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Once you've committed to a media cabinet, the next choice is filter depth: 4 inches or 5 inches (sometimes marketed as 6"). The difference seems minor but affects cost, airflow, and maintenance cadence.
What the inch difference means
Depth directly affects three things: surface area, airflow restriction, and lifespan. Deeper media = more folds = more surface for dust to land on.
| Depth | Typical MERV | Lifespan | Price |
|---|---|---|---|
| 4 inch | MERV 11-13 | 6-9 months | $20-35 |
| 5 inch | MERV 11-16 | 9-12 months | $30-55 |
| 6 inch | MERV 13-16 | 12 months | $40-70 |
When 4" makes sense
The 4-inch form factor is the most common. It's the default for Honeywell's FC100 series and many generic cabinets. Pick 4-inch when:
- Your existing cabinet is built for 4"
- You have standard air quality needs (no severe allergies)
- Budget is a priority
- You're willing to change every 6-9 months
When 5" (and 6") makes sense
The 5/6-inch form factor is premium territory — typical for Aprilaire, Trane CleanEffects, and Lennox PureAir systems. Choose when:
- Your cabinet was built for 5" or 6" (most common with Aprilaire)
- You want MERV 13+ performance consistently
- You want annual (not semi-annual) changes
- Premium air quality justifies premium cost
Cabinet compatibility matters
You can't swap a 4" filter for a 5" in the same cabinet. The cabinet dictates depth. Common pairings:
- Honeywell FC100A (4" cabinet): 20x25x4, 16x25x4, 20x20x4
- Aprilaire 2200/2400 (6" cabinet): takes Aprilaire 201 or 413
- Aprilaire 1210 (6" cabinet): takes Aprilaire 213
- Trane CleanEffects (5" cabinet): takes proprietary BAYFTAH23M
- Lennox PureAir (5" cabinet): takes X8308 or X6672
Frequently asked questions
Can I convert from a 4" to 5" cabinet?
Yes, but requires replacing the cabinet — same cost as original install. Usually not worth it unless you have specific needs.
Do 5" filters restrict airflow more than 4"?
Counterintuitively, no. More media means more surface area, so a 5" MERV 13 typically flows better than a 4" MERV 13.
Are generic 4"/5" filters as good as OEM?
For fit and MERV rating, many generics perform comparably. For Aprilaire and Trane CleanEffects cabinets, OEM is worth the premium.
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