Jan 8, 2026
2 min read
beginner
Quick RefFilter Sizes Cheat Sheet
Most common filter sizes, how to measure, where to buy, and replacement schedules at a glance.
Most Common Sizes#
| Size | Where It's Common |
|---|---|
| 20x20x1 | #1 most popular |
| 16x25x1 | #2 most popular |
| 20x25x1 | #3 most popular |
| 16x20x1 | #4 most popular |
| 20x25x4 | Whole-house media filters |
| 16x25x4 | Whole-house media filters |
How to Measure#
- Turn off HVAC
- Remove current filter
- Measure length × width × depth
- Round UP each measurement to nearest inch
- That's your nominal size (what you order)
Where to Buy#
- Hardware stores: Home Depot, Lowe's, Ace
- Big box: Costco, Walmart, Target
- Online: Amazon, FilterBuy, discount filter sites
Pro tip: Buy in bulk. A 6-pack of pleated filters costs much less per filter than buying singles.
Replacement Schedule#
| Situation | 1" Filters | 4" Filters |
|---|---|---|
| Normal | 60-90 days | 6 months |
| With pets | 30-60 days | 3 months |
| Allergies | 30-45 days | 3 months |
| Multiple pets | 20-30 days | 6-8 weeks |