usually measured under controlled test conditions. Real installations introduce ducts, fittings, filters and grills that add resistance and reduce airflow. Static pressure and installation details can significantly change delivered CFM compared with the nameplate value, so fan curves are essential for accurate design.
The calculator can help you understand and approximate ventilation requirements, but code compliance depends on the specific regulations in your jurisdiction and on detailed design details. Always confirm values against applicable building codes and standards or consult an HVAC professional.
You can start with typical ranges and experiment. Lower ACH values mean less frequent air changes and lower required CFM, while higher ACH increases both air quality and fan size. For precise values, use design guides, standards or local codes for your room type and occupancy.