Compound interest is one of the more important ideas in personal finance, and it is simpler than it sounds. Unlike simple interest, which is only calculated on the original amount you saved or borrowed, compound interest is calculated on that original amount plus any interest already added. Over time, this creates a snowball effect, since interest starts earning its own interest.
A simple, illustrative example
Imagine, purely for illustration and not as a real or promised rate, that you set aside a sum of money and it grows by a fixed percentage each year, with the interest added back to the balance instead of withdrawn. In year one, you earn interest only on your original amount. In year two, you earn interest on the original amount plus the interest from year one. In year three, the interest is calculated on an even larger balance. The number itself is not the point; the pattern of growth building on previous growth is.
Why the details change the outcome
Several factors affect how compounding plays out in real life, and they matter more than people often expect:
- How often interest is added, since more frequent compounding can add up faster
- Whether interest is withdrawn or left in place to keep compounding
- How long the money stays untouched, since time is what compounding needs
- Fees or charges, which can quietly reduce the effect of compounding
Compounding works in both directions. It can help a saver's balance grow, but it can also make debt, such as an unpaid credit card balance, grow faster than expected if interest keeps being added to what is already owed.
The takeaway
Compound interest rewards time and consistency more than large one-off amounts. It is a useful concept for understanding both savings products and debt, but actual rates, terms and outcomes vary between providers and are never guaranteed. This article explains the mechanism only; it is not advice to open any particular account or product, so compare terms and do your own research before deciding what, if anything, is right for you.
On why two calculators can give different answers, see Why financial calculators disagree with each other
