What is a compound interest calculator?
A compound interest calculator shows how a sum of money grows when it earns interest, and that interest then earns interest of its own. It's the single most important concept in long-term saving and investing: the earlier you start and the longer you leave money invested, the more the curve bends upward. This tool adds optional regular deposits so you can model a real savings or investing plan, not just a one-time lump sum. Once you can see how your money grows, use the FIRE Calculator to translate that growth into an early-retirement target, or the Savings Goal Calculator to plan toward a specific milestone.
How to use this calculator
- Enter your starting amount — the money you have invested today (use 0 if you're starting from scratch).
- Add a regular contribution and choose monthly or yearly. This is the deposit you plan to make every period.
- Set the interest rate, number of years, and compounding frequency, then press Calculate to see your future balance, total contributions, and total interest earned.
Example
Suppose you start with $10,000, add $500 every month, and earn a 7% annual return compounded monthly for 20 years. You'll have contributed $130,000 of your own money ($10,000 + $500 × 240 months), but your balance grows to roughly $300,000. The extra ~$170,000 is pure compound interest — money your money made for you.
The math, in plain English
The classic compound interest formula is A = P(1 + r/n)nt: a starting principal P grows by a rate r that compounds n times a year for t years. The magic lives in the exponent — each period you earn interest on a slightly bigger balance than the period before, so growth accelerates instead of staying flat.
Regular contributions add a second layer. Every deposit you make also begins compounding for however many years remain, so an early $500 is worth far more at the finish line than a $500 added near the end. That's the real reason starting sooner beats trying to catch up later with larger amounts.
Worked example: $10,000 + $500/month at 7%
Here's how the default scenario — $10,000 to start, $500 added monthly, 7% compounded monthly — grows over 20 years. Watch the interest column overtake your own contributions somewhere around year 15:
| After | You contributed | Interest earned | Balance |
|---|---|---|---|
| 5 years | $40,000 | $9,973 | $49,973 |
| 10 years | $70,000 | $36,639 | $106,639 |
| 15 years | $100,000 | $86,971 | $186,971 |
| 20 years | $130,000 | $170,851 | $300,851 |
You put in $130,000 of your own money over 20 years but end with about $300,000 — the other ~$171,000 is interest. Run the same plan for 30 years instead of 20 and the interest portion dwarfs your contributions entirely. Time is the ingredient doing the heavy lifting.
Common mistakes with compound interest
- Using nominal instead of real returns. A 7% return with 3% inflation is really about 4% in today's buying power. For long-range planning, subtract expected inflation so a big headline number doesn't fool you.
- Ignoring inflation entirely. $300,000 in 20 years won't buy what $300,000 buys today. Still useful — just not as much as the raw figure suggests.
- Chasing compounding frequency. Daily versus monthly compounding barely moves the result at the same rate. The rate, the amount, and the time horizon matter far more.
- Starting late and expecting to catch up. Because early dollars compound longest, a decade of delay usually can't be undone by simply saving more later.
Rules of thumb
- The Rule of 72: divide 72 by your annual return to estimate the years it takes money to double. At 7% that's about 10 years; at 9%, about 8.
- Time beats timing. Consistent contributions left alone for decades typically outperform attempts to jump in and out at the "right" moment.
- Automate it. A fixed monthly transfer turns compounding into a background process you never have to think about.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and on the interest it has already earned. Over time this creates exponential growth, because each period you earn interest on a slightly larger balance.
How is compound interest calculated?
The core formula is A = P(1 + r/n)nt, where P is the starting principal, r is the annual rate as a decimal, n is how many times interest compounds per year, and t is the number of years. When you add regular contributions, each deposit also compounds for the remaining time.
Does more frequent compounding earn more money?
Yes, but the difference is small. Daily compounding earns slightly more than monthly, which earns slightly more than yearly, for the same nominal rate. The gap is usually a fraction of a percent.
What is the difference between APR and APY?
APR is the nominal annual rate before compounding. APY (annual percentage yield) is the effective rate after compounding is applied. APY is always equal to or higher than APR for the same account.
Are contributions added before or after interest?
This calculator adds each contribution at the end of the period (an ordinary annuity), the standard convention. Contributing at the start of each period would earn slightly more interest.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes your money to double. At a 7% return, money doubles in roughly 10 years; at 9%, about 8 years. It's an approximation, but a handy one for sanity-checking long-term growth without a calculator.
Does inflation affect my compound interest?
Yes. Compound interest grows the dollar amount, but inflation erodes what each dollar buys. If your investment returns 7% and inflation runs 3%, your "real" return — the growth in actual purchasing power — is closer to 4%. For long-range plans, consider using an inflation-adjusted rate so the future balance reflects real spending power, not just a larger number.