Airflow Calculator
Calculate CFM, velocity, and friction loss
Inside diameter of round duct
Standard residential: 600 fpm
Total length of the duct run
Air Velocity
—
fpm
CFM
—
CFM
Friction Loss
—
in w.g.
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Understanding airflow helps you verify that your ductwork delivers the right amount of conditioned air. CFM, velocity, and friction loss together describe how a duct system performs.
- Enter the inside diameter of your round duct
- Set the air velocity (residential: 600 fpm)
- Enter the total duct run length
- Review CFM delivered and the friction loss to size your blower
Expert Insights
Smart analysis for your HVAC project
CFM Delivery Check
CFM = area x velocity. AI confirms whether your duct delivers enough air for the room — typically 1 CFM per sq ft of floor area.
Friction Loss Tracking
Longer ducts and higher velocity increase friction loss. AI estimates pressure drop so you can size blowers and avoid weak airflow.
Noise Control
Velocities above 900 fpm get noisy. AI flags high-velocity results and suggests larger ducts for quiet residential comfort.
Calculation Formula
Duct Area (sq ft) = pi x (Diameter / 2)^2 / 144
CFM = Duct Area x Velocity
Friction Loss = ceil(Duct Length x 0.06 x Velocity / 1000 x 100) / 100 (in w.g.)
Airflow Performance by Duct Type
| Duct Type | Friction Factor | Max Velocity | Noise Level | Best For |
|---|---|---|---|---|
| Sheet Metal (Smooth) | Low | 1,500 fpm | Low | Main supply trunks |
| Flex Duct (Wire Helix) | High | 900 fpm | Medium | Short branch runs |
| Duct Board | Medium | 1,200 fpm | Low | Quiet returns |
| Spiral Round Metal | Very Low | 2,000 fpm | Very Low | High-flow runs |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- ACCA (Air Conditioning Contractors of America) — Manual D Residential Duct Systems. Learn more
- ASHRAE — Fundamentals Handbook — Duct Friction Charts. Learn more
- AMCA International — Air Movement and Control Standards. Learn more
Frequently Asked Questions
How do you calculate CFM from duct size?
CFM = duct area (sq ft) x velocity (fpm). For a round duct, area = pi x (diameter/2)^2 / 144. An 8-inch duct at 600 fpm delivers about 209 CFM.
What is friction loss in ductwork?
Friction loss is the pressure drop caused by air rubbing against duct walls. It increases with duct length, velocity, and smaller diameters, reducing delivered airflow.
What is a good CFM for a room?
A typical room needs 1 CFM per sq ft of floor area. Kitchens and bathrooms may need 50-100 CFM of exhaust, while bedrooms need 50-75 CFM of supply air.