Total Interest Paid Over a Loan

Multiply the payment by the number of payments, subtract what you borrowed, and what remains is the price of the money. The arithmetic is trivial and the answer is often shocking: 360 payments of $1,199.10 come to $431,676, so a $200,000 mortgage at 6% costs $231,676 in interest. You pay for the house more than twice, and the second time you get no house.

This is the number to look at when a lender offers a longer term to "lower your payment". Stretching a truck loan from 48 months to 72 does lower the monthly figure, but it raises the total handed over, because interest is charged on a balance that now falls more slowly. Read the payment and the total interest together, never one alone.