Compound Interest Calculator

The starting amount — called the principal — is the lump sum you invest today. This could be existing savings, an inheritance, a CD rollover, a bonus, or any one-time investment you want to put to work. There's no minimum: the formula works the same whether you start with $500 or $500,000. Even a small principal grows substantially over decades. For example, $5,000 invested at 7% annual interest for 30 years grows to $38,061 without adding a single additional dollar. Starting early matters far more than starting big — $5,000 at age 25 outperforms $10,000 at age 35 because it captures 10 extra years of compounding. The key insight: time in the market is more valuable than the size of your initial deposit.

For savings accounts and high-yield accounts, use the APY (annual percentage yield) listed by your bank — the APY already accounts for compounding, so you don't need to adjust it. For stock market projections, the S&P 500 has returned roughly 7–10% annually (nominal) over long periods; 7% is a conservative long-run estimate after inflation. Compounding frequency determines how often earned interest is added back to your balance. Monthly compounding earns slightly more than annual because interest is reinvested 12 times per year instead of once. On a $10,000 deposit at 5% for 10 years, daily compounding ($16,487) outpaces annual compounding ($16,289) by $198 — a real but modest difference. For most savings products, monthly compounding is the standard; for stocks and mutual funds, you can treat growth as continuous and use any frequency.

This is where compound interest truly gets powerful. Regular contributions don't just add linearly — each dollar you contribute also starts compounding from the moment it's deposited. A $200/month contribution at 7% for 30 years adds over $1.2 million to your final balance compared to making no contributions at all. Even small contributions add up fast. Adding just $50/month to $10,000 at 7% for 20 years pushes the final balance from $38,697 (no contributions) to $64,491 — a $25,794 gain from contributions that cost you only $12,000 total out of pocket. The calculator shows your total contributions alongside interest earned so you can see exactly how much of your growth came from your money working vs. money you added.

Time is the most powerful variable in the compound interest formula. The interest you earn in later years is larger than all the interest from the early years combined — this is the exponential curve in action. Extending from 20 to 30 years on $10,000 at 7% grows the result from $38,697 to $76,123, almost doubling the outcome by adding just 10 more years. Use the preset buttons (1, 5, 10, 20, 30 years) for quick comparisons, or type any custom value up to 100 years. A useful exercise: run the same principal and rate at 10, 20, and 30 years in sequence. You'll see that the jump from 20 to 30 years is larger than the jump from 10 to 20 years — proof that the longer you wait, the faster the balance grows. This is why financial advisors consistently emphasize starting early rather than investing more later.