What is compound interest?
Compound interest is interest earned on interest. When you invest money, you earn a return on your initial investment. In the next period, you earn a return on your initial investment PLUS the return you already earned. Over time, this snowballs—your money grows exponentially, not linearly.
The opposite of compound interest is simple interest, where you earn interest only on your initial principal. With simple interest, $10,000 at 7% for 30 years earns $21,000 in interest. With compound interest, it earns $66,000—more than three times as much.
The formula
The compound interest formula for a lump sum is:
A = P(1 + r/n)^(nt)
Where: A = final amount, P = principal (initial investment), r = annual interest rate (decimal), n = number of times interest is compounded per year, t = number of years.
For regular monthly contributions, the formula gets more complex, but the concept is the same: each month's contribution starts earning returns, and those returns earn returns, and so on.
The real numbers for a nurse
Let us walk through a realistic example. Sarah is a 30-year-old registered nurse making $78,000/year. She starts contributing $500/month to her 403(b) retirement account. Her investments earn an average of 7% per year (the historical average for a diversified stock portfolio, adjusted for inflation).
| Age | Total Contributed | Account Value | Interest Earned |
|---|---|---|---|
| 30 (start) | $0 | $0 | $0 |
| 35 | $30,000 | $36,000 | $6,000 |
| 40 | $60,000 | $87,000 | $27,000 |
| 45 | $90,000 | $167,000 | $77,000 |
| 50 | $120,000 | $290,000 | $170,000 |
| 55 | $150,000 | $473,000 | $323,000 |
| 60 | $180,000 | $566,000 | $386,000 |
Notice the pattern: in the first 10 years (age 30–40), Sarah's account grows by $87,000. In the last 5 years alone (age 55–60), it grows by $93,000. The growth accelerates because there is more money earning returns. This is the power of compounding.
Why starting early matters more than saving more
The most important factor in compound interest is time—not the amount you invest. Consider two nurses:
- Nurse A starts investing at age 25, saves $300/month for 10 years, then stops. Total contributed: $36,000. At age 60 (7% return), account value: ~$405,000.
- Nurse B waits until age 35 to start, then saves $300/month for 25 years. Total contributed: $90,000. At age 60 (7% return), account value: ~$254,000.
Nurse A contributed less than half as much money, but ends up with 60% more—because her money had 10 extra years to compound. This is why starting early is the single most important thing you can do for your retirement.
Where nurses should invest for compound growth
- 403(b) / 401(k) with employer match. If your hospital offers a matching contribution, contribute at least enough to get the full match. This is free money—a 50% or 100% instant return on your investment. Many hospitals match 50% of the first 6% you contribute.
- Roth IRA. After getting the full employer match, contribute to a Roth IRA if you qualify (income limits apply). Roth IRAs grow tax-free, and withdrawals in retirement are tax-free. For 2024, the contribution limit is $7,000/year ($8,000 if age 50+).
- Additional 403(b) / 401(k) contributions. If you have extra money after maxing out the Roth IRA, increase your 403(b)/401(k) contributions. For 2024, the limit is $23,000/year ($30,500 if age 50+).
- Taxable brokerage account. If you have maxed out all tax-advantaged accounts and still have money to invest, open a taxable brokerage account. You will pay taxes on dividends and capital gains, but there are no contribution limits and you can withdraw at any time without penalty.
What to invest in
For most nurses, the simplest and best approach is a low-cost, diversified portfolio of index funds. You do not need to pick individual stocks or time the market. A target-date fund (which automatically adjusts its allocation as you approach retirement) is the ultimate "set it and forget it" investment.
Look for funds with low expense ratios (under 0.10% is good, under 0.05% is great). High fees are the silent killer of compound returns—a 1% annual fee can cost you hundreds of thousands of dollars over a 30-year career.