Compound Interest Explained, With the Ten-Year Head Start Problem
Compound interest pays returns on past returns. Starting ten years earlier can double the result, even with less money put in. Here is the math.
Simple interest pays you only on what you put in. Compound interest pays you on what you put in and on the interest already earned. Over short periods, the difference is small. Over decades, it becomes most of the result.
Simple vs. compound
$10,000 earning 5% a year for 30 years:
Compounding adds about $18,200 over simple interest. Nothing extra was deposited; the interest itself earned interest.
The head start problem
Two hypothetical savers each invest $300 a month at an assumed 6% annual return, compounded monthly, until age 65.
| Starts at 25 | Starts at 35 | |
|---|---|---|
| Years investing | 40 | 30 |
| Total contributed | $144,000 | $108,000 |
| Balance at 65 | about $597,400 | about $301,400 |
Show the numbers behind this chart
| Balance at 65 | Total contributed | |
|---|---|---|
| Starts at 25 | $597,447 | $144,000 |
| Starts at 35 | $301,355 | $108,000 |
The earlier saver contributes only $36,000 more but ends with nearly $300,000 more. The extra ten years at the start are the most valuable ones, because that money has the longest time to compound.
To catch up, the saver who starts at 35 would need to invest about $595 a month, roughly double, to reach the same balance at 65.
Why time matters more than amount
Compound growth accelerates. In the 40-year example, the balance after 30 years is about $301,400. The final ten years add about $296,100 more, nearly as much as the first thirty years combined. Most of the growth arrives at the end, which means the years you can't get back are the early ones.
What works against compounding
- Fees compound too, in reverse; see what a 1% fee really costs.
- Inflation reduces what the final balance can buy; see inflation and real returns.
- Taxes on gains each year slow growth, which is why tax-advantaged accounts matter.
- Withdrawals interrupt the process, especially early ones.
- Debt compounds against you. A credit card balance at 24% doubles in about three years if unpaid; see the Rule of 72.
Compounding frequency
Interest can compound annually, monthly, or daily. More frequent compounding produces slightly more, which is why savings accounts quote APY. See APR vs. APY. But the frequency matters much less than the rate and the time.
The takeaway
Compound interest rewards time more than anything else. Starting earlier, even with smaller amounts, often beats starting later with larger ones. And the same force works against you on high-interest debt and fees.
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