Tee fittings are not all the same size. Depending on your application, you should find the appropriate sized tee fitting that will suit your needs. There are several type of tee fittings, such as unequal tees. Try to determine first what kind you need, and the size of the pipes that you are fitting into the tee. It would be best if you could bring a sample of the pipe to your local hardware store. This is so you can show the sales person what you are working with, so he can make the appropriate recommendation for your application. Don’t just buy tee fittings blindly, because they come in different forms and sizes.