What is Compound Interest?
Compound interest is the interest calculated on the initial principal and also on the accumulated interest of previous periods. Essentially, it is "interest on interest," which allows your savings or investments to grow exponentially over time rather than linearly.
Albert Einstein famously called compound interest the "eighth wonder of the world." Our calculator helps you visualize this power by showing how small, regular contributions can transform into significant wealth over a long-term horizon.
How to Use the Compound Interest Calculator
To project your investment growth, follow these five steps:
- 1Enter Your Initial InvestmentInput the starting amount of money you have ready to invest or save today.
- 2Set Regular Monthly AdditionsConsistency is key. Even a small monthly contribution can drastically change the final outcome due to compounding.
- 3Select the Time HorizonChoose how many years you plan to stay invested. Compound interest becomes more powerful the longer the timeframe.
- 4Input Expected Interest RateEnter the estimated annual return. For reference, the S&P 500 has historically averaged around 10% before inflation.
- 5Select Compounding FrequencyChoose whether interest is added daily, monthly, or annually. Frequent compounding accelerates growth.
Frequently Asked Questions
What is the formula for compound interest?
The formula for compound interest is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal balance, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years.
How does compounding frequency affect my savings?
The more frequently interest compounds, the faster your money grows. For example, interest that compounds daily will result in a slightly higher final balance than interest that compounds monthly or annually, even if the interest rate (APR) is identical.
What is the "Rule of 72"?
The Rule of 72 is a quick way to estimate how long it will take for your money to double at a fixed interest rate. Simply divide 72 by your annual interest rate. For example, at a 6% interest rate, your money will double in approximately 12 years (72 / 6 = 12).
Does compounding daily instead of monthly make a big difference?
Not much. With the default inputs, daily compounding ends about $54 above monthly over 20 years, and annual compounding ends about $482 below. The rate and the years matter far more.
What is the effective annual rate?
It is the actual yearly growth after compounding, calculated as (1 + r/n)^n - 1. A 7% rate compounded monthly has an effective annual rate of about 7.23%.
The Rule of 72
How long to double your money?
Calculation: 72 / Interest Rate = Years to Double.
Methodology by Frank Sterling (pen name of LifeCalc's founder). Not financial advice. Data last verified 2026-09-01.
How this calculator works
Compound interest means you earn interest on your earlier interest, not just on the original amount. The calculator starts with your initial deposit and, for each year, steps through every compounding period. In each period it adds that period's share of your monthly contribution (monthly amount x 12 / periods per year) and then multiplies the balance by (1 + r/n), where r is the annual rate and n is compounding periods per year (365 daily, 12 monthly, 4 quarterly, 1 annual).
Contributions are added at the start of each period, so each one earns interest for that period. The effective annual rate is (1 + r/n)^n - 1, the yearly growth after compounding. The comparison line labeled simple interest applies the annual rate to the deposits without letting interest earn more interest, which shows how much of the final balance comes from compounding.
For a single lump sum with no contributions, the result reduces to the familiar A = P x (1 + r/n)^(n x t). Total interest earned is the final balance minus everything you put in.
Assumptions & defaults
- Initial deposit $10,000, monthly contribution $500, annual rate 7%, compounded monthly, over 20 years.
- The rate is constant and applied every year with no taxes, fees or inflation. Real investments fluctuate; a savings account or CD rate is fixed only for a short time, and stock returns are neither steady nor guaranteed.
- Results are nominal dollars. To see what the balance might buy in today's terms, subtract expected inflation from the rate or use an inflation calculator.
Worked example
With the defaults, after 20 years the calculator gives:
| Total contributions ($10,000 + $500 x 240 months) | $130,000 |
|---|---|
| Final balance | $302,370 |
| Interest earned | $172,370 |
| Effective annual rate (7% compounded monthly) | 7.23% |
Compounding frequency matters less than most people expect. The same inputs compounded daily end at about $302,424 and compounded annually at about $301,888, a difference of under $540 over 20 years. The rate, the time and the amount you contribute matter far more: at 6% the balance is about $265,278, at 8% about $345,742, and if you contribute $250 a month instead of $500 it is about $171,379.
Time is the strongest lever. Holding the same inputs, the balance is about $107,144 after 10 years, $302,370 after 20 and $694,709 after 30; the second decade adds more than the first, and the third more than the second. A lump sum of $10,000 with no contributions grows to about $38,697 in 20 years at 7%, so in this example most of the ending balance comes from contributions and their growth.
Limitations: when to talk to a professional
A calculator like this shows arithmetic, not a forecast. Investment returns vary and can be negative in any year; a 7% figure is an assumption and not a promise. Interest in a taxable account is taxed, and inflation reduces what the money buys.
Which account you use changes the outcome: a high-yield savings account or CD has a stated, lower rate, while stocks and funds have higher expected but uncertain returns, and tax-advantaged accounts like a 401(k) or Roth IRA have contribution limits and withdrawal rules. A fee-only financial planner or a tax professional can help you choose accounts and an allocation that fit your goals and time horizon.
If you are carrying high-interest debt, the same math works against you, and paying it off may beat any return you could expect from investing.