CFM Calculator
Estimate room airflow from air changes per hour, or calculate duct CFM from duct size and air velocity.
Results
Actual airflow also depends on the fan curve, duct losses, filters, dampers, and static pressure.
How to use this calculator
- 01Choose room ACH mode or duct velocity mode.
- 02Enter room dimensions and target ACH, or enter duct size and air velocity.
- 03Review the airflow result in CFM, L/s, and m³/h.
- 04Read the area, volume, airflow density, and mode-specific result details.
If you are sizing heating equipment instead of airflow, use the Furnace Size Calculator. If you need a heating or cooling load estimate, use the BTU Calculator.
If you already know the required airflow and need a duct size, use the Duct Size Calculator.
If you know a fan’s airflow and want the room air change rate, use the Air Changes per Hour Calculator.
Understanding the math
Room mode uses room volume and target ACH. Duct mode multiplies duct area by air velocity. The conversions show the same airflow in liters per second and cubic meters per hour.
room airflow = room volume x ACH / 60 ACH = airflow x 60 / room volume duct airflow = duct area x velocity
Actual airflow also depends on the fan curve, duct losses, filters, dampers, and static pressure.
Ventilation quick reference
Use these ACH ranges as a starting point for planning. Project requirements, equipment ratings, and local code can call for different airflow.
| Space | Example area | Typical target |
|---|---|---|
| Bedroom | 120–200 sq ft | 4–6 ACH |
| Living room | 250–500 sq ft | 4–6 ACH |
| Kitchen | 120–250 sq ft | 6–12 ACH |
| Bathroom | 50–120 sq ft | 8–10 ACH |
| Garage / workshop | 300–800 sq ft | 6–10 ACH |
| Basement | 500–1,200 sq ft | 3–6 ACH |
Frequently asked questions
How do you calculate CFM?
For room ventilation, use CFM = room volume x ACH / 60. For duct airflow, use CFM = duct area x air velocity.
How many CFM do I need for a room?
It depends on room volume and target air changes per hour. A 20 ft x 15 ft room with an 8 ft ceiling at 6 ACH needs about 240 CFM.
How do I calculate CFM for a room?
Measure length, width, and ceiling height to get room volume. Choose a target ACH, then multiply volume by ACH and divide by 60.
How do I calculate CFM of a fan?
If the fan has a rated airflow, use that rating. If you are measuring airflow through a duct or opening, use CFM = area x velocity.
What is the difference between CFM and ACH?
CFM is airflow volume per minute. ACH is how many times the room air is replaced per hour. The same CFM gives a higher ACH in a smaller room.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, and reference values follow the active unit system where applicable.