Grease Duct Size Calculator
What size grease duct do you need? Enter your hood's CFM, or its length and type, and get the recommended round duct diameter with the resulting air velocity.
Velocity by duct size
Tap a row to compare a different size.
| Diameter | Velocity (FPM) | Status |
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How duct size is calculated
Velocity (FPM) = CFM ÷ duct area (sq ft). For round duct, area = π × (diameter in inches ÷ 12)² ÷ 4. The calculator recommends the largest stocked size that keeps velocity at or above 1,500 FPM. That keeps you in the 1,500–2,500 FPM design range on the quieter, lower-static-pressure side while still moving air fast enough to carry grease.
Common questions
What size grease duct do I need?
It depends on your hood's exhaust CFM. Divide CFM by the duct's cross-sectional area to get velocity. Most designers target 1,500–2,500 FPM. For example, 1,800 CFM through a 14" round duct (1.069 sq ft) is about 1,680 FPM.
What is the minimum air velocity for a grease duct?
The International Mechanical Code (Section 506.3.4) and NFPA 96 require at least 500 FPM in grease ducts serving Type I hoods. Below that, grease can settle in the duct and create a fire hazard.
Can grease duct velocity be too high?
Yes. Above roughly 2,500 FPM the system gets noisy and static pressure rises, so the exhaust fan has to work harder. If velocity is high, step up one duct size.
How much CFM does my hood need?
Your hood's label CFM governs. Without one, the IMC sets minimums per linear foot, such as 300 CFM/ft for a wall canopy over medium-duty equipment. A 6 ft wall canopy over fryers would need at least 1,800 CFM.
What if my hood has two duct collars?
Split the total CFM between the collars and size each duct for its share. Select "2 collars" above to do this automatically.
For estimating only. Hood-length mode uses IMC Section 507 minimum exhaust rates for unlisted hoods; listed hoods should use the CFM on their label. Final duct design must follow your local code and authority having jurisdiction.