Duct Sizing Calculator
Proper duct sizing ensures adequate airflow, quiet operation, and energy efficiency. Undersized ducts create high velocity, noise, and static pressure; oversized ducts waste material and space. This calculator determines round duct diameter and equivalent rectangular sizes from CFM requirements.
Airflow Requirements
Based on ACCA Manual D methodology. Actual duct design must account for total system static pressure, fittings, and equipment capacity. A licensed HVAC contractor should perform the full duct design.
Duct Sizing Fundamentals
The core relationship between airflow, duct area, and velocity:
CFM = Area (ft²) × Velocity (FPM)
Therefore: Area = CFM ÷ Velocity, and Diameter = √(4 × Area / π)
Recommended Velocities by Location
| Duct Location | Max Velocity (FPM) | Noise Level |
|---|---|---|
| Supply branch (bedroom) | 500–600 | Very quiet |
| Supply branch (living areas) | 600–700 | Quiet |
| Supply trunk | 700–900 | Moderate |
| Return trunk | 600–800 | Moderate |
| Commercial supply | 1,000–1,500 | Acceptable |
Common Room CFM Requirements
| Room Type | Typical CFM |
|---|---|
| Bedroom (150 sq ft) | 75–100 |
| Living Room (300 sq ft) | 150–250 |
| Kitchen | 150–200 |
| Bathroom | 50–75 |
| Master Suite | 150–200 |
Frequently Asked Questions
What size duct for 400 CFM?
At the residential standard velocity of 700 FPM, 400 CFM requires approximately a 10-inch round duct or equivalent rectangular (8×12 inches). See the calculator above for exact sizing based on your friction rate and duct type.
Can I use flex duct everywhere?
Flex duct is common for branch runs but has higher friction than sheet metal. It should be pulled taut (not kinked or compressed) and kept to short runs (under 25 feet). Most codes require rigid duct for trunk lines and plenums.