What Is Compound Interest?
Compound interest is interest calculated on both your original principal AND the interest you've already earned. Unlike simple interest (which only pays on principal), compound interest snowballs — your earnings generate their own earnings.
Simple interest on $10,000 at 8% for 10 years = $8,000 total interest earned.
Compound interest on $10,000 at 8% for 10 years = $11,589 total interest earned — 45% more, with zero extra work.
The longer you wait to start investing, the more you pay in opportunity cost. A 25-year-old who invests $5,000/year until age 35 (10 years, $50,000 total) and stops will have MORE at retirement than a 35-year-old who invests $5,000/year all the way to 65 (30 years, $150,000 total) — assuming the same 8% return.
The Compound Interest Formula
A = P(1 + r/n)^(nt)
A = Final amount · P = Principal (initial investment) · r = Annual interest rate (decimal) · n = Compounding periods per year · t = Time in years
For a $10,000 investment at 8% compounded monthly for 10 years:
A = 10,000 × (1 + 0.08/12)^(12×10) = 10,000 × (1.00667)^120 = $22,196
Real-World Examples
| Investment | Rate | 5 Years | 10 Years | 20 Years | 30 Years |
|---|---|---|---|---|---|
| $5,000 | 5% | $6,381 | $8,144 | $13,266 | $21,610 |
| $10,000 | 7% | $14,026 | $19,672 | $38,697 | $76,123 |
| $10,000 | 8% | $14,898 | $22,196 | $49,268 | $109,357 |
| $25,000 | 10% | $40,263 | $64,844 | $168,187 | $436,235 |
| $500/mo | 8% | $36,983 | $91,473 | $294,510 | $745,180 |
Daily vs Monthly vs Annual Compounding
Compounding frequency matters — the more frequently interest compounds, the faster your money grows. On $10,000 at 8% for 10 years:
| Compounding Frequency | Final Amount | Extra vs Annual |
|---|---|---|
| Annually (once/year) | $21,589 | — |
| Quarterly (4×/year) | $21,911 | +$322 |
| Monthly (12×/year) | $22,196 | +$607 |
| Daily (365×/year) | $22,253 | +$664 |
The difference between monthly and daily compounding is small, but the difference between annual and monthly compounding is significant at larger amounts and longer timeframes.
Calculate Your Exact Compound Interest
Enter your principal, rate and time period. See year-by-year growth and exactly how much interest you'll earn.
Use Free Calculator →The Rule of 72 — Quick Mental Math
The Rule of 72 lets you quickly estimate how long it takes to double your money: divide 72 by your annual interest rate.
- At 6%: 72 ÷ 6 = 12 years to double
- At 8%: 72 ÷ 8 = 9 years to double
- At 10%: 72 ÷ 10 = 7.2 years to double
- At 12%: 72 ÷ 12 = 6 years to double
This means $10,000 invested at 8% becomes $20,000 in 9 years, $40,000 in 18 years and $80,000 in 27 years — purely through compounding, with no additional contributions.
Best Accounts for Compound Interest in 2026
- High-Yield Savings Accounts (HYSA) — 4.5–5.2% APY in the US. FDIC insured, zero risk. Best for emergency funds and short-term goals.
- S&P 500 Index Funds — historical average ~10% annually. Best long-term compounder available to retail investors. Available through any brokerage.
- Roth IRA (US) — compound interest grows completely tax-free. $7,000/year contribution limit (2026). Ideal for retirement savings.
- ISA (UK) — £20,000/year allowance, all gains completely tax-free. Stocks & Shares ISA invested in index funds is the UK equivalent.
- 401(k) with employer match — free money on top of compounding. Always contribute at least enough to get the full employer match.
Frequently Asked Questions
What is the difference between APY and APR?
APR (Annual Percentage Rate) is the simple interest rate without compounding. APY (Annual Percentage Yield) accounts for compounding and represents the actual return you earn over a year. When comparing savings accounts or investments, always compare APY — it's the true measure of what you'll earn.
How often does the S&P 500 compound?
Stock market returns compound continuously as share prices move. When you reinvest dividends (which most index funds do automatically), you compound quarterly when dividends are paid. The key is to stay invested and reinvest all dividends rather than taking them as cash.
Is compound interest taxable?
In most countries, yes — interest and investment gains are taxable in the year they're earned or realized. In the US, interest in regular savings accounts is taxed as ordinary income. Capital gains on investments held over a year are taxed at lower long-term capital gains rates. Tax-advantaged accounts (Roth IRA, 401k, ISA) shelter compound growth from taxes.
What is the best compound interest investment in Europe?
In Europe, low-cost ETFs tracking the MSCI World or S&P 500 index are widely considered the best compound interest vehicles. Available through platforms like Trade Republic, Degiro, and Interactive Brokers. Investing within an ISA (UK), PEA (France) or equivalent tax-advantaged wrapper maximizes after-tax compounding.