Compound Interest Calculator
How Does Compound Interest Work?
Compound interest means you earn interest on your interest — not just on your original investment. Over time, this creates an exponential growth effect often called the "snowball effect." The longer you invest, the more powerful compounding becomes. Albert Einstein reportedly called compound interest "the eighth wonder of the world."
Historical Investment Returns 2026
| Investment | Avg Annual Return | Risk Level | $10k grows to (30yr) |
|---|---|---|---|
| S&P 500 Index | ~10.5% | Medium-High | ~$202,000 |
| MSCI World Index | ~9.2% | Medium | ~$143,000 |
| 60/40 Stock/Bond Mix | ~7.5% | Medium | ~$87,000 |
| Conservative Bonds | ~5.0% | Low | ~$43,000 |
| High-Yield Savings | ~4.5% | Very Low | ~$37,000 |
| Cash/Savings Account | ~1–2% | None | ~$13,500 |
Past performance does not guarantee future results. For illustrative purposes only.
Why Starting Early Beats Investing More
Compound interest rewards time more than it rewards the amount you invest. Consider two people who each earn a 7% annual return. Anna invests $200 a month from age 25 to 35 (just 10 years, $24,000 total) and then stops. Ben invests the same $200 a month from age 35 all the way to 65 (30 years, $72,000 total). At 65, Anna — despite investing a third as much — often ends up with a larger pot, because her early money had an extra decade to compound. The lesson: the best day to start was years ago; the second best day is today.
What Slows Compounding Down
- Fees. A 1% annual fee sounds small but can eat 25%+ of your final pot over 40 years — favour low-cost index funds.
- Inflation. Growth is only "real" above the inflation rate. A 7% return with 3% inflation is really about 4% in spending power.
- Cashing out early. Interrupting compounding — or panic-selling in a downturn — resets the snowball and locks in losses.
- Taxes on gains. Using tax-sheltered accounts (ISA, 401(k), IRA) lets more of each year's growth stay invested.
Frequently Asked Questions
What is 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 your annual return rate. At 10% return, your money doubles in 7.2 years. At 6%, it takes 12 years. At 4%, it takes 18 years. This shows why higher returns have such a dramatic impact over time.
How much should I invest monthly to become a millionaire?
At a 10% annual return (S&P 500 historical average): investing $200/month for 40 years grows to ~$1.2 million. Investing $500/month for 30 years grows to ~$1.1 million. Starting earlier is far more powerful than investing more — time is your greatest asset.
What is the difference between simple and compound interest?
Simple interest is calculated only on your original principal. Compound interest is calculated on your principal plus all previously earned interest. On a $10,000 investment at 10% for 30 years: simple interest gives you $40,000. Compound interest gives you $174,000 — more than 4x more.
How often is interest compounded?
Interest can compound daily, monthly, quarterly, or annually. More frequent compounding means faster growth. Most investment accounts and index funds effectively compound continuously. This calculator uses monthly compounding, which is standard for most investment and savings accounts.
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How compound interest works
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⚠️ Disclaimer: Results are estimates only and do not constitute financial, tax, or legal advice. Tax laws change frequently — always verify with official sources (IRS, HMRC) and consult a qualified professional before making decisions.
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