To calculate the monthly payment for a particular loan amount:
Enter the loan amount, number of payments, and interest amount.
Then click on "monthly payment."
Try this example:
Enter 100000 as loan amount, 120 as number of payments
(12 payments a year for 10 years), and 9.0 as interest rate.  Click on "monthly payment."
Answer should be 1293.98
Important:  Do not enter commas !  Write over the 0s that are in the fields now.

To calculate the particular loan amount that a certain payment will pay off:
Enter the number of payments, interest rate, and monthly payment. 
Then click on "loan amount."
Try this example:
Enter 120 as number of payments (12 payments a year for 10 years), 
and 9.0 as interest rate, and 1000 as the monthly payment.  
Click on "loan amount."
Answer should be 77281.21
(This is the loan amount that a 1000 monthly payment will pay off in 120 payments at 9.5%.
Important:  Do not enter commas !  Write over the 0s that are in the fields now.