How it works — and what it really is

FilterByZip gives you an estimate, not a measurement. Here is exactly how it's built, so you can judge how much to trust it.

Step 1 — your zip code becomes a county

We match your zip code against a public postal dataset (GeoNames, CC-BY 4.0) covering 40,000+ U.S. zip codes. If we can't find your exact zip, we fall back to the state your zip prefix belongs to.

Step 2 — your county gets a hardness figure

Water hardness — dissolved calcium and magnesium, measured in grains per gallon (gpg) — varies enormously across the U.S. We use regional classifications published by the U.S. Geological Survey, supplemented with hardness figures from published municipal water-quality reports for major metro counties. Where county-level data isn't available, we use the state average.

What this is not: a live reading from your water utility, or a test of your tap water. Hardness varies within counties, and homes on private wells or with water softeners will differ. For a precise figure, check your utility's annual water-quality report or test your water.

Step 3 — hardness becomes a schedule

Most refrigerator filters are rated for roughly 200–300 gallons or 6 months, whichever comes first. Harder water carries more mineral and sediment load, so filters clog and lose effectiveness sooner. We adjust the manufacturer-standard 6 months like this:

• Very hard (over 10.5 gpg) — every 4 months
• Hard (7–10.5 gpg) — every 5 months
• Moderately hard (3.5–7 gpg) — every 6 months
• Soft (under 3.5 gpg) — every 7 months

If you tell us your household size, we adjust once more: 5+ people means more water through the filter (subtract a month); 1–2 people means less (add a month). The result is never shorter than 3 months or longer than 8.

Why we ask about your fridge brand

Only to link you to the right replacement filter search. Brand doesn't change the schedule.

The honest summary

Every number on this site is an estimate based on regional water-hardness data. It's a well-reasoned starting point — better than the flat "every 6 months" on the box — but your manufacturer's manual and your own water conditions always win.