Compound Interest: The Most Powerful Force in Finance.

There is a Chinese proverb that gets quoted on every personal finance blog: the best time to plant a tree was twenty years ago. The second best time is today. Compound interest works exactly like that. The best time to start investing was the first day you earned a paycheque. The second best time is today, regardless of your age, regardless of your income, regardless of how much you can afford to put in.

The entire secret to compounding is one word: time. Its not the size of your contributions, not the rate of return, not the platform you invest with, just time. Which means the most important financial decision most Canadians ever make is not what to invest in or how much, but whether to start at all. The habit of contributing something, anything, every month beats waiting for the right moment, the right amount, or the right level of income.

Most articles about compound interest open with a formula. This one will not, because almost nobody actually cares about the formula. What people care about is whether starting now will change anything. Whether they have missed the window. Whether $50 a month is even worth bothering with. The answer to all three questions is the same answer: yes, start today, and the amount matters far less than the habit you are building.

The story of two investors

Two Canadians both want to retire at 65. They invest the same amount each month, $100, into the same low-cost S&P 500 index ETF inside a TFSA. The only difference is when they start.

Investor A starts at age 20, contributes $100/month for 45 years, and stops at 65.

Investor B starts at age 30, contributes $100/month for 35 years, and stops at 65.

Both use the S&P 500's long-run nominal return of about 10% annualized. Here is what they each end up with:

Investor A puts in only $12,000 more than Investor B across their entire lifetime. They end up with about $670,000 more at retirement.

Said differently: the extra $12,000 Investor A contributed in their twenties grew into roughly $668,000 by age 65. The same $12,000 contributed in their fifties would have grown into just over $20,000. Same dollars, same investment, completely different outcome.

The contributions you make in your 20s do more long-term work than the contributions you make in your 30s, so start early.

It is not how much you start with. It is that you start.

The single most common reason people put off investing is that they think their starting amount is too small to matter. They wait until they have $10,000 saved or they wait for a raise, or they wait until after the next big expense. None of these triggers ever quite arrives, and the doubling cycles get skipped in the meantime.

Start with what you can. $25 a month from age 20at the S&P 500's 10% historical return compounds to about $262,000 by 65. $50 a month reaches about $525,000. $10 a month — the cost of one streaming subscription — still reaches over $104,000. None of these are retirement-level wealth on their own, but every single one of them is far more than $0, and every single one of them builds the habit that scales up as your income does.

The habit is the asset, not the dollar amount. A Canadian who starts at 22 contributing $25 a month and increases that to $200 a month by 30 (still well below the TFSA annual room) ends up with a fundamentally different retirement than someone who waits until they can “afford to invest properly” and starts at 35 with $500 a month. The first person built a habit. The second person built a delay.

Why time matters more than the amount

The instinct most people have is “I'll catch up by saving more later when I earn more”. The math says this is almost always wrong. Compare a third investor who starts late but contributes triple:

Investor C waits until age 40, then contributes $300/monthfor the next 25 years until 65. Same S&P 500, same 10% annualized return.

Total contributed by Investor C: $90,000 (almost double what Investor A put in across 45 years). Final balance: about $398,000. Investor A still wins by over $650,000, despite contributing $36,000 less in total.

Time in the market beats timing the market

A second instinct people have is to wait for “a good time to enter the market”. The 2020 COVID crash is the cleanest evidence this does not work. The S&P 500 peaked on February 19, 2020 at 3,386. By March 23 — just 33 days later — it bottomed at 2,237, a drop of 34%. Every headline that month was about the end of the bull market, the global recession, the worst crash since 2008. Anyone watching from the sidelines was certain the bottom was still ahead.

Here is what makes timing impossible. On March 13, mid-crash, the S&P 500 delivered a single-day gain of +9.3% — the largest one-day rally since 2008 at the time. Anyone who treated that as the bottom and jumped back in would have watched the market fall another 17.5% over the next ten days. The actual bottom came on March 23. The very next day, March 24, the index posted another +9.4% single-day gain, and thatone stuck. Both March 13 and March 24 rank among the ten largest single-day gains in S&P 500 history, and they happened ten days apart with a 17.5% drop sandwiched between them.

By August 18, less than five months after the bottom, the index had recovered to a new all-time high. The S&P 500 finished 2020 up about 16%, in a calendar year that contained one of the fastest crashes ever recorded.

The lesson is mechanical. The biggest down days and the biggest up days cluster in the same few weeks during every crisis, and you cannot tell them apart in real time. Anyone who sold in March 2020 to “wait it out” missed both — they avoided the worst days and the recovery days at the same time, because those days were adjacent on the calendar. Long-run studies show that missing just the ten best trading days of any given decade cuts compound returns by roughly half. Staying invested through everything, even crashes that feel like the end of the world, is the only strategy that reliably catches the rebounds.

The S&P 500's nominal returns since 1928 average about 10% annualized despite years like 2008 (-37%), 2002 (-22%), and the brief COVID drop in 2020. Any single year can be wildly different. The compounding math relies on the long-run average, not on whichever number this calendar year happens to deliver. If your horizon is decades, your job is to keep contributing and wait.

