Grease Duct Design Calculator

Grease duct sizing is really a velocity problem: NFPA 96 requires airflow fast enough (at least 1,500 feet per minute) to keep grease vapor moving instead of condensing on duct walls, but slow enough (no more than 2,500 FPM) to avoid excess noise and erosion. This calculator targets 1,800 FPM — a middle-of-the-range value — and derives the required duct cross-sectional area from your exhaust CFM using Area = CFM / Velocity, converting square feet to square inches along the way. From that area it computes both a round-duct diameter (rounded up to the nearest even inch, since round duct is typically stocked in even sizes) and a rectangular alternative, searching a list of standard sheet-metal widths for a width/height pair that meets the required area while staying within NFPA 96's 4:1 maximum aspect ratio for rectangular grease duct. Because the round diameter gets rounded up, the calculator re-derives the actual resulting velocity from that rounded size — it's worth checking this number stays inside the 1,500-2,500 FPM window rather than trusting the target velocity alone.

Cleanout openings are tallied per code: one for every 12 feet of horizontal run, one at each elbow (direction change), plus one at the fan connection. Static pressure is estimated using a friction rate of 0.08 inches water gauge per foot of straight duct, with each elbow counted as 15 additional equivalent feet — a standard method for accounting for the extra resistance elbows add to airflow. Note this tool doesn't distinguish horizontal from vertical runs when counting cleanouts, so a duct with significant vertical rise may need additional cleanouts beyond what's shown here.