What if the faucet adapter doesn't fit
Quick-connect faucet adapter is a standard item in the plumbing industry. It fits most, but not all faucets on the market. It will not fit newer kitchen faucets that do not have threaded aerator, such as the pull-out ones and the pull-down ones. Aerator on these faucets are pressed in. They do not have threads.
Our quick-connect has both 15/16"-27 male and 55/64"-27 female threads.
If your faucet has threaded aerator, but the size doesn't fit, you need to do the following:
Take the existing aerator and the quick-connect to a hardware store. In most cases, you'll find a small adapter to convert the non-standard thread to standard thread.
Here's an example:
A popular faucet brand is Pfister. Pfister's aerator has 13/16"-27 male thread. To attach the quick-connect, you will need to buy a 13/16" x 55/64" male x male aerator adapter from a hardware store such as Home Depot. The 13/16" top thread goes into the faucet spout. The 55/64" bottom thread goes into the quick-connect. You need to remove the top washer of the quick-connect to reveal the 55/64" female thread.
Check to see if you have other water supplies
Read here
If nothing else works, consider replace your faucet with a new one that has standard threaded aerator, i.e, 15/16" male.