I bet on falling rates. I lost on both sides at once.

A variable-rate mistake of mine, the after-tax math almost nobody runs, and the rule of thumb that quietly tells you to buy high.

Years ago I decided interest rates were probably heading lower, so I took a variable-rate mortgage. For about two years I looked clever. Then rates climbed, and I found out I was wrong.

The painful part wasn’t just that my borrowing got more expensive. My fixed-income investments were falling at the same time. Rising rates hit both sides of the balance sheet at once. That experience is the whole reason I think “should I pay off debt or invest?” deserves more than the one-line answer the internet keeps giving.

The standard Canadian answer goes: compare your interest rate to your expected return, invest if you can beat the rate, and factor in your peace of mind. That’s not wrong. It’s just written for a kind of debt most of the people asking don’t actually have, and it skips the part that quietly costs the most.

First, what kind of debt is it? Because two very different things get called “student debt.” A federal Canada Student Loan has been interest-free since April 2023 — no new interest accrues. Against a zero-percent cost, any positive return wins, so the math rarely argues for rushing to repay. But most high earners asking this question aren’t carrying that. A doctor, dentist, or lawyer typically has a professional student line of credit — a revolving bank loan, often well into six figures, priced at or near the prime rate. The rate floats, and the interest generally isn’t tax-deductible. The internet’s “student loan” advice wasn’t written for it.

Paying down isn’t a return. It’s a cost you remove. If a line of credit costs 4.5%, every dollar you put against it is a dollar that’s no longer costing 4.5% a year, guaranteed. Keep the balance and invest instead, and you’re making a bet: that your portfolio out-earns the loan. The gap between the two is the only part you actually pocket. And that gap is thinner than it looks because the comparison that counts is after tax. The line-of-credit interest usually isn’t deductible, while gains inside a TFSA are tax-free and an RRSP contribution throws off a refund. So it’s after-tax return versus after-tax cost, not headline versus headline.

Here’s the trap. “Invest if you can beat your rate” holds up, but only if “beat your rate” means your long-run expected return, not whatever the market did this month. Apply it to the present moment and it inverts: in a correction your portfolio is down, so the rule whispers “stop investing”, which is exactly when future returns tend to be highest. Near a peak it says “pile in”, when returns tend to be lowest. Followed literally, a moment-by-moment check nudges you to buy high and skip the dips. The one number worth reacting to is your rate, because it’s a known figure you can observe…not a market you’re trying to time. (That’s the lose-lose from my mortgage: rates and prices move together, so reacting to the moment can hurt on both sides at once.)

And then there’s the part the math can’t settle. The spread is arithmetic. How you sleep isn’t. Carrying a big balance to chase a slim edge is a real weight for some people, and clearing it anyway isn’t irrational — it’s buying certainty, and certainty has a price worth naming. There’s also an asymmetry the spreadsheet misses: a paid-down balance can’t be sold in a panic, but an invested dollar can. For anyone who’d bail in a downturn, simply repaying the debt can beat a spread they’d have earned on paper and then handed back at the bottom.

So the real answer is that “pay down or invest” is the wrong first question. Start with the kind of debt. Run the comparison after tax. Anchor it to the long run, not this month’s headlines. Then decide what you can live with. The arithmetic narrows the choices; the last call is yours.

I wrote the full version — with the FP Canada return assumptions, the actual bank line-of-credit rates, and the tax mechanics laid out step by step — on the site: Pay Off Student Loans or Invest in Canada? The Real Math →

And the reason any of this is easy to see for your own numbers — the balance, the rate, the accounts, and the spread in one view — is the thing I built. Take a look at YouGotThis →