Investing Basics
Stocks, bonds, index funds, and the compounding math that makes it all worth doing.
What 'investing' actually means
Investing means putting money into an asset with the expectation it grows in value over time, in exchange for taking on some risk (the value can also go down, especially in the short term).
The basic building blocks
• Stocks: a small ownership slice of a company. Higher potential growth, but more volatile.
• Bonds: essentially a loan to a government or company that pays you interest. Generally more stable, lower expected returns than stocks.
• Index funds / ETFs: a single fund that holds hundreds or thousands of stocks (or bonds) at once, like the S&P 500. Instead of picking individual companies, you own a small slice of the whole market. This is how most everyday investors should build their portfolio — low fees, automatic diversification, no stock-picking required.
Risk and time horizon
The general principle: the longer you have until you need the money, the more risk (and more stock exposure) you can afford to take, because you have time to ride out downturns. Money you need in the next 1–3 years (a house down payment, a wedding) shouldn't be in the stock market — keep it in cash/savings instead. Money you won't touch for 10+ years (retirement) can afford to be mostly invested in stocks.
Compound interest: the core idea
Compound interest means you earn returns not just on the money you put in, but also on the returns you've already earned. Over long stretches of time, that snowball effect is dramatic — it's why investing consistently starting in your 20s beats investing much larger amounts starting in your 40s.
A concrete example: starting at 25 vs. 35
Sarah starts investing $300 a month at age 25. Her friend Mike puts off investing until he's 35, then also starts putting away $300 a month. Both of them keep contributing every month until they retire at 65, and both average an 8% annual return. The only difference between them is a 10-year head start:
| Sarah, starts at 25 | Mike, starts at 35 | |
|---|---|---|
| Years contributing | 40 years | 30 years |
| Total contributed | $144,000 | $108,000 |
| Growth earned | $903,000 | $339,000 |
| Balance at 65 | $1,047,000 | $447,000 |
Sarah's 10-year head start costs her only $36,000 in extra contributions, but by 65 she ends up with about $600,000 more than Mike. Almost all of that gap is time doing the work, not effort or amount.
The takeaway: don't wait for the "perfect" amount to start investing — starting small today beats waiting to start big later.
Try it yourself
Play with the calculator below — change the starting amount, monthly contribution, rate of return, and years, and watch how much of the final balance comes from your own contributions versus growth.
Compound Interest Calculator
Timeline
Contributions
Advanced: change contribution over time
Model a Coast FI-style plan (drop contributions to $0 at some point), a raise, or any other change to how much you invest partway through the timeline.