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Put the monthly payment in this box.
For example, if the monthly payment is $40, put 40 without the dollar sign in the box.

Next, put in this box the annual interest rate.
If the interest rate is 5.25% put 5.25 in the box without the % sign.

Next, choose how many months you want to pay back the loan.

And click this button.

The total amount paid is:

The total interest paid is:

You can do this again with different numbers. Just hit the clear button.