What Is Compound Interest? (How It Works For and Against You)

Compound Interest vs. Simple Interest

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Example: $10,000 at 5% for 10 years - Simple interest: $10,000 × 5% × 10 = $5,000 in interest → $15,000 total - Compound interest (annually): $10,000 × (1.05)^10 = $16,289 total (+$1,289 more) - Compound interest (monthly): $10,000 × (1 + 0.05/12)^120 = $16,470 total (+$1,470 more) The more frequently interest compounds, the more you earn (or owe).

The Rule of 72

The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate. At 4% APY (high-yield savings), your money doubles in 72 ÷ 4 = 18 years. At 7% (stock market average), it doubles in 72 ÷ 7 ≈ 10 years. At 24% (credit card APR), your debt doubles in 72 ÷ 24 = 3 years.

Compound Interest Working For You (Savings)

Scenario	10 Years	20 Years	30 Years
$10K at 4.5% APY (HYSA)	$15,530	$24,117	$37,453
$500/mo at 4.5% APY	$76,408	$191,473	$381,846
$500/mo at 7% (investing)	$86,919	$261,220	$606,438

Compound Interest Working Against You (Debt)

Credit card debt at 24% APR compounds daily. If you carry a $5,000 balance and only make minimum payments (~$100/month), it takes 94 months (7.8 years) to pay off and costs $4,311 in interest — nearly doubling the original debt. This is why paying off high-interest debt is the highest guaranteed return on your money.

Frequently Asked Questions

How does compound interest work?

Compound interest calculates interest on both the original principal and the accumulated interest from previous periods. For example, $1,000 at 10% annual compound interest: Year 1 earns $100 (total $1,100). Year 2 earns $110 (10% of $1,100, total $1,210). Year 3 earns $121 (10% of $1,210, total $1,331). The interest earned grows each year because you're earning interest on a larger base.

What is the best example of compound interest?

The best example of compound interest working for you is long-term investing. $10,000 invested at 7% annual return (historical stock market average) grows to $76,123 in 30 years — without adding a single dollar. The best example working against you is credit card debt: $5,000 at 24% APR with minimum payments takes 8 years to pay off and costs $4,000+ in interest.

How often does compound interest compound?

It depends on the account or loan. High-yield savings accounts typically compound daily. CDs may compound daily or monthly. Bonds typically pay simple interest. Credit cards compound daily. More frequent compounding means slightly more interest earned (or owed). Daily compounding at 4.5% APY yields slightly more than monthly compounding at the same rate.

What is the difference between APR and APY in compound interest?

APR (Annual Percentage Rate) is the stated rate before compounding. APY (Annual Percentage Yield) accounts for compounding and shows the true annual return. A savings account with 4.46% APR compounding daily has an APY of 4.56%. For savings, APY is the more accurate measure. For loans, APR is used (and is actually the more accurate cost measure when it includes fees).