Concepts
Compound interest is the most important concept in personal finance and the easiest to ignore. Here's the actual math, with Canadian numbers, that shows why every year of delay matters.
By Suroy Thamotharam
Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether he actually said it doesn’t matter. The math is true.
But “compound interest” gets thrown around in finance content so often that it’s lost meaning. Most people nod, agree it’s important, then ignore it for another year while they decide between TFSA and RRSP.
Let me make it concrete.
Compound interest means: interest earns interest on itself.
Year 1, you invest $1,000 at 7%. End of year, you have $1,070. You earned $70.
Year 2, your $1,070 earns 7%. End of year, you have $1,144.90. You earned $74.90 - more than year 1, because the $70 you earned in year 1 is now ALSO earning interest.
Year 3, you have $1,225.04. You earned $80.14.
Year 10, you have $1,967.15. Your money has nearly doubled.
Year 30, you have $7,612.26. Your money has grown 7.6x.
Year 40, you have $14,974.46. Your money has grown nearly 15x.
And that’s with ONE deposit, never adding more. If you keep adding, the curve goes nearly vertical.
Here’s where it gets real for Canadians. Let me run three scenarios:
Invests $500/month from age 25 to age 65 (40 years). Average return: 7%. Total contributed: $240,000. Final balance at 65: $1.31 million.
Same monthly contribution. Invests $500/month from age 35 to age 65 (30 years). Average return: 7%. Total contributed: $180,000. Final balance at 65: $613,000.
Same monthly contribution. Invests $500/month from age 45 to age 65 (20 years). Average return: 7%. Total contributed: $120,000. Final balance at 65: $264,000.
Person A contributed $60,000 more than Person B but ended up with $700,000 more.
Person B contributed $60,000 more than Person C but ended up with $349,000 more.
The 10 extra years at the START are worth way more than the same 10 years at the END. Because each early dollar gets longer to compound.
The reason starting early is so powerful is the shape of the compound interest curve.
For the first 10-15 years, your balance grows slowly. You feel like you’re shoveling money into a void. The compound interest you’ve earned is small relative to your contributions.
Then around year 15-20, the curve starts to bend. Your annual interest starts to exceed your annual contributions. Your money starts working harder than you do.
By year 30, most of your wealth comes from interest, not contributions. The contributions you made in your 20s are doing the heavy lifting - they’ve had 30 years to compound.
This is why the personal finance community is obsessed with starting young. It’s not because compound interest is more powerful when you’re young (it’s the same math at any age). It’s because you only get one chance at the “time” half of the equation.
You can always increase your contributions. You can never go back and invest at age 25 if you didn’t.
Don’t panic.
The honest math is: starting late costs money but doesn’t doom you. You just need to contribute more to make up for the lost compounding time.
If you’re starting at 35, you can still catch up by increasing your contribution. If you’re starting at 45, you need to either contribute aggressively or accept a smaller retirement balance.
If you’re starting at 55, the math gets brutal. You’re better off planning to work longer, downsize, or both.
Even smaller amounts compound dramatically over long timeframes.
$100/month from 25 to 65 at 7% = $262,000.
$200/month from 25 to 65 at 7% = $525,000.
$300/month from 25 to 65 at 7% = $787,000.
Even $50/month from 25 to 65 = $131,000.
The lesson: small amounts started young beat large amounts started late.
If you can’t afford $500/month, contribute $50. The habit matters more than the amount. You can always increase later as your income grows. The compounding clock starts when you deposit the first dollar.
Compound interest works against you too. This is why high-interest debt is so dangerous.
$5,000 of credit card debt at 20% APR, only paying minimums (3% of balance), takes about 20 years to pay off and costs you about $5,800 in interest - more than the original balance.
The credit card company is using compound interest against you. Every month, you pay interest on the previous month’s interest. Same math, opposite direction.
This is why paying off high-interest debt is mathematically equivalent to a guaranteed investment return of the interest rate. Killing a 20% debt is a 20% guaranteed return. Even the stock market doesn’t reliably do that.
If you have credit card debt at 8%+ AND you’re not capturing an employer RRSP match, pay the debt first. The math is unambiguous.
This trips people up. There’s no single “right” return rate to assume for planning purposes.
If you want to be more conservative, use 5%. If you want to be more aggressive, use 8%. Anything above 10% is wishful thinking for a typical portfolio.
A useful shortcut: divide 72 by your return rate to get the years it takes to double your money.
This is why the difference between a 1% MER mutual fund and a 0.1% MER ETF matters. If you’re earning 7% in fund A and 7.9% in fund B (same gross return, lower fee), fund B doubles in 9 years vs fund A’s 10. Over 40 years, that’s 4.4 doublings vs 4 doublings - a 32% larger final balance.
Fees are the silent destroyer of compound returns. They scale the wrong direction.
Once you understand compounding, the 4% rule (Safe Withdrawal Rate for retirement) makes intuitive sense.
If your portfolio grows at 7% per year on average, you can withdraw 4% per year and your portfolio will still grow (because the remaining 3% real growth covers inflation and keeps the principal intact).
Inverted: your “retirement number” is your annual expenses × 25.
This is the connection between compound interest and financial independence. The compound growth of your portfolio is what sustains the 4% withdrawal indefinitely.
For your specific FI number, try the Nomad Number Calculator.
A few habits destroy compound returns faster than slow contributions:
If you sell when the market drops 30% and re-enter when it recovers 30%, you missed the recovery and locked in the loss. Stay invested. Automate so you don’t have to think about it.
A 2% MER mutual fund vs a 0.2% MER ETF, over 40 years on a $100,000 portfolio, costs you over $400,000. Pick low-cost ETFs.
Trading in a non-registered account triggers capital gains taxes. Long-term holds in TFSA + RRSP are tax-efficient. Use the registered accounts first.
Studies show individual investors who trade frequently underperform buy-and-hold investors by 4-7% per year. Less is more.
The biggest one. If you stop adding to your investments for 5 years, the compounding catch-up is brutal. Keep automated contributions running through every market and every life event.
The math doesn’t care about your reasons for delaying. Compound interest is the most powerful force in personal finance, but it requires time more than money to work.
Time is the one thing you can never buy back later, no matter how much you earn. Use the years you have left.
Tools mentioned in this article
About Suroy
University Finance degree (with distinction). 13+ years personal investing. 10 paid coaching clients before going public. Financial Coach based in Toronto.