I have had a two stage Omnifilter under my sink for many years. The diameter is not listed in the product description. Omni has decided to replace all of its systems with large diameter 1/4 inch filters with the result that filters are no longer available for its older systems. I now own an Aquasana system that actually tastes a lot better, filters out more pollutants, and costs a little less than a new Omni. I recommend you take a look at the Aquasana 3-Step System.
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