Simple vs. compound interest

The difference, in one sentence

Simple interest only ever pays you on the money you put in. Compound interest pays you on your money and on the interest that money already earned. Same deposit, same rate — the second one keeps building on a bigger and bigger base.

Put 10,000 pesos at 10% a year. With simple interest you earn 1,000 pesos every single year, forever. With compound interest you earn 1,000 the first year, then 1,100 the second, then 1,210 the third — because now you’re earning on 11,000, then on 12,100.

Year three, the gap is 310 pesos and it looks like a rounding error. That’s why most people stop paying attention right here. Keep going, though, and after twenty years the simple version holds 30,000 pesos while the compound version holds about 67,000. Same deposit, same rate, more than double the money.

Time does more work than the amount

This is the part that surprises people. Compounding is not really rewarding you for being rich — it’s rewarding you for being early.

Someone who invests 2,000 pesos a month from age 25 to 35 and then stops completely, contributing nothing for the next thirty years, ends up ahead of someone who starts at 35 and contributes every month until 65 — at 8% a year, the rate the calculator further down starts at. The second person puts in three times more money and still finishes behind. The only difference is that the first person’s money had ten extra years to multiply.

That rate is not a footnote to the story, it is half of it. Run the same two people at 6% or less and the comparison flips: there the one who keeps contributing for thirty years wins. Time is the bigger lever, and the instrument you pick decides how far that lever moves.

The practical takeaway isn’t “invest a fortune”. It’s that a small amount started now beats a large amount started later, and no amount of enthusiasm at 40 buys back a decade you skipped at 25.

The curve is boring, then it isn’t

Compounding is not a straight line, and that’s exactly why people quit. For the first several years the growth looks almost flat, indistinguishable from just saving. Most of the visible progress in that stretch comes from your own deposits, not from returns.

The interesting part happens later, when the interest your money generates starts to be bigger than what you’re adding yourself. Getting to that point requires doing something unglamorous for years while it looks like nothing is happening. That, not math, is the actual difficulty.

The same force, pointed at you

Credit cards compound too. A card at 45% annual interest that you keep paying the minimum on isn’t charging you 45% of the original purchase — it’s charging interest on the interest, month after month. (In Mexico the full cost of a card, fees included, is the CAT that every bank must show you — and Banxico requires that number to be calculated without the IVA on interest and fees, so the real bite is bigger than the figure you compare. The credit card lesson does that arithmetic.)

That’s why a balance can feel like it never moves even though you pay every month: the minimum payment often barely covers what compounding just added. Attacking expensive debt is not “playing it safe” instead of investing. It’s turning off a compounding machine that’s running against you at a rate no investment reliably matches.

Three things that quietly break it

1. Withdrawing along the way. Every time you pull money out, you reset the base that everything else was building on. Compounding needs to be left alone.
2. Fees. A 2% annual commission sounds trivial next to a 9% return. Over twenty years it quietly takes about a third of what you would have ended up with, and over thirty it takes more than four pesos out of every ten — because it compounds too, in the other direction.
3. Inflation. If your money earns 4% and prices rise 5%, you are compounding backwards in real terms. The number in the account grows and what it can buy shrinks.

The short version

Compound interest is just interest earning interest. It rewards time far more than it rewards size, it looks disappointing for years before it looks impressive, and it works identically well against you when you owe money. Start earlier than feels necessary, leave it alone, and pay off anything expensive first.