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Compound Interest Calculator

See how your money grows with compound interest and regular contributions. Includes year-by-year projection and investment breakdown.

Investment Details

$
$
%
yr

Projected Growth

Future Value

$343,778

after 20 years at 8%

Total Invested$130,000
Interest Earned$213,778
Return on Investment164%
● Invested 38%● Earned 62%
YearInvestedEarnedBalance
1$16,000$1,055$17,055
2$22,000$2,695$24,695
3$28,000$4,970$32,970
4$34,000$7,932$41,932
5$40,000$11,637$51,637
6$46,000$16,148$62,148
7$52,000$21,531$73,531
8$58,000$27,859$85,859
9$64,000$35,210$99,210
10$70,000$43,669$113,669
11$76,000$53,329$129,329
12$82,000$64,288$146,288
13$88,000$76,655$164,655
14$94,000$90,546$184,546
15$100,000$106,088$206,088
16$106,000$123,419$229,419
17$112,000$142,685$254,685
18$118,000$164,049$282,049
19$124,000$187,684$311,684
20$130,000$213,778$343,778

Projections are estimates based on a fixed rate of return. Actual investment returns vary and are not guaranteed. Past performance does not predict future results.

Compound Interest Formula

Compound interest is calculated using this formula:

A = P × (1 + r/n)^(n×t)

Where A = final amount, P = principal, r = annual interest rate (decimal), n = compounds per year, and t = time in years.

For a $10,000 investment at 8% compounded monthly for 20 years: A = 10,000 × (1 + 0.08/12)^(12×20) = $49,268. Adding $500/month in contributions brings the total to over $320,000 — illustrating the power of consistent investing.

The Power of Starting Early

Time is the most important variable in compound interest. An investor who starts at 25 and invests $300/month at 8% until age 65 accumulates about $1,006,000. An investor who starts at 35 with the same contributions only reaches about $440,000 — less than half — despite investing for only 10 fewer years.

This is why financial advisors consistently recommend starting to invest as early as possible, even with small amounts, rather than waiting until you can invest more.

Frequently Asked Questions

What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (applied only to the principal), compound interest grows exponentially — which is why Einstein reportedly called it the eighth wonder of the world.
How often should interest compound for the best returns?
The more frequently interest compounds, the higher your final balance. Daily compounding produces slightly more than monthly, which produces more than annually. In practice, the difference between daily and monthly compounding is small for most investment horizons.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your investment. Divide 72 by your annual interest rate. At 8% annual return, your investment doubles in approximately 72 ÷ 8 = 9 years.
What annual return rate should I use for stocks?
The S&P 500 has historically returned an average of approximately 10% per year before inflation (about 7% after inflation). For conservative planning, many financial advisors suggest using 6%–8% as a realistic long-term average for a diversified stock portfolio.
How do monthly contributions affect compound interest?
Regular contributions dramatically accelerate wealth accumulation due to compounding. For example, investing $10,000 at 8% for 30 years grows to about $100,627. Adding just $200/month on top of that results in approximately $370,000 — more than 3.5x the outcome.