CFM Required Calculator
Ventilation is often specified in air changes per hour, the number of times the entire volume of air in a room is replaced each hour. To turn that target into a fan or duct airflow, you convert it to cubic feet per minute, and that is what this calculator does. The airflow needed equals the room volume in cubic feet multiplied by the desired air changes per hour, divided by 60 to move from per-hour to per-minute. Different spaces call for different change rates: a bathroom or kitchen needs many changes to clear moisture and odors, while a bedroom needs far fewer. Enter the room volume, or compute it from length, width and height, and the air-change target, and the tool returns the required airflow in CFM, the figure you use to select an exhaust fan or size a supply. It also shows the airflow per 1,000 cubic feet for quick scaling. Ventilation standards and local codes set minimum rates for health and comfort; this tool simply helps you translate a chosen rate into practical equipment sizing. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator defaults.
Required airflow is room volume x air changes per hour / 60. A 2,000 cubic foot room at 6 air changes per hour needs 200.00 CFM.
CFM required formula
CFM = volume x ACH / 60
volume = room volume in cubic feet
ACH = target air changes per hour
divide by 60 to convert per hour to per minute
Air changes per hour means the room volume is replaced that many times each hour. Dividing by 60 converts the hourly figure to the cubic-feet-per-minute rating used to select fans.
Worked example
Suppose the room volume is 2,000 cubic feet and you want 6 air changes per hour.
- Multiply volume by ACH: 2,000 x 6 = 12,000 cubic feet per hour
- Convert to per minute: 12,000 / 60 = 200.00 CFM
The required airflow is 200.00 CFM. These are the calculator's default inputs, so the result matches the widget exactly.
CFM Required Calculator: frequently asked questions
What are air changes per hour?
Air changes per hour, or ACH, is how many times the full volume of air in a space is replaced with fresh or conditioned air in one hour. A rate of six means the air is fully exchanged six times an hour. Higher rates suit spaces that generate moisture, heat or odors.
How many air changes does a room need?
It varies by use: bathrooms and kitchens need high rates to remove moisture and cooking byproducts, while living spaces and bedrooms need fewer. Ventilation standards and local codes set minimums for health. Choose a rate appropriate to the room, then size the fan to the resulting CFM.
How do I find room volume?
Multiply the room's length by its width by its ceiling height, all in feet, to get cubic feet. For irregular rooms, split the space into rectangular sections, compute each volume, and add them. Enter the total volume into the calculator.
Why divide by 60?
Air changes are stated per hour, but fans are rated in cubic feet per minute. Dividing the hourly air volume by 60 converts it to a per-minute figure that matches fan ratings, so you can select equipment directly.
Does this guarantee good indoor air quality?
Meeting an air-change target is a key part of ventilation, but air quality also depends on filtration, fresh-air intake, humidity control and pollutant sources. The Department of Energy and ventilation standards provide broader guidance on healthy, efficient ventilation.
Official sources
- Ventilation and home energy efficiency guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.