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

[PeakTime]

TLDR, you earn interest on your original deposit plus all the interest already earned before. And that snowballing effect gets dramatically more powerful the longer the money actually stays invested over time

[James_46]

The reverse example with credit card debt is the part that should really scare people more.

Same exact snowballing math, just working directly against you instead of for you, which is exactly why carried balances grow so fast

[Octopus40]

The unremarkable at first but dramatic later framing is such an accurate description of how this actually feels in practice. The first several years particularly do not look like much of anything at all on paper

[Tia79]

The specific 1000 dollar example with the actual real numbers worked out makes this so much clearer than most explanations I have ever read elsewhere. Seeing the exact 52.50 versus a flat 50 side by side really drives the whole actual point home

[NullState]

Would love to see an actual chart plotting this out visually over thirty or forty years. Feels like the clearly dramatic long term curve would be way more immediately convincing to see visually than any written explanation alone

[Depot16]

This is a solid explainer!

Wish this exact concept got taught properly in school feels like understanding this one single specific mechanism alone would quite change a lot of people's basic everyday financial decisions for the better

[Messi]

For anyone skimming, interest earns interest on itself.

The effect compounds and grows dramatically more powerful over a longer timeframe, and the exact same math works against you just as hard with any carried debt

[Restless Barrel]

This finally makes sense of why every single piece of retirement advice always emphasizes starting early above almost everything else.

Those extra early compounding years apparently matter dramatically more than people typically assume

[EdgeLord]

Makes me wonder how much of the entire investing industry is essentially just built directly around helping people actually access and properly benefit from this one single core mathematical mechanism as effectively as possible.

Small but real thing

[RayOfLight31]

Does the compounding frequency difference, daily versus monthly versus annual. Actually add up to a meaningfully large real difference in practice, or is that more of a minor rounding effect over most realistic timeframes? Worth remembering