Round Duct Sizing Reference
Enter duct diameter and airflow (CFM) to get velocity (FPM), friction rate (in.w.g./100 ft), noise level, and SMACNA design guidance. Friction calculations assume galvanized steel per ASHRAE Manual of Fundamentals.
Small Ducts (4"–8")
Branch ducts for individual rooms and zone diffusers.
- 4" Round Duct Calculator — Small branch runs, bath fans, spot ventilation
- 6" Round Duct Calculator — Standard bedroom branch — 50–150 CFM range
- 8" Round Duct Calculator — Living area branch or small trunk — 100–300 CFM
Medium Ducts (10"–16")
Main trunk lines and large zone branches.
- 10" Round Duct Calculator — Medium trunk line — 200–500 CFM
- 12" Round Duct Calculator — Large trunk line — 300–800 CFM
- 14" Round Duct Calculator — Large commercial branch or residential main trunk
Frequently Asked Questions
What is the maximum velocity for a bedroom supply duct?
SMACNA recommends a maximum of 500 FPM for bedroom supply ducts to maintain quiet operation. At higher velocities, occupants will hear air rushing noise. For living areas and hallways, 700 FPM is acceptable. Mechanical rooms and return air plenums can operate at 900 FPM.
What friction rate should I design to?
Most residential duct systems are designed to 0.10 in.w.g. per 100 ft of equivalent length. Some higher-static air handlers allow 0.15 in.w.g./100 ft. Always check the air handler rated external static pressure and subtract coil, filter, and grille pressure drops to find available duct friction budget.
Should I use flex duct or hard duct?
Hard galvanized steel or spiral duct is preferred for trunk lines. Flex duct is acceptable for branch runs up to 14 ft with minimal bends. Compressed or kinked flex duct dramatically increases friction — always fully extend and support flex duct. Flex duct has 15–20% more friction than equivalent hard duct due to its corrugated interior.