I put $5,000 into a Tangerine mutual fund and forgot about it. A few months later, I logged in and saw $5,520. I genuinely couldn’t remember whether I’d invested $5,000 or $5,500. So I went back through my bank statements to check.

It was $5,000. The $520 was growth. Free money. Money that appeared because my money was sitting somewhere that made more money.

I know $520 doesn’t sound like a lot. But at that point in my life, I had never made $520 without doing something for it. No job, no side project, no favour. Just money making money. That was the moment investing became real for me.

That $520 was my introduction to compound interest. And while Einstein probably never actually called it the eighth wonder of the world (that quote is almost certainly made up), the math behind it is genuinely one of the most important things you can understand about money.

None of this is financial advice. But the concept of compounding is worth understanding even if you’re nowhere near ready to invest.

Simple interest vs. compound interest

Simple interest is straightforward. You invest $1,000, it earns 5% per year, you get $50 every year. After 10 years, you have $1,500. The $50 never changes because it’s always calculated on the original $1,000.

Compound interest works differently. In year one, you earn 5% on $1,000, so you have $1,050. In year two, you earn 5% on $1,050, which is $52.50. In year three, 5% on $1,102.50. Each year, you earn interest on your interest.

After 10 years, compound interest gives you $1,629 instead of $1,500. That $129 difference doesn’t look dramatic yet. But the gap between simple and compound interest doesn’t grow in a straight line. It curves. And the longer you wait, the steeper the curve gets.

After 30 years, that same $1,000 at 5% compounded becomes $4,322. With simple interest, it would be $2,500. The compounding added nearly $2,000 in extra growth, all generated by earlier growth earning its own returns.

The rule of 72

There’s a shortcut for estimating how long it takes your money to double. Divide 72 by your annual return rate.

At 6% growth, your money doubles roughly every 12 years. At 8%, every 9 years. At 10%, about every 7 years.

This means $10,000 invested at 8% becomes $20,000 in 9 years, $40,000 in 18 years, and $80,000 in 27 years. Each doubling happens on a larger base. The first doubling added $10,000. The third doubling added $40,000. Same timeframe, four times the dollar growth.

That’s compounding in action. The early years feel slow, then the curve bends upward and the numbers start moving fast.

Why starting early matters more than starting big

Here’s the comparison that changed how I think about time and money.

Start age	Monthly contribution	Years investing	Total contributed	Value at age 65 (7% annual return)
20	$200	45	$108,000	$526,000
25	$200	40	$96,000	$359,000
30	$200	35	$84,000	$243,000
35	$200	30	$72,000	$163,000

Look at the difference between starting at 20 and starting at 30. The person who started at 20 contributed $24,000 more in total but ended up with $283,000 more. Those 10 extra years of compounding generated far more wealth than the extra contributions themselves.

I started investing at 22, which is earlier than most people. And I still think about the four years between 18 and 22 when my money sat in a chequing account doing nothing. Those were good market years. Four years of compounding I’ll never get back.

I’m not saying that to make anyone feel bad about their timeline. Not everyone can invest at 18, or 22, or even 30. Money is a real constraint, not just a mindset issue. But for anyone who has the ability to start and just hasn’t, the math is clear: the best time to start is now.

Why $200 a month beats $20,000 later

People sometimes put off investing because they’re waiting to have a bigger lump sum. They think $200 a month isn’t “enough” to matter. The table above says otherwise.

$200 a month from age 25 turns into $359,000 by age 65. The total you contributed was only $96,000. The other $263,000 is compounding. That’s nearly three dollars of growth for every dollar you put in.

Waiting until you have $20,000 saved up and investing it all at once at age 35 gives you about $150,000 by age 65 at the same 7% return. Starting small and early beats starting big and late. It’s not even close.

This is why time in the market keeps showing up as one of the most important concepts in investing. The amount you invest matters, but it matters less than how long that money has to compound.

The part nobody talks about

Compounding works against you too. Credit card interest compounds. Debt that carries interest on interest grows the same way investments do, just in the wrong direction.

A $5,000 credit card balance at 20% interest, if you only make minimum payments, can take over 20 years to pay off and cost you more than $5,000 in interest alone. The same force that grows your investments can bury you in debt. Paying down high-interest debt before investing isn’t a detour. It’s the mathematically correct order of operations for most people.

What compounding can’t do

Compounding is powerful, but it’s not magic. It doesn’t eliminate risk. It doesn’t guarantee returns. The stock market doesn’t go up every single year, and some years it goes down significantly. The 7% average return used in the examples above is a long-term historical average, not a promise.

What compounding does is reward patience. A bad month or even a bad year matters less and less the longer your time horizon. The math favours people who start early and stay invested, even through the rough stretches.

The takeaway

Compounding is the reason a 25-year-old investing $200 a month can retire wealthier than a 40-year-old investing $500 a month. It’s the reason fees that look small, like 1% versus 0.2%, can cost you tens of thousands over a lifetime. And it’s the reason the first step matters more than the size of that first step.

My $520 in “free money” from Tangerine was tiny. But it showed me what was possible if I gave my money time. That understanding is worth more than any single investment I’ve ever made.

Compound interest: the only chart that matters