How Does Compound Interest Work?

Compound interest is one of those terms everyone has heard but few people could actually explain with numbers. It's also the mechanism behind almost every long-term investing projection, including the ones on this site — so it's worth understanding properly rather than just accepting it as "a good thing."

The basic idea

Compound interest means earning returns on your returns, not just on your original amount. Each period, any growth from previous periods stays in the account and starts generating its own growth alongside the original balance — rather than being set aside or withdrawn.

This is different from simple interest, where you only ever earn a return on the original amount, and each period's gain stays flat.

A small worked example

Here's $1,000 growing at a flat 8% a year, compared side by side under compound growth and simple growth, so you can check the numbers yourself:

In year one, the two are identical — there's no prior growth yet to compound. By year three, compound growth has pulled ahead by about $20, because year two's $80 gain itself earned a further 8% in year three, on top of the original $1,000. That gap is small here because the example is short and the amount is small — over decades and larger balances, it becomes far more significant.

Why time matters more than people expect

Compounding is slow at first and accelerates later — the growth in year 20 is being calculated on a much larger balance than the growth in year 1, even at the same rate. This is the mathematical reason "start early" is such common advice for long-term investing: it's not that early money is special, it's that it simply has more years to compound.

Compounding frequency: monthly vs. annual

Interest or growth can compound on different schedules — annually, monthly, daily, and so on. For the same nominal annual rate, compounding more frequently produces a slightly higher result, because each period's growth starts earning its own growth a little sooner. Our calculators use monthly compounding, both because it matches how monthly contributions are added and because it's a common simplification for modeling long-term stock market growth. See the methodology page for the exact formulas.

What compound interest isn't

A stock market investment like VOO doesn't pay a fixed, guaranteed rate the way a savings account or bond does — its value moves with the market and can go down as well as up in any given year. When a calculator applies compound-growth math to an assumed annual return, that's a modeling simplification for exploring "what if" scenarios, not a promise about what will actually happen. The assumption is yours to choose; the outcome is not guaranteed.