Loan calculation methodology

This first release models a fixed-rate loan with monthly payments and equal total payment amounts.

Monthly interest equals the balance multiplied by the annual interest rate divided by 12. Interest is rounded half up to the nearest cent.

Principal equals the monthly payment minus that month's interest, plus any extra principal payment. The final payment is adjusted to bring the balance exactly to zero.

If the required payment does not exceed the monthly interest, the calculator reports that the loan will not amortize under the entered terms.