What this calculator does
CFM, cubic feet per minute, is the standard way airflow is rated for fans, extraction systems, range hoods and HVAC ducting. This cfm airflow calculator works it out two ways: from a duct or opening's cross-sectional area and the air velocity moving through it, or from a room's volume and a target number of air changes per hour.
There is no single "correct" velocity or air-change rate to assume, because they depend entirely on the application and the code or manufacturer guidance that applies: a bathroom extractor, a kitchen range hood and a server room ventilation system all target very different figures. That is why both inputs are left free here rather than fixed to one assumed value.
The formula
The area method multiplies the duct or opening's cross-sectional area, in square feet, by the air velocity in feet per minute: CFM = area x velocity. The room-volume method instead converts room volume to cubic feet and multiplies by the target air changes per hour, dividing by 60 to convert from changes per hour to changes per minute: CFM = volume x ACH / 60.
| Term | Meaning |
|---|---|
| CFM | Cubic feet per minute, the standard unit of airflow rate in the United States and widely used elsewhere in HVAC work. |
| Air velocity | How fast the air is moving through a duct or opening, typically in feet per minute (ft/min). |
| ACH | Air changes per hour: how many times the full volume of air in a room is replaced every hour. |
The inputs explained
| Field | What to enter |
|---|---|
| Calculate from | Choose whether you know the duct area and air velocity, or the room size and a target air-change rate. |
| Duct width (in) | Duct or opening width, used with height to work out the cross-sectional area. |
| Duct height (in) | Duct or opening height. |
| Air velocity (ft/min) | Air velocity through that opening, from a manufacturer spec sheet, anemometer reading or design guideline for the application. |
| Room length (m) | Room length, for the air-changes method. |
| Room width (m) | Room width, for the air-changes method. |
| Ceiling height (m) | Ceiling height, for the air-changes method. |
| Target air changes per hour (ACH) | Target air changes per hour: check the guidance or code that applies to the space (bathrooms, kitchens and plant rooms typically call for different rates). |
When to use it
Sizing an extractor fan from a duct spec
A fan or duct is often rated by the manufacturer with a velocity figure at a given static pressure; combined with the duct's cross-section, that gives the actual CFM it will deliver in practice.
Meeting a ventilation code requirement
Building codes and design guides for bathrooms, kitchens and plant rooms are often expressed as a minimum air changes per hour; working from room volume and that target ACH gives the fan CFM needed to meet it.
Comparing two fan options
Two fans quoted at different CFM ratings can be compared directly against either the duct velocity they would produce or the air changes per hour they would achieve in a specific room, rather than the CFM figure alone.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How airflow changes with duct size at a fixed velocity
A fixed 600 ft/min velocity, across a range of duct cross-sections.
| Duct width (with 8 in height) | Airflow | Duct cross-sectional area |
|---|---|---|
| 6 in | 200 CFM | 0.333 sq ft |
| 8 in | 267 CFM | 0.444 sq ft |
| 10 in | 333 CFM | 0.556 sq ft |
| 12 in | 400 CFM | 0.667 sq ft |
| 16 in | 533 CFM | 0.889 sq ft |
| 20 in | 667 CFM | 1.111 sq ft |
How the required airflow changes with target air changes per hour
A fixed room volume, across a range of target air-change rates.
| Target air changes per hour | Required airflow |
|---|---|
| 2 ACH | 57 CFM |
| 4 ACH | 113 CFM |
| 6 ACH | 170 CFM |
| 8 ACH | 226 CFM |
| 10 ACH | 283 CFM |
| 15 ACH | 424 CFM |
Questions
What air velocity should I use for a duct?
It depends on the duct type and application: supply ducts, return ducts and flexible ducting each have their own recommended velocity ranges to keep noise and pressure loss within reason. Check the manufacturer's guidance for the specific duct and fan being used rather than assuming a single figure.
What air changes per hour should a room have?
It varies by room type and the applicable code: bathrooms, kitchens, laundries and plant rooms commonly call for different minimum rates. Check the building code or design guide that applies to the specific space rather than using one fixed number everywhere.
Why does the area method need square feet when everything else is metric?
CFM itself is defined in feet, so duct dimensions in inches are converted to a cross-sectional area in square feet before multiplying by velocity in feet per minute. The room-volume method instead lets you work in metres and converts the volume to cubic feet internally.
Can I use this for a range hood or bathroom extractor?
Yes, either method applies: use the duct dimensions and rated velocity if you have a manufacturer spec, or the room volume and a target ACH if you are working from a ventilation guideline for that room type.
For the reverse question, working out air changes per hour from a known fan rating, see the air changes per hour calculator.