Compound Interest Calculator
See how your money grows with compound interest. Calculate future value and total interest earned across different compounding frequencies.
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Understanding Compound Interest
Compound interest is interest calculated on both your original principal and the interest you have already earned. It is often called "interest on interest," and it is the mechanism that lets savings and investments grow exponentially rather than in a straight line. Given enough time, compounding can turn steady contributions into a much larger sum.
The concept is simple, but its effect is powerful. Each period, your earnings are added to the balance, and the next period's interest is calculated on that larger balance. The longer this cycle repeats, the more dramatic the growth becomes.
Simple vs compound interest
Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus accumulated interest, so it accelerates over time. On a long time horizon, the gap between the two becomes enormous, which is why compounding is often described as one of the most important forces in personal finance.
How compounding frequency matters
The more frequently interest is compounded, the more total interest you earn on the same rate. Daily compounding yields slightly more than monthly, which yields more than annual, because interest is added to your balance sooner and immediately starts earning its own interest. The differences are modest year to year but add up over long periods.
Time is the biggest lever
Of all the inputs, time has the largest impact. Money invested early has far more years to compound, so starting sooner often matters more than the exact rate or amount. This is why beginning to save in your twenties, even with small sums, can outpace much larger contributions started decades later.
The Rule of 72 and regular contributions
The Rule of 72 is a quick shortcut: divide 72 by your annual rate of return to estimate how many years it takes to double your money. At 6%, that is roughly 12 years. Adding regular contributions supercharges the effect, because every new deposit becomes fresh principal that compounds alongside your existing balance.
Frequently asked questions
- Compound interest is interest earned on both your original principal and on the interest already added to it. Because each period’s interest starts earning its own interest, savings grow faster over time than they would with simple interest.
- The more often interest is compounded — daily versus monthly versus annually — the more you earn, because interest is added to the balance sooner and starts compounding earlier. The difference is modest at low rates and grows more noticeable at higher rates and over long periods.
- Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus accumulated interest, so the balance grows on a curve that steepens over time.
- A quick estimate is the Rule of 72: divide 72 by the annual interest rate to approximate the number of years to double. At 6% that is about 12 years. It is an approximation, so use this calculator for a precise figure.