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Calcimator

Grease Duct Design Calculator

Size grease exhaust ductwork based on CFM, run length, and NFPA 96 code requirements for velocity, material, and cleanout spacing.

Inputs

Results

Round Duct Diameter (in)

18

Rectangular Width (in)8
Rectangular Height (in)30
Duct Velocity (FPM)1,698
Cleanout Openings6
Est. Static Pressure (in. w.g.)4.4
Est. Duct Weight (lbs)263
Rect Side15.49
Gauge Label18 ga Stainless Steel
How to Use This Calculator
  1. Enter the total exhaust CFM from the hood.
  2. Set the total duct run length in feet and the number of 90-degree elbows.
  3. Select duct material: 1 = carbon steel, 2 = stainless steel.
  4. Review the recommended round duct diameter and verify velocity is within NFPA 96 range of 1,500-2,500 FPM.
  5. Use the cleanout count and estimated static pressure for code compliance and fan sizing.

How the result changes with Exhaust CFM

Exhaust CFMRound Duct Diameter (in)
5,09024
17,56544
32,53558
45,01068

What each input means

Exhaust CFM
Total exhaust airflow from the hood in cubic feet per minute.
Duct Run Length (ft)
Total straight-line duct length from hood to exhaust fan.
Number of Elbows
Number of 90° elbows or direction changes in the duct run.
Duct Material
1 = 16-gauge Carbon Steel, 2 = 18-gauge Stainless Steel.

What each result means

Round Duct Diameter (in)
Recommended round duct diameter (rounded to nearest even inch).
Rectangular Width (in)
Rectangular duct width option.
Rectangular Height (in)
Rectangular duct height option.
Duct Velocity (FPM)
Actual air velocity. NFPA 96 requires 1500-2500 FPM.
Cleanout Openings
Required access/cleanout openings per NFPA 96 (every 12 ft + at each elbow).
Est. Static Pressure (in. w.g.)
Estimated duct static pressure for fan sizing.
Est. Duct Weight (lbs)
Approximate total duct weight for structural support planning.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Exhaust CFM = 3000, Duct Run Length (ft) = 25, Number of Elbows = 2, Duct Material = 2 = 4 input(s) provided
  2. Calculate Round Duct Diameter
    Round Duct Diameter
    18 = 18
  3. Calculate Rectangular Width
    Rectangular Width
    8 = 8
  4. Calculate Rectangular Height
    Rectangular Height
    30 = 30

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