How it works — and what it really is

Here is exactly how FilterByZip builds your schedule — every data source and every step.

Step 1 — we find your area

You type your zip code. We use it to look up your county.

Step 2 — we check how hard your water is

Hard water has extra minerals in it. Those minerals build up inside your filter and clog it. We look up a hardness number for your county. The higher the number, the harder your water, and the faster your filter fills up.

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 — we tell you when to change it

A fridge filter can handle a set amount of water before it stops working. Hard water uses that up faster. Soft water makes it last longer. We start with the standard six months printed on the box and move it earlier or later based on your water. Tell us how many people are in your home and we adjust for that too.

Where the data comes from

Your zip code is matched against a free public list of U.S. zip codes (GeoNames, CC-BY 4.0) covering 40,000+ of them. Where we have a hardness number for your county, it comes from the published water-quality report of a utility serving that county. Everywhere else, we fall back to a statewide estimate we compiled ourselves — a rougher number, and your result says so when that's what you're getting.

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 here is an estimate from regional water-hardness data, not a reading from your tap. But the six months printed on the box is the same number given to every household in America, from Phoenix to Seattle. This one at least accounts for where you actually live.