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Compound interest is one of the most powerful forces in personal finance. Understood well, it builds wealth over decades. Ignored, it quietly destroys it through growing debt.
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What Is Compound Interest?
Compound interest is interest calculated on the initial principal AND on the accumulated interest from previous periods. It is the opposite of simple interest, which only applies to the principal.
Simple interest example: $10,000 at 5% per year = $500/year every year.
Compound interest example: $10,000 at 5% compounded annually:
- Year 1: $10,500
- Year 2: $11,025
- Year 10: $16,289
- Year 30: $43,219
That extra $26,930 over simple interest is earned doing nothing — just letting time work.
The Compound Interest Formula
A = P(1 + r/n)^(nt)
- A = final amount
- P = principal
- r = annual interest rate (decimal)
- n = times compounded per year
- t = time in years
Compounding Frequency Matters
| Frequency | $10,000 at 5% after 10 years |
|---|---|
| Annually | $16,289 |
| Monthly | $16,470 |
| Daily | $16,487 |
The difference between annual and daily compounding is modest. The bigger lever is time and rate.
The Rule of 72
Divide 72 by your annual return to estimate how long it takes to double your money.
- 4% return: doubles in 18 years
- 6% return: doubles in 12 years
- 8% return: doubles in 9 years
- 12% return: doubles in 6 years
Compound Interest Working Against You
Credit cards often charge 20-29% APR compounded daily. A $5,000 balance at 24% APR with only minimum payments takes over 20 years to pay off and costs more than $6,000 in interest.
Where Compound Interest Works For You
- High-yield savings accounts: 4-5% APY compounding daily
- Index funds and ETFs: Reinvested dividends compound over decades
- 401(k) and IRA: Tax-deferred compounding accelerates growth
- CDs: Fixed rate, guaranteed compounding for a set term
Starting Early Is the Real Advantage
Investor A puts $5,000/year from age 25-35, then stops. Investor B puts $5,000/year from age 35-65. Both earn 7% per year.
- Investor A: contributed $50,000 — ends with ~$602,000
- Investor B: contributed $150,000 — ends with ~$472,000
Investor A contributed less and ends up with more. Time is the dominant factor.
Frequently Asked Questions
What is compound interest?
Interest earned on both your original principal and accumulated interest. It grows exponentially rather than linearly.
How often does compound interest compound?
Depends on the account. Savings accounts typically compound daily; loans often compound monthly.
What is the Rule of 72?
Divide 72 by your annual interest rate to estimate how many years to double your money. At 6%, that is 12 years.
Does compound interest work against you?
Yes, on debt. Credit card interest compounds daily at high rates, making unpaid balances grow quickly.
What is APY vs APR?
APY reflects compounding and shows the true annual yield. APR does not. Use APY to compare savings accounts.
Information as of May 2026. This is for educational purposes only and not personalized financial advice. Consult a licensed professional for your specific situation.