THE MOST IMPORTANT IDEA IN MONEY
The magic of compound interest
A small amount of money. Started early. Left alone. Becomes a fortune. This is the single most important idea I want my kids to understand — and the math is so beautiful it’s almost unfair.
THE THOUGHT EXPERIMENT
Would you rather have $1,000,000 right now, or one penny doubled every day for 30 days?
Most adults pick the million. Most kids pick the million too. They’re both wrong.
Day 30 result
$5,368,709
One penny, doubled every day for 30 days, becomes $5.4 million. That’s compound interest — but extreme. Real-world investing isn’t this aggressive. It’s slower. But it’s the same idea: growth on top of growth.
What compound interest actually means
Most adults think they understand compound interest. Then they sit down with a calculator and discover they don’t. The math is counter-intuitive because human brains expect addition. Compound interest is multiplication.
Simple interest is when you earn money on the original amount. Put in $100, earn 10% per year, get $10 every year. After 30 years: $400.
Compound interest is when you earn money on the original amount AND on the money you’ve already earned. Put in $100, earn 10% per year, but each year your “original amount” grows. After 30 years: $1,744. Same starting money, same 10%. Just a different rule for what counts as “original.”
The difference between $400 and $1,744 is the magic. It’s the reason a 22-year-old saving $200/month will out-earn a 32-year-old saving $400/month — even though the 32-year-old put in twice as much money.
Run the math yourself
Try different numbers. Watch what happens to the final balance. This is the calculator we wish every Canadian saw at 18.
Typical historical Canadian balanced portfolio: 6–8% annual return. The S&P/TSX has averaged ~7% over decades. Inflation runs ~2%, so the real-dollar return is roughly 5%.
Final balance
$525,454
Total you put in
$96,000
Interest earned (the magic)
$429,454
Year-by-year balance — watch the curve bend upward.
Three real Canadian scenarios
All assuming 7% average annual return, in a TFSA (so the growth is tax-free).
EARLY STARTER
$200/mo from age 18
Started at 18. Stops contributing at 28. Never adds another dollar. Lets it sit until 65.
Total contributed: $24,000
$520,000
at age 65
LATE STARTER
$400/mo from age 35
Started at 35. Contributes twice as much per month. Keeps contributing until 65.
Total contributed: $144,000
$490,000
at age 65
MAX TFSA
$7,000/yr from age 18
Maxes the TFSA every year ($583/mo). Doesn’t touch a dollar of it. Retires at 65.
Total contributed: $329,000
$2.1M
at age 65, tax-free
The lesson: The early starter put in $24,000 over 10 years and beat the late starter who put in $144,000 over 30 years. That’s the power of compound interest. It’s not about how much money you have. It’s about how long that money has to grow.
FOR PARENTS
How to teach this to a kid
AGES 6-9
The chocolate bar test
Hand your kid one chocolate bar. Tell them: “You can eat it now, or wait one week. If you wait, you get two chocolate bars. Wait two weeks, four chocolate bars.” Watch them think. Then tell them: that’s exactly how money works in a bank account. Patience multiplies.
AGES 9-12
The penny doubling game
Ask: “Would you rather have $1 million now, or one penny that doubles every day for 30 days?” Let them think. Then write the doubling on a sheet of paper, day by day. By day 21 they’re shocked. By day 30 they understand compound interest better than most adults.
AGES 12-15
Run their numbers
Sit them at the calculator above. Have them put their actual age into the “years invested” field (assume retirement at 65). Plug in $25/month — what would they do? $100/month? They’ll see their future bank balance in front of them. Most teens never have. It’s a defining moment.
AGES 15-18
Open the TFSA (when they turn 18)
The day your kid turns 18 — or 19 in some provinces — open a TFSA with them. Even $25/month, automatically, beats every other financial gift you can give. The contribution room starts that year. Use it.
FOR PARENTS
Want to teach this to your kids?
We built a kid-friendly version of this lesson — plain English, real numbers a 12-year-old can relate to, and a free 3-page printable activity (“The Magic of Money”) you can do together on a Saturday morning.
Compound Interest for Kids →The earliest dollar is the most valuable dollar
Get the Canadian Tax-Free Wealth Playbook — the free 12-page guide to the TFSA, FHSA, and RRSP accounts that turn compound interest into a real life.
Get the playbook → Teach your kids