Fundamentals
What is compound interest?
When your money earns interest, and that interest then earns interest of its own, your wealth grows on a curve, not a line. Time is the variable doing the heavy lifting.
5 min read
The idea, in one paragraph
Compound interest is what happens when the returns on your investments start producing returns of their own. Each year, you earn a return not just on the money you originally put in, but also on every dollar of return those earlier years already produced. The longer you stay invested, the more dramatic the effect becomes, and at some point the curve of your portfolio simply lifts off.
It is the single most important idea in long-term investing. A modest amount, left alone for thirty years at typical equity-market returns, will multiply many times over without a single extra dollar from you. Active stock-picking matters. Asset allocation matters. But neither of them comes close to the leverage that simply staying invested provides.
Mechanically, the formula is simple: your starting amount, multiplied by one plus your annual rate, raised to the number of years you stay invested. Add monthly contributions and the math becomes a sum of many smaller compounding streams, each new dollar starting its own clock. Behind the formula hides an extraordinary fact, though: those exponents are doing the real work. That is why every additional year in the market is worth more than the year before it.
See it for yourself
Move the sliders to see how time, rate of return, and starting amount each change the shape of the curve. The default values reflect the long-term real return of the S&P 500.
What the curve actually shows
- Time is the most powerful variable. Doubling your starting amount roughly doubles your final balance. Doubling your time horizon, at 7% per year, multiplies it by about eight, because compounding feeds on itself.
- Starting ten years earlier roughly doubles the final outcome. The first decade often feels slow, but it sets the base on which every later year multiplies.
- Small amounts compound. A daily five-dollar coffee, redirected into a market-tracking fund, grows to roughly $182,000 after thirty years. The amount feels small. The result does not.
- Compound growth is non-linear. The straight line on the chart shows what you would earn from simple interest alone. The gap between that line and the curve is everything compounding adds.
- Compounding works against you, too. The same math that grows wealth on the way up erodes purchasing power when inflation runs unchecked, which is why investing, not just saving, matters in countries with persistent inflation.
Why this matters for LATAM investors
For investors in Mexico, Brazil, Colombia, Peru and Chile, compound growth is more than an abstraction. Local inflation has eroded peso, real and sol savings for decades, while a US-dollar-denominated broad-market fund has compounded at roughly 7% in real terms over the long run. That gap is why so many LATAM households now treat USD-denominated index investing as a baseline for retirement, not a luxury.
The earlier the curve starts bending, the larger the buffer you build between your future self and the next inflation shock. Starting young is a privilege, but starting today is a decision available to anyone, and the math rewards even modest beginnings, provided they are not abandoned along the way.
See compound interest in action
Three broad market-tracking ETFs that have historically delivered the kind of long-horizon returns the curve above is built on. Use them as a reference point, not a recommendation.
Where to start
If the math above made sense, the natural next step is choosing a fund that captures the broad market efficiently. Our ETF starter list is a good place to begin.