How does compound interest actually work?

[Brad79]

Compound interest means you earn interest not just on your original deposit. But also on all the interest that deposit has already accumulated in previous periods, which is the key structural difference that separates it from simple interest, where you would only ever earn interest on that original starting amount and nothing more.

Say you deposit 1000 dollars at a 5 percent annual interest rate. After the first year you have 1050 dollars. In year two, that 5 percent gets calculated on the full 1050 dollars rather than just the original 1000, giving you 52.50 dollars in interest that specific year rather than another flat 50, and that small extra amount keeps growing larger every single subsequent year as the process repeats itself.

The effect looks genuinely unremarkable over just a couple of years, but it becomes dramatically more powerful the longer that money is actually left to sit and compound. Over several decades, the compounding effect on the accumulated interest itself can genuinely end up contributing more total growth to your balance than all of your original individual deposits combined.

How often interest actually compounds also meaningfully matters. Daily or monthly compounding grows a balance slightly faster than annual compounding at that exact same stated interest rate, since interest starts earning its own additional interest sooner and more frequently throughout each year.

This exact same mathematical mechanism works identically in reverse against you with debt. Credit card interest compounds in the exact same way, which is precisely why carrying a balance for a long period can grow so much faster and larger than most people initially and intuitively expect it to