CFM Calculator
This CFM calculator, also known as a cubic feet per minute calculator, works out the airflow a room needs for proper ventilation. Enter the room size and air changes per hour and the CFM calculator gives you the airflow to size a fan, plus a CFM to m3/hr converter and a duct sizing reference.
Results
The CFM Calculator is part of our Ventilation collection. Browse more ventilation to size, convert, and check your work alongside this tool.
What the CFM calculator does
The CFM calculator measures airflow in cubic feet per minute, the standard unit for sizing fans and ventilation. As a cubic feet per minute calculator it turns a room's volume and its target air changes per hour into the exact airflow you need. Getting CFM right keeps a space fresh, controls humidity and odours, and helps you pick a fan and duct that move the air.
CFM formula
The CFM calculator uses the room volume times the air changes per hour, divided by 60:
L, W and H are in feet and ACH is the air changes per hour. For metric equipment the CFM to m3/hr converter multiplies by 1.699, so 100 CFM is about 170 m3/hr.
CFM calculator example
A 15 × 12 ft room with a 9 ft ceiling at 6 air changes per hour:
Running the cubic feet per minute calculation the other way, 162 CFM converts to about 275 m3/hr with the CFM to m3/hr calculator.
How to use the CFM calculator
Enter the room length, width, and ceiling height in feet, then the target air changes per hour for the room type. The CFM calculator multiplies these into the required airflow. Use the duct sizing chart CFM section below to match that airflow to a duct, and the CFM to m3/hr converter if your equipment is rated in metric.
Reading the CFM calculator result
Choose a fan rated at or slightly above the CFM figure, allowing for duct losses that reduce real airflow. Then use the duct sizing chart CFM guide to pick a duct that carries that airflow without noise. Higher CFM suits kitchens and bathrooms; living spaces need less.
When to use the CFM calculator
- Sizing bathroom and kitchen exhaust fans with the CFM calculator
- Matching airflow to a duct using the CFM duct chart
- Converting airflow with the CFM to m3/hr calculator
- Planning whole-room ventilation as a cubic feet per minute calculator
Common CFM calculation mistakes
- Ignoring duct losses. Real airflow falls below the fan rating; size the CFM with headroom.
- Skipping the duct chart. The right CFM with an undersized duct still starves the room.
- Mixing units. Keep the room in feet, then use the CFM to m3/hr converter for metric gear.
Tips for accurate CFM results
- Pick a fan slightly above the CFM calculator result to cover losses.
- Use the duct sizing chart CFM to keep air velocity quiet.
- Convert CFM to m3/hr when your fan is rated in metric.
Why the CFM calculator matters for duct sizing
Airflow and duct size are two halves of the same job, which is why a good CFM calculator pairs with a duct chart. Once the cubic feet per minute calculation gives your target airflow, the duct has to carry it without the air moving so fast it whistles and wastes energy. Undersized duct chokes the fan and drops the real CFM well below its rating; oversized duct wastes space and money. The duct sizing chart CFM guide above bridges the two, matching each airflow to a duct that moves it quietly.
CFM and metric airflow
Much of the world rates fans in cubic meters per hour, so the CFM to m3/hr converter is essential when your calculation is in CFM but the equipment is metric. The factor is fixed: one CFM equals 1.699 m3/hr, and the reverse CMH to CFM calculator divides by the same number. Keeping both units in view lets you shop for a fan whether it is labeled in cubic feet per minute or cubic meters per hour, without ever guessing at the conversion.
CFM duct chart
A quick duct sizing chart for CFM, matching airflow to round duct diameter at typical velocity:
| Airflow (CFM) | Round duct | m3/hr |
|---|---|---|
| 50 CFM | 4 in | 85 |
| 100 CFM | 5 in | 170 |
| 160 CFM | 6 in | 272 |
| 300 CFM | 8 in | 510 |
CFM Calculator FAQ
What is a CFM calculator?
A CFM calculator, or cubic feet per minute calculator, works out the airflow a room needs from its volume and air changes per hour. It tells you the fan size to keep the space properly ventilated.
How do I convert CFM to m3/hr?
Multiply CFM by 1.699. The CFM to m3/hr converter built into this calculator does it automatically, so 100 CFM is about 170 m3/hr, and the CFM to CMH calculator works the same way.
How do I use a CFM duct chart?
Find your required CFM, then read across the duct sizing chart CFM to the duct diameter that carries it at a safe velocity. The chart above pairs common airflows with round duct sizes.
How many CFM do I need?
It depends on room size and use. The cubic feet per minute calculator gives the figure; kitchens and bathrooms need higher airflow than bedrooms because they produce more moisture and odours.