The Rule of 72 — your mental shortcut

The only piece of compound-interest math worth memorising is the Rule of 72. Divide 72 by your annual rate of return and the result is roughly how many years it takes your money to double.

At the S&P 500's historical 10%, money doubles about every 7.2 years. At a more conservative 7%, every 10.3 years. At 4% (closer to a Canadian high-yield savings account), every 18 years. At 2% (long-run inflation), every 36 years.

Use the rule to sanity-check any retirement projection. $50,000 invested at 25 should be worth about $100,000 by 32, $200,000 by 40, $400,000 by 47, and roughly $800,000 by 54 if left alone. Every doubling is the same gap of years; the dollar amount the doubling represents gets larger and larger.

Why TFSAs and RRSPs make compounding even better

Every number above assumes the investment growth is not being taxed each year. That only holds inside a registered account.

In a non-registered account, the CRA taxes interest, dividends, and realised capital gains every year. Even if you reinvest everything, that annual drag reduces your effective return by roughly your marginal tax rate applied to each year's yield. A 10% raw return at a 30% marginal rate compounds closer to 7% after tax. Over 45 years, that drag turns Investor A's $1,050,000 into about $380,000 — which is, by coincidence, exactly where Investor B landed by waiting until 30 to start. Annual tax drag inside a non-registered account costs you roughly the same amount as losing a full decade of compounding.

Inside a TFSA, RRSP, or FHSA, no annual tax is charged on growth. Every dollar of return stays in the account and gets to compound. The TFSA is the cleanest illustration: contributions are after-tax dollars, but every dollar of growth is permanently sheltered, including the withdrawals. For a long-horizon investor, this is one of the highest risk-free returns available.

The inflation footnote

Every dollar figure in this post is nominal — the actual number that will appear in the account in 45 years, not adjusted for inflation. Inflation eats purchasing power, so a million dollars in 2071 will not buy what a million buys in 2026.

At 3% average inflation (roughly the long-run Canadian average), Investor A's $1,050,000 in 2071 will buy about what $278,000 buys today. That is still a life-changing outcome from $54,000 of contributions. But it is not the same as $1,050,000 of 2026 money landing in 2071.

The reverse of this also matters, and it is the more important point: if you don't invest at all, you are not standing still — you are going backwards. $10,000 sitting in a chequing account at 0% interest loses purchasing power to inflation every year. After 45 years at 3% inflation, that same $10,000 buys what about $2,645 buys today. Three quarters of its value, gone, without you spending a dollar. Even a high-interest savings account paying 2.5% is running a small loss in real terms because inflation outpaces the rate. The choice is not between investing (risky) and not investing (safe). The choice is between compounding with you and inflation compounding against you. Doing nothing is not neutral.

Compound interest works in reverse too

The same engine that grows your TFSA is what makes high-interest debt destructive. Credit-card interest compounds against you every month. A $5,000 balance at 21% annual interest (the average Canadian credit-card rate), carried for ten years with no payments, balloons to over $40,000.

This is the reason almost every personal-finance writer, advisor, and bank gives the same advice: pay off high-interest debt before you invest a single dollar. The math is straightforward. Paying off a credit-card balance at 21% is the financial equivalent of earning a guaranteed, risk-free 21%compound return on the same money — better than any stock-market average, better than anything a financial advisor can responsibly offer you. The S&P 500 has averaged about 10% annualized. Eliminating a 21% credit-card balance more than doubles that return, with zero risk and zero waiting.

Put plainly: every dollar you throw at a 21% credit-card balance is doing more work for you than every dollar you put into an index fund, until the balance is gone. Once it is, the engine flips. Take the same monthly amount you were paying toward debt and redirect it into a TFSA-held S&P 500 ETF. Now the compounding works for you instead of against you, and the habit you built paying down debt becomes the habit that builds your retirement.

The misconceptions worth clearing up

Three patterns come up repeatedly.

“I'll just save more later when I earn more” . Investor C tried this and lost by $650,000. Earning more later does not buy you back the doubling cycles you skipped early.

“A 10% return is unrealistic”. The S&P 500's long-run annualized nominal return since 1928 is about 10%. Any single year can be wildly different, but a global-equity ETF held inside a TFSA over a multi-decade horizon has historically delivered close to that figure. Cut it to 7% if you want to be conservative; the difference between starting at 20 and starting at 30 is enormous at either rate.

“I need a financial advisor to invest”. A low-cost S&P 500 or all-world index ETF held inside a TFSA, RRSP, or FHSA, with monthly automatic contributions, is a strategy any Canadian can set up in an afternoon. Advisor fees of 1-2% per year compound against you the same way market returns compound for you.

The best time to start is today

Everything in this post points to the same action: open a registered account, set up an automatic monthly contribution for whatever amount you can afford right now (even if that is $25), and let it run. The compounding engine does not care what your starting amount is. It cares whether the money goes in this month, and the month after that, and the month after that, for as many years as you can give it.

If you don't have a TFSA, RRSP, or FHSA open yet (or you want one separate from your bank), Wealthsimple opens all three for free with no minimum balance and no monthly fee. You can set up an automatic monthly contribution in a few minutes and the compounding engine starts running tonight. I use Wealthsimple personally for my TFSA, RRSP, and FHSA, and it's the platform I recommend to friends starting their first registered account.

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