What Is Compound Interest?
Compound interest means earning interest on your interest — each period's gains are added to the principal, and the next period's interest is calculated on the larger balance. Einstein reportedly called it the "eighth wonder of the world."
- Simple interest: Interest calculated on the original principal only — doesn't grow over time
- Compound interest: Interest added to principal each period, so future interest is calculated on a larger base
A = P × (1 + r/n)^(n×t)
Where: A = final amount, P = principal, r = annual rate, n = compounds per year, t = years
Example: $10,000 at 7% annual return, 40 years:
- Simple interest: $10,000 + ($10,000 × 7% × 40) = $38,000
- Compound interest: $10,000 × (1.07)^40 = $149,745
Same starting amount — compound interest delivers nearly 4× more. The longer the time horizon, the more dramatic the gap.
Real Investment Growth Examples
Assuming 7% annual return (close to long-term stock market average), starting with $10,000 and adding $200/month:
| Years | Total Contributed | Final Value | Gains |
|---|
| 10 | $34,000 | $48,213 | $14,213 |
| 20 | $58,000 | $116,652 | $58,652 |
| 30 | $82,000 | $243,994 | $161,994 |
Use tool.tl's compound interest calculator to model your own numbers.
The Rule of 72: Quick Mental Math
Divide 72 by your annual return rate to estimate how many years it takes to double your money:
- 4% return: 72 ÷ 4 = 18 years to double
- 6% return: 72 ÷ 6 = 12 years to double
- 8% return: 72 ÷ 8 = 9 years to double
- 10% return: 72 ÷ 10 = 7.2 years to double
Why Starting Early Matters So Much
- Time is the most powerful variable: Starting at 25 vs 35 — with the same monthly contributions — can result in 3–4× more wealth by retirement, simply from 10 extra years of compounding
- Reinvest dividends: Don't take out earnings — reinvest them immediately to keep the compounding engine running
- Consistency beats timing: Regular monthly contributions (dollar-cost averaging) outperform most attempts to time the market over long periods
Frequently Asked Questions
Is the calculator free?
Yes — tool.tl's compound interest calculator is completely free. Customize principal, rate, time horizon, monthly contributions, and compounding frequency.
What investments use compound interest?
Stock index funds and ETFs (with dividend reinvestment), savings accounts, certificates of deposit, money market funds, and bonds all benefit from compounding. Real estate rental income reinvested also compounds over time.
Should I account for inflation?
Yes. If your investment returns 7% but inflation runs at 3%, your real purchasing power grows at about 4%. Use your real return rate (nominal return minus inflation) to estimate true wealth growth in today's dollars.