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Prepaying a loan: Shouldn't the interest be recalculated like a shorter loan?
You seem to think that you are mostly paying interest in the first year because of the length of the loan period. This is skipping a step.  You are mostly paying interest in the first year because your principle (the amount you owe) is highest in the first year.  You do pay down some principle in that first year; this reduces the principle in the second year, which in turn reduces the interest owed. Your payments stay the same; so the amount you pay to principle goes up in that second year. This continues year after year, and eventually you owe almost no interest, but are making the same payments, so almost all of your payment goes to principle. It is a bit like "compounded interest", but it is "compounded principle reduction"; reducing your principle increases the rate you reduce it. As you didn't reduce your principle until the 16th year, this has zero impact on the interest you owed in the first 15 years. Now, for actual explicit numbers.  You owe 100,000$ at 3% interest.  You are paying your mortgage annually (keeps it simpler) and pay 5000$ per year. The first year you put 3000$ against interest and 2000$ against principle. By year 30, you put 145$ against interest and 4855$ against principle.  because your principle was tiny, your interest was tiny.