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Finance calculators here cover interest, loan payments, and the difference between simple and compound growth, the mechanics behind a savings account, a credit card balance, or a mortgage schedule. These are educational tools for understanding how the numbers behave, not financial advice, and none of them account for your personal tax situation, fees, or risk tolerance.
Simple interest is calculated only on the original principal, every period, for the life of the loan or deposit. A $1,000 balance at 5 percent simple interest earns exactly $50 a year, every year, for as long as the money sits there. Compound interest is calculated on the principal plus whatever interest has already accumulated, so each period's interest is calculated on a slightly larger base than the one before it.
Over a short period the difference is small, but it grows with time. The same $1,000 at 5 percent compounded annually earns $50 in year one, same as simple interest, but $52.50 in year two, because the second year's interest is calculated on $1,050, not $1,000. Over a 30 year horizon that gap compounds into a genuinely different outcome, which is the entire premise behind long-term retirement saving.
Interest can compound annually, monthly, daily, or continuously, and the more often it compounds, the more total interest accumulates for the same stated annual rate, because each compounding period adds a little more to the base the next period is calculated on. A 5 percent annual rate compounded monthly yields more over a year than the same 5 percent compounded just once, though the gap between monthly and daily compounding is small enough that it rarely changes a real-world decision by much.
APR, the annual percentage rate, is the stated yearly interest rate before accounting for compounding within the year. APY, the annual percentage yield, factors compounding in and reflects what you'd actually earn or pay over a full year. The two are equal only when interest compounds exactly once a year; any more frequent compounding makes APY higher than APR for the same underlying rate, which is why a savings account advertises its APY, the more flattering number, while a credit card advertises its APR.
An amortizing loan, a standard mortgage or auto loan, charges interest on the outstanding balance each period, and early in the loan that balance is close to the full amount borrowed, so a large share of each payment goes to interest rather than principal. As the balance shrinks over time, less of each fixed payment is needed to cover interest, and more goes toward reducing the principal, which is why the same monthly payment builds equity slowly at first and much faster in a loan's later years.