Compound Interest
Einstein probably didn't say it (the quote is misattributed). But the principle is still true: compound growth is the most powerful force in personal finance — and it rewards the patient. This page walks through how it works, why time matters more than the size of your contribution, and the limitations of the simple formula.
Not Financial Advice
Every example and projection on this page uses constant annual returns for illustration. Real market returns vary year to year — sometimes by 30% or more. The math shows the principle, not a forecast. Past performance doesn't guarantee future results.
What compound interest actually is
Simple interest pays you a fixed percentage of your original deposit, every year, forever. A $10,000 deposit at 5% simple interest earns $500/year. After 30 years, you've earned $15,000 in interest on top of your principal, for a total of $25,000.
Compound interest pays you interest on the interest. That same $10,000 at 5% compounded annually becomes $10,500 after year 1, then $11,025 after year 2 (5% of $10,500), then $11,576 after year 3, and so on. After 30 years, you'd have about $43,219 — nearly double the simple-interest outcome, even though the rate is identical. The difference is just that the interest keeps getting reinvested instead of being paid out.
In the real world, no bank pays you 5% on a savings account these days (the closest is high-yield savings at 4-5% as of 2026). But stock market returns, over long horizons, are essentially compound interest at a higher rate. The S&P 500 has returned roughly 10% annually before inflation, 7% after inflation, over any 30+ year window in the historical record. That 7%, compounded over 30 years, is the engine behind most retirement portfolios.
See it in action
The formula
r = annual rate ÷ 12 (monthly periods) | n = years × 12 (total months)
The Rule of 72
Divide 72 by your annual return rate to get the approximate years to double your money. No calculator needed.
| Return Rate | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 7% | 10.3 years |
| 8% | 9 years |
| 10% | 7.2 years |
Time beats returns
A 22-year-old who invests $200/month at 7% until age 65 accumulates roughly $1,034,000. A 32-year-old who invests $400/month — double the amount — at the same rate accumulates roughly $986,000. Starting ten years earlier beats saving twice as much. That's the Coast FIRE insight in a nutshell.
The math behind "ten years earlier is worth double the savings"
The reason isn't intuitive, but it's mechanical. The 22-year-old's contributions have 43 years to compound. The 32-year-old's contributions have only 33 years. Because compound growth is exponential, those extra 10 years on the early money matter more than the doubled monthly contribution. The early money does almost all the heavy lifting by retirement age.
If you want to see the effect for yourself, try the calculator above with the same monthly contribution but different starting ages. Add 10 years to the timeline and watch the final value more than double. The compounding isn't linear; the curve steepens as the timeline extends.
What the calculator doesn't show
The compound interest calculator on this page uses a constant annual return. Real markets don't do that. In any given year, the S&P 500 might return +30% or -40%. The long-run average is 7% after inflation, but the year-to-year path is bumpy. A 30-year horizon smooths out most of the bumps, but a 5- or 10-year horizon is much more variable.
Other things the simple formula misses:
- Inflation. A 7% nominal return with 3% inflation is really a 4% real return. The calculator shows nominal dollars; your actual purchasing power is lower.
- Taxes. Money in a taxable brokerage account gets taxed on dividends and capital gains. Tax-advantaged accounts (401(k), IRA, Roth IRA) shelter this. The "true" return depends on which account holds the money.
- Fees. A 1% annual fee compounds against you. Over 30 years, a 1% fee on a $100,000 portfolio can cost you 25% of your final balance.
- Sequence of returns. If the market crashes in your first 5 years of retirement, the damage to your portfolio is permanent. If it crashes in year 25, you can ride it out. The calculator doesn't model this.
- Contribution limits. 401(k)s and IRAs have annual contribution caps. You can't actually invest unlimited amounts in tax-advantaged accounts.
What "7% real return" means in practice
The 7% figure that shows up in every retirement calculator is a historical average, not a forecast. It's the approximate real (after-inflation) return of a diversified US stock portfolio over any 30+ year period in the data. It assumes you:
- Stay invested through crashes (don't sell in 2008 or 2020)
- Hold a diversified mix (the S&P 500, or a total market index fund)
- Keep fees low (index funds, not actively managed funds)
- Don't try to time the market
If you do all of those things, the long-run average is genuinely around 7% real. But individual 30-year windows have varied — some have been 5% real, some 9%. The future is uncertain; 7% is a reasonable central estimate, not a guarantee.
What about crypto, real estate, and other "alternative" investments?
Real estate has its own return profile — a mix of appreciation, rental income, and leverage effects. It can outperform stocks in some periods, underperform in others. Adding real estate to a portfolio can reduce volatility if the correlations are low, but it also adds complexity (managing tenants, repairs, illiquidity).
Crypto and similar speculative assets have wildly variable historical returns. Bitcoin, for example, has had multi-year periods of 10x returns followed by 80% drawdowns. Including them in a long-term projection is mostly speculation — there's no 30-year history to anchor a return estimate.
The compound interest principle applies to any appreciating asset. The question isn't "does this asset compound?" — it's "what's a realistic long-run return, and what risks am I taking to get it?" The 7% number for stocks comes with 30+ years of supporting data. Other assets come with less.
Practical takeaways
- Start now, even small. A dollar invested at 25 is worth 4-5x a dollar invested at 35 at the same rate over 30 years.
- Keep fees low. Use index funds. A 1% annual fee compounds against you for decades.
- Stay invested. The market will crash. Historically, it's recovered every time, but only if you stayed invested through the crash.
- Max tax-advantaged accounts first. 401(k) match, then Roth IRA, then taxable brokerage.
- Don't anchor on a single return number. 7% is a planning estimate, not a forecast. Build a plan that works at 5% real too.
Related reading
Compound interest is the engine, but the Coast FIRE calculator uses it to find your specific number. The Safe Withdrawal Rate guide covers the other side: how to draw the money down without running out. And the Progress Tracker lets you see your current portfolio value vs. where compound growth can take it.
Why Coast FIRE rewards early savers
Every dollar you save in your 20s or 30s that sits in the market for 30 years is worth 3–4x a dollar saved in your 40s. Hitting your Coast FIRE number early matters more than squeezing extra savings later.
This calculator uses constant annual returns. Market returns vary year to year. For educational illustration only, not financial advice. See our Editorial Policy for how we approach this content.