Compound Interest Calculator
See how your money grows over time with the power of compound interest. Plan savings, investments, and retirement with monthly contributions.
Final Balance
Total Contributions
Total Interest Earned
Interest as % of Final Balance
Growth Over Time
| Year | Starting Balance | Contributions | Interest | Ending Balance |
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"What If" Rate Comparison
| Interest Rate | Final Balance | Total Interest | Difference |
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Understanding Compound Interest
What is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is only calculated on the principal, compound interest causes your money to grow exponentially over time. This is why Albert Einstein reportedly called it "the eighth wonder of the world." The more frequently interest is compounded (daily vs. annually), the faster your investment grows.
A = P(1 + r/n)nt + PMT × [((1 + r/n)nt - 1) / (r/n)]
Where P = principal, r = annual rate, n = compounds per year, t = years, PMT = periodic contribution
The Rule of 72
The Rule of 72 is a quick mental math shortcut to estimate how long it takes for an investment to double. Simply divide 72 by the annual interest rate. For example, at 8% annual return, your money doubles in approximately 72 / 8 = 9 years. At 6%, it takes about 12 years. This rule works best for rates between 6% and 10%.
4%
~18 years
6%
~12 years
8%
~9 years
10%
~7.2 years
Simple vs Compound Interest
Simple interest earns the same fixed amount each period, while compound interest earns interest on your interest, creating a snowball effect. The longer your time horizon, the more dramatic the difference becomes.
| $10,000 at 8% | Simple | Compound | Advantage |
|---|---|---|---|
| 5 years | $14,000 | $14,693 | +$693 |
| 10 years | $18,000 | $21,589 | +$3,589 |
| 20 years | $26,000 | $46,610 | +$20,610 |
| 30 years | $34,000 | $100,627 | +$66,627 |
The Power of Starting Early
Time is your most powerful asset when it comes to compound interest. Consider two investors who both contribute $300/month at 8% annual return:
Investor A (starts at 25)
Invests for 40 years until age 65
Total contributed: $144,000
Final: ~$1,054,000
Investor B (starts at 35)
Invests for 30 years until age 65
Total contributed: $108,000
Final: ~$447,000
Investor A contributed only $36,000 more but ended up with over $607,000 more thanks to an extra decade of compounding.
Frequently Asked Questions
How does compounding frequency affect my returns?
The more frequently interest compounds, the more you earn. Daily compounding yields slightly more than monthly, which yields more than quarterly or annual compounding. For example, $10,000 at 8% for 10 years: annual compounding gives $21,589, while daily compounding gives $22,255 — a difference of $666.
What is a realistic annual return rate?
The historical average annual return of the S&P 500 is approximately 10% before inflation (about 7% after inflation). High-yield savings accounts offer 4-5%, bonds typically yield 3-6%, and real estate averages 8-12%. Use a conservative estimate for long-term planning.
Does this calculator account for taxes and inflation?
This calculator shows nominal (before-tax, before-inflation) returns. To approximate after-inflation returns, subtract the inflation rate (typically 2-3%) from your interest rate. For tax-advantaged accounts like 401(k)s or IRAs, the nominal figure is more accurate until withdrawal.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the stated rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding. For example, an APR of 8% compounded monthly results in an APY of about 8.30%. This calculator uses the APR you enter and applies compounding based on your chosen frequency.
How much should I invest monthly for retirement?
A common guideline is to save 15-20% of your gross income for retirement. Use this calculator to test different scenarios. For example, starting at age 25 with $500/month at 8% could grow to over $1.7 million by age 65. The key is to start as early as possible to maximize the compounding effect.