At 1500 fpm (7.62 m/s), a single commercial kitchen exhaust can move over 10,000 cubic feet of grease-laden air per minute—and if your duct is even slightly oversized, that velocity drops, grease condenses on the walls, and you've got a fire hazard waiting for a spark. Getting the math right isn't optional; it's code.
The Formula
The core relationship is the continuity equation for duct flow:
A = Q / V
Where:
- A = required duct cross-sectional area (m² or ft²)
- Q = exhaust airflow (m³/s or CFM)
- V = target transport velocity (m/s or FPM)
Why this form? In a steady, incompressible flow, mass flow rate is constant. For grease-laden air, we need a minimum velocity to keep particulates entrained. Below ~1500 FPM, grease droplets settle; above ~2500 FPM, pressure drop and noise spike. The equation directly gives the area needed to achieve that velocity for a given flow.
Once we have area, round duct diameter follows from geometry:
D = sqrt(4 * A / π)
For rectangular ducts, if you fix the width, the required height is:
H = A / W
where W is the chosen width. After sizing, the actual velocity is back-calculated:
V_actual = Q / A_actual
This check ensures the velocity falls in the acceptable range.
Worked Example 1: Round Duct, Metric
Inputs:
- Airflow: 6000 m³/h (1.667 m³/s)
- Target velocity: 8 m/s (1575 FPM)
- Rectangular width: not used (round duct)
Step 1: Required area
A = 1.667 / 8 = 0.2084 m²
Step 2: Round diameter
D = sqrt(4 * 0.2084 / π) = 0.515 m = 515 mm
Step 3: Actual velocity (same as target since round)
V_actual = 1.667 / 0.2084 = 8.0 m/s → 1575 FPM (within 1500–2500 FPM) ✓
Result: 515 mm round duct is appropriate.
Worked Example 2: Rectangular Duct, Imperial
Inputs:
- Airflow: 12000 CFM
- Target velocity: 2000 FPM
- Rectangular width: 24 inches (2 ft)
Step 1: Required area
A = 12000 / 2000 = 6 ft²
Step 2: Rectangular height
H = 6 / 2 = 3 ft = 36 inches
Step 3: Actual velocity
A_actual = 2 * 3 = 6 ft² → V_actual = 12000 / 6 = 2000 FPM ✓
Result: 24" x 36" rectangular duct works perfectly.
What Engineers Often Miss
First, the continuity equation assumes uniform velocity. In reality, grease duct flow profiles are not flat; a safety margin of 10% on velocity is wise. Second, rectangular ducts with aspect ratios above 4:1 create dead zones where grease accumulates—always check the height-to-width ratio stays below 4. Third, the calculator gives a first-pass size, but code requires fire-rated enclosure, welded seams, and cleanout doors every 20 feet—these add constraints that may force a different size.
Try the Calculator
Plug your own numbers into the Grease Duct Sizing Calculator to get round diameters and rectangular dimensions instantly.
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