simple-cfm-airflow-calculator
Free CFM calculator for HVAC ductwork. Enter duct dimensions and air velocity to find airflow rate in cubic feet per minute for rectangular and round ducts.
CFM Calculator
Calculate Cubic Feet per Minute (CFM) of airflow based on duct dimensions and air velocity.
Result
Calculation Formula
CFM = Air Velocity (FPM) × Area (sq ft)
CFM = 1,000 × (1.00 × 1.00) = 1,000.00
Documentation
CFM airflow calculator
A CFM airflow calculator finds the volume of air moving through a duct each minute. CFM stands for cubic feet per minute, the standard unit for airflow in heating, ventilation, and air conditioning (HVAC) systems. The calculator takes two inputs, air velocity and duct size, and multiplies them to get the airflow rate.
What CFM measures
CFM measures the volume of air flow, not its speed. Speed alone is measured in feet per minute (FPM). A duct with fast air but a small opening can carry the same volume as a duct with slower air and a larger opening. CFM combines both factors into a single number.
A higher CFM value means more air is moving through the duct. HVAC engineers use CFM to size fans, choose duct dimensions, and check whether a room receives enough fresh air.
CFM formula
The basic formula is:
CFM = air velocity (FPM) × duct cross-sectional area (square feet)
The area depends on the duct's shape.
For a rectangular duct, with width and height measured in inches:
Area = (width ÷ 12) × (height ÷ 12)
For a round duct, with diameter measured in inches:
Area = π × (diameter ÷ 2 ÷ 12)²
Dividing by 12 converts inches to feet, since duct dimensions are usually measured in inches while airflow area is measured in square feet.
How to calculate CFM for a rectangular duct
- Measure the duct's width and height in inches.
- Find the air velocity in feet per minute, either measured with an instrument or taken from equipment specifications.
- Divide each dimension by 12 to convert it to feet.
- Multiply width by height (both in feet) to get the area in square feet.
- Multiply the area by the velocity to get CFM.
Example
A duct measures 12 inches wide and 8 inches high. Air moves through it at 1000 feet per minute.
Area = (12 ÷ 12) × (8 ÷ 12) = 1.000 × 0.667 = 0.667 square feet
CFM = 1000 × 0.667 = 666.67, which rounds to 667 CFM
How to calculate CFM for a round duct
- Measure the duct's diameter in inches.
- Find the air velocity in feet per minute.
- Convert the diameter to a radius in feet: radius = diameter ÷ 2 ÷ 12.
- Find the area with the circle formula: area = π × radius².
- Multiply the area by the velocity to get CFM.
Example
A round duct has a 10-inch diameter. Air moves through it at 800 feet per minute.
Radius = 10 ÷ 2 ÷ 12 ≈ 0.41667 feet
Area = π × (0.41667)² ≈ 0.545 square feet
CFM = 800 × 0.545415 = 436.33, which rounds to 436 CFM
Typical air velocities in HVAC ducts
Actual velocity depends on the system, but these ranges are common in practice.
| Duct type | Typical velocity |
|---|---|
| Supply duct | 800–1200 FPM |
| Return duct | 600–800 FPM |
| Exhaust duct | 1000–1500 FPM |
| Fresh air intake | 400–600 FPM |
These figures are general guidelines, not fixed rules. Actual design values depend on the building, the equipment, and local codes.
Where CFM calculations are used
CFM figures guide many parts of building design and equipment selection.
- Sizing supply and return ducts in homes and offices
- Choosing bathroom and kitchen exhaust fans
- Checking that clean rooms and labs meet required air-change rates
- Sizing cooling airflow for server rooms
- Confirming that a building meets ventilation codes
Frequently asked questions
What does CFM mean?
CFM stands for cubic feet per minute. It measures the volume of air that passes through a duct or opening in one minute.
What is the difference between CFM and FPM?
FPM, feet per minute, measures how fast air moves. CFM measures how much air moves, combining speed with the size of the duct opening. CFM equals FPM multiplied by the duct's cross-sectional area in square feet.
How do I calculate CFM for a rectangular duct?
Convert the width and height from inches to feet, multiply them together to get the area, then multiply the area by the air velocity in feet per minute.
How do I calculate CFM for a round duct?
Convert the diameter to a radius in feet, square the radius, multiply by pi to get the area, then multiply the area by the air velocity.
How much CFM does a room need?
It depends on the room's size, use, and how often the air inside should be replaced. A common rough guideline for residential rooms is about 1 CFM per square foot of floor area, though actual requirements vary by application and local building code.
How do I convert CFM to metric units?
One CFM equals about 0.0283 cubic meters per minute.