This balloon mortgage calculator allows you to vary both the term and interest rate then calculates the mortgage balance owed (balloon) at the end of the payment term.
A balloon mortgage is usually rather short, with a term of 5 years to 7 years, but the payment is based on a term of 30 years. The advantage is they usually offer a lower interest rate and can be easier to qualify for than a traditional 30-year-fixed mortgage. However, beware of the risk because you will either need to refinance or pay off your outstanding balance at the end of the term.