Compound interest is a simple idea: when you earn interest on a savings account, that interest gets added to your balance. Next period, you earn interest on the new, larger balance. Over time, this snowballs.
The opposite — simple interest — pays interest only on your original principal, not on accumulated interest. Almost no real-world savings products use simple interest anymore, but the comparison illustrates why compounding matters.
A 30-year example
Imagine you put $10,000 in a savings account paying 5% per year and never touch it for 30 years.
With simple interest (5% of $10,000 paid each year, no compounding):
- After 30 years: $10,000 + (30 × $500) = $25,000
With compound interest (5% APY, compounded monthly):
- After 30 years: $44,677
Same starting balance, same nominal rate, same time period — but compounding adds an extra $19,677 to your final balance. The interest earned in the final year alone (~$2,150) is more than four years of simple interest combined.
Why the curve gets steeper over time
In the early years, your interest is small because your balance is small. But each year's interest gets added to next year's earning base. The growth curve looks linear at first and then bends sharply upward.
Looking at the same $10,000 at 5% APY:
| Year | Balance | Interest that year |
|---|---|---|
| 1 | $10,512 | $512 |
| 5 | $12,834 | $621 |
| 10 | $16,470 | $797 |
| 20 | $27,126 | $1,313 |
| 30 | $44,677 | $2,162 |
| 40 | $73,584 | $3,562 |
By year 30, your annual interest is more than 4× what it was in year 1. By year 40, it's nearly 7×.
This is why financial planners emphasize starting early. The first $10,000 invested in your 20s is worth several times more at retirement than the same $10,000 invested in your 50s, even if you earn the exact same rate of return.
Time vs rate: which matters more?
A common question: would you rather earn 8% APY for 20 years, or 4% APY for 40 years?
- $10,000 at 8% APY for 20 years = $48,754
- $10,000 at 4% APY for 40 years = $49,308
Almost identical results — but this glosses over an important point. In the second scenario, your money was working for twice as long. Real-world savings rates rarely sit at 8% for two decades, and even if they did, the second scenario gives you 20 extra years to add contributions.
In practice, the longer time horizon almost always wins.
The contribution effect
The math gets more interesting once you add regular contributions. $200/month added to a $10,000 starting balance at 4.5% APY for 30 years:
- Total contributions: $72,000 ($200 × 12 × 30)
- Final balance: about $230,000
- Interest earned: about $148,000
The interest earned is more than double what you actually deposited. This is the case even in a relatively conservative savings scenario; in a tax-advantaged retirement account averaging 7-8% returns, the multiplier is much larger.
What slows compound interest down
Three things significantly reduce the compounding effect:
- Withdrawals. Taking money out resets the compounding clock on that portion of your balance.
- Taxes on interest. Interest in a regular savings account is taxed as ordinary income each year. In a federal 22% tax bracket, your effective rate on a 5% APY account is more like 3.9%. Tax-advantaged accounts (Roth IRA, 401k, HSA) avoid this drag.
- Inflation. A 5% APY in a 3% inflation environment is really only earning you 2% in real purchasing power.
For long-term saving goals, the right comparison isn't "what rate am I earning" but "what real, after-tax rate am I earning."
Practical takeaways
- Time matters more than rate, almost always. Start saving as early as you can, even small amounts.
- Don't withdraw from compounding accounts unless you have to.
- Tax-advantaged accounts (Roth IRA, 401k, HSA) compound faster than regular savings because interest isn't taxed annually.
- Even relatively low APYs on FDIC-insured accounts produce significant returns over decades — you don't need risky investments to benefit from compounding.
To project a specific compounding scenario for your own savings, use our APY calculator.