While lenders have been required (for loans closed after July '99) to cancel Private Mortgage Insurance (PMI) when the balance goes under 78% of the price of purchase, they do not have to cancel automatically if the loan's equity is above 22%. (There are some exceptions -like some "high risk' loans.) But if your equity reaches 20% (regardless of the original purchase price), you have the legal right to cancel PMI (for a mortgage loan closed past July 1999).
Keep a running total of payments
Analyze your statements often. Also stay aware of how much other homes are purchased for in your neighborhood. You've been paying mostly interest if your mortgage loan closed fewer than 5 years ago, so your principal probably hasn't gone down much.
When you determine you have achieved at least 20 percent equity in your home, you can begin the process of canceling your Private Mortgage Insurance. You will need to notify your mortgage lender that you want to cancel PMI. Lending institutions ask for documentation verifying your eligibility at this point. The best proof there is can be found in a state certified appraisal using form URAR-1004 (Uniform Residential Appraisal Report), required by most lenders before canceling PMI.