HVAC Filter Sizing Calculator Formula

Understand the math behind the hvac filter sizing calculator. Each variable explained with a worked example.

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Formulas Used

Min Filter Face Area

face_area_needed = min_face_area

Filters Needed

num_filters = single_filter_area > 0 ? ceil(min_face_area / single_filter_area) : 0

Actual Face Velocity

actual_velocity = single_filter_area > 0 ? system_cfm / (ceil(min_face_area / single_filter_area) * single_filter_area) : 0

Variables

VariableDescriptionDefault
system_cfmSystem Airflow(CFM)1200
max_face_velocityMax Face Velocity(ft/min)300
filter_widthFilter Width(in)20
filter_heightFilter Height(in)25
min_face_areaDerived value= max_face_velocity > 0 ? system_cfm / max_face_velocity : 0calculated
single_filter_areaDerived value= filter_width * filter_height / 144calculated

How It Works

Filter Sizing

Face Area = CFM / Face Velocity

Filters = Face Area / Single Filter Area

Keeping face velocity under 300 ft/min ensures good filtration and low pressure drop. Higher velocities reduce filter life and may blow dust through the media.

Worked Example

1,200 CFM system, 300 ft/min max, using 20x25 inch filters.

system_cfm = 1200max_face_velocity = 300filter_width = 20filter_height = 25
  1. 01Min face area = 1200 / 300 = 4.0 sq ft
  2. 02Filter area = 20 x 25 / 144 = 3.47 sq ft
  3. 03Filters = ceil(4.0 / 3.47) = 2
  4. 04Actual velocity = 1200 / (2 x 3.47) = 173 ft/min

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