What can I use to determine the proper size of faucet aerator to buy to replace the existing ones in my american standard faucet?
Any sizing chart online won't be accurate because the sizes are so close together. Your best bet is to take your old aerator to a home store. They usually have a template gauge you can screw your aerator into to determine the correct replacement size. Taking it to a plumbing supply store would also work. They could tell just by looking at it.