How to use this calculator
Enter your initial deposit (the lump sum you're starting with), your planned monthly contribution, the annual interest rate you expect to earn, and how long you plan to invest. Choose your compounding frequency, monthly is most common for savings accounts and investment accounts.
The calculator shows your projected final balance, how much of it came from your own contributions versus interest earned, and a year-by-year breakdown so you can see the snowball effect in action. Use the “what if” comparison to see how even small rate changes affect your outcome.
The power of compound interest
Albert Einstein reportedly called compound interest “the eighth wonder of the world.” Whether or not he actually said it, the math backs up the hype. Compound interest means you earn interest on your interest, and over long time periods, this creates exponential growth.
Here's a concrete example: if you invest $10,000 and add $300 per month at a 7% annual return, after 10 years you'll have about $72,000. After 20 years: roughly $178,000. After 30 years: about $390,000. Notice how the growth accelerates, you earned more in the last 10 years than in the first 20 combined. That's compounding at work.
The three biggest levers are: how much you invest, what rate you earn, and, most importantly, how long you let it compound. Time is the ingredient you can't buy back, which is why starting early matters so much.
The Rule of 72
The Rule of 72 is a quick mental math shortcut for estimating how long it takes your money to double. Simply divide 72 by your annual interest rate. The result is the approximate number of years to double your investment.
| Annual Rate | Years to Double | Example |
|---|---|---|
| 4% | 18 years | $10,000 → $20,000 |
| 6% | 12 years | $10,000 → $20,000 |
| 8% | 9 years | $10,000 → $20,000 |
| 10% | 7.2 years | $10,000 → $20,000 |
| 12% | 6 years | $10,000 → $20,000 |
This rule works best for rates between 4% and 12%. It's not exact, but it's close enough for quick back-of-the-envelope calculations when you're comparing investment options or setting savings goals.
Frequently asked questions
What is compound interest and how does it work?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns on the original amount, compound interest earns interest on interest, creating exponential growth over time. The longer your money compounds, the faster it grows.
How does compounding frequency affect my returns?
More frequent compounding means interest is calculated and added to your balance more often, which slightly increases your total return. Monthly compounding earns more than annual compounding at the same rate because each month's interest starts earning interest sooner. However, the difference is relatively small, the interest rate and time horizon matter far more.
What is a realistic rate of return?
The S&P 500 has historically returned about 10% annually before inflation, or roughly 7% after inflation. High-yield savings accounts currently offer 4 to 5%. CDs and bonds typically return 3 to 6%. For conservative long-term planning, using 6 to 7% for stocks or 4 to 5% for savings accounts is reasonable.
How much should I save each month to become a millionaire?
It depends on your timeline and rate of return. At 7% annual returns: saving $500/month gets you to $1 million in about 33 years. Saving $1,000/month gets there in about 25 years. Starting with a $50,000 lump sum and adding $500/month reaches $1 million in about 27 years. The earlier you start, the less you need to save each month.