20x20x4 Air Filter Actual Size Chart (19.5"x19.5"x3.63" vs 19.88"x19.88"x4.38")
A '20x20x4' air filter comes in two incompatible thicknesses: standard 4-inch return grille filters measure 19.50" x 19.50" x 3.63" (3-5/8"), whereas Honeywell F100/F200 and Lennox media cabinet '20x20x4 / 20x20x5' cartridges measure 19.88" x 19.88" x 4.38" (4-3/8"). Buying the 4.38" media cartridge for a 3.63" return grille causes the filter to stick out 3/4 inch.
⚠ CRITICAL FIT RULE: Measure your existing frame thickness before ordering. If your slot is 3-5/8" (3.63") deep, buy a standard 20x20x4 pleated filter. If you have a Honeywell FC100A1011 or Lennox X6672 media cabinet, you need the 4-3/8" (4.38") deep cartridge. Order exact-fit multi-packs at HVACDirect with coupon 9QOS73CI2VEW.
Verified Reddit / X Live Monitor Fit Complaint:
r/HomeImprovement — '20x20x4 return filter sticks out 3/4 inch'