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.
References & Methodology
- Air Conditioning Contractors of America (ACCA), "Manual D: Residential Duct Systems," 2014. — Duct sizing methodology based on friction rate and available static pressure.
- ASHRAE, "Fundamentals Handbook," Chapter 21: Duct Design, 2021. — Equal friction and static regain methods for duct sizing; friction loss charts.
- Sheet Metal and Air Conditioning Contractors National Association (SMACNA), "HVAC Duct Construction Standards," 2005. — Duct material specifications and construction guidelines.
References
- 10 CFR Part 431 — Energy Efficiency Program for Certain Commercial and Industrial Equipment (eCFR)
- 25 to rates that vary by region of conditioned-air energy
- 2 to 3 units of heat energy for every 1 unit of electrical energy consumed
- 10 CFR Part 431 — Energy Efficiency Program: Commercial and Industrial Equipment
- 10 CFR Part 433 – Energy Efficiency Standards for New Federal Commercial and Multi-Family High-Rise
- A Brief Guide to Mold, Moisture, and Your Home
- 2021 International Energy Conservation Code, as referenced by the Utah Uniform Building Code Commiss
- 2 CFR Part 200 — Uniform Administrative Requirements, Cost Principles, and Audit Requirements for Fe