Compound Interest Calculator
See how your savings or investments grow over time when interest earns interest. Add a starting balance, optional monthly contributions, and watch the magic of compounding.
Future balance
—
Total contributions
—
Interest earned
—
Starting balance
—
Year-by-year breakdown
| Year | Contributions | Interest earned | End balance |
|---|
How compound interest works
Compound interest is interest earned on both your original money and the interest it has already earned. Over time, your balance grows faster and faster — that’s the “snowball effect” Albert Einstein reportedly called the eighth wonder of the world.
- Starting balance — the lump sum you have invested at the start (your principal).
- Monthly contribution — extra money you add every month. Even small amounts compound into large sums given enough time.
- Annual interest rate — the yearly return on your money. Historical S&P 500 returns average around 7% after inflation; high-yield savings accounts typically pay 4–5%.
- Years to grow — your investment horizon. Time is the single most powerful variable in compounding.
- Compounding frequency — how often interest is calculated and added back. Most savings accounts compound daily; many investments are quoted as monthly. The more frequent the compounding, the slightly higher the result.
Tip: Doubling your monthly contribution roughly doubles your interest earnings, but doubling the number of years can quadruple them. Starting early beats starting big.
This calculator assumes a constant interest rate and that contributions are made at the end of each month. Real-world returns vary year to year. This is not financial advice.
Compound interest is the reason small amounts of money saved early can grow into life-changing sums later — and it’s also why debt left unpaid snowballs faster than most people expect. This calculator shows you both sides: how much your savings or investment will grow over time, and exactly how much of that final number is interest earned versus money you actually put in.
The core idea is simple. With simple interest, you earn interest only on your original deposit. With compound interest, you earn interest on your deposit and on the interest you’ve already earned. The longer your money sits, the more dramatic the difference becomes.
Starting balance — the lump sum you’re investing or saving today. Even $0 works if you’re starting from scratch and relying on regular contributions.
Monthly contribution — the amount you add each month on top of your starting balance. Consistent contributions often matter more than a large starting amount.
Annual interest rate — your expected yearly return. For a high-yield savings account, this might be 4–5%. For a broad stock market index fund, the historical average is around 7% after inflation. Be conservative with estimates.
Years to grow — how long you leave the money untouched. This is the most powerful variable in the formula. Doubling your rate of return helps — but doubling your time in the market often does more.
Compounding frequency — how often interest is calculated and added to your balance. Daily compounding produces slightly more than monthly, which produces slightly more than annually. For most savings accounts and investments, monthly or daily is standard.
The year-by-year breakdown table shows exactly how your balance, contributions, and interest earned stack up at the end of each year — open it to see the full picture of how your money grows over time.
The results show your total balance, total contributions, and total interest earned. That gap between contributions and interest is the real story — it shows how much the math is working in your favor without you doing anything extra.
The most important takeaway: starting early matters far more than starting with a lot. Someone who invests $200/month from age 25 to 65 will typically end up with significantly more than someone who invests $400/month from age 40 to 65, even though the late starter put in more total dollars. Time is the one input you can’t buy back.
If you have a specific dollar target in mind, the Savings Goal Calculator tells you exactly how long it will take to get there.
For long-term wealth building, run the same numbers through the Investment Return Calculator to see projected ROI and inflation-adjusted value.
Putting compounding to work for retirement? The Retirement Savings Calculator shows whether you’re on track.
This calculator assumes a constant interest rate and that contributions are made at the end of each month. Real-world returns vary year to year. This is not financial advice.