All our filters are designed to last up to 5 years under normal residential use. You can check the size of your existing filter by looking for a label on the bottom of the unit. Model HM400 indicates standard size filter. Still not sure which filter you need? Our Standard size filters measure 14.5” high. Feel like an upgrade? All standard size units are interchangeable with all standard size filters. So, if you purchased a HealthMate® and would like to upgrade to a HealthMate Plus® you can replace your old HealthMate® filter with a new HealthMate Plus® filter.