Pay Down Mortgage or Invest? The Math, Explained

Why this is the question I get asked most

It usually shows up after a raise, a tax refund, or the kids moving out — a stretch of months where the bank balance keeps creeping up and the homeowner starts to wonder what to do with it. The mortgage is sitting there, visible and emotional. The TFSA is sitting there, abstract and quiet. The internet has loud answers on both sides. So they call us, because we sit in the middle of both worlds.

Here's how we actually think about it.

What pay-down really earns you

Every dollar of principal you prepay on your mortgage permanently stops accruing interest at your contract rate for the rest of the amortization. On a 5.49% mortgage, a $1,000 prepayment saves you exactly $1,000 × 5.49% = $54.90 in year-one interest, and compounds from there. There's no tax on that savings (you weren't earning anything to be taxed). There's no fee. There's no variance.

That's the magic of the pay-down side: it's a guaranteed, after-tax, after-fee return equal to your mortgage rate. In a world where 5-year GICs pay 4%, a guaranteed 5.49% is genuinely hard to find anywhere else.

What investing really earns you

Long-run averages for a balanced 60/40 or 70/30 portfolio land somewhere in the 6%–8% nominal range. Most fee-only planners use 7% as a "plan-with" assumption, not as a forecast. The actual year-to-year experience is wildly different — 2022 was painful, 2023 and 2024 rebounded hard, and any 5-year window contains at least one stomach-turning drawdown.

So the comparison isn't "5.49% guaranteed vs 7% guaranteed." It's "5.49% guaranteed vs ~7% expected with real variance." That gap — the variance — is where the personal part of the decision lives.

The tax wrapper changes everything

Here's the wrinkle the math YouTubers usually skip. Where you invest matters at least as much as what you invest in:

• TFSA: 7% in stays 7%. No tax on growth, no tax on withdrawal. This is the wrapper where the invest side wins cleanly.
• RRSP: Tax-deferred. You get a refund today, pay tax on withdrawal. Works when retirement marginal rate is lower than today — i.e., most employed Canadians.
• Non-registered: Dividends taxed annually, capital gains on disposition. A 7% nominal return easily drops to ~5% after-tax for a middle-income Ontario or Atlantic Canadian earner.

Run the same comparison without considering the wrapper and the invest-side numbers look way more attractive than they actually are.

The honest math

Let's run a realistic case. Same $400,000 mortgage at 5.49%, 25-year amortization. Client has $500/month of extra cash, 15-year horizon, plenty of TFSA contribution room.

• Pay-down path: $500/month against principal cuts roughly 7 years off the mortgage and saves a meaningful chunk of lifetime interest. End of year 15: mortgage paid off or nearly so; investments unchanged.
• Invest path (TFSA @ 7%): $500/month at 7% compounded for 15 years lands around $158,000. Mortgage continues on its original schedule.

In raw dollars the invest path is usually ahead — sometimes by $30K, sometimes by $80K — but the entire gap depends on whether the 7% materializes. Drop the assumed return to 5% and the two paths are roughly tied. Drop it to 3% and pay-down wins outright.

When pay-down clearly wins

• Mortgage rate above 6%. The guaranteed return rises, and the invest side has to clear a higher bar to win.
• Short horizon (under 10 years). Variance dominates short windows. The pay-down certainty is worth more than the chance of higher returns.
• No TFSA room left. Without the tax wrapper, the invest-side return takes a 20–30% haircut.
• Behavioural risk. If you'll touch the investments to remodel the kitchen or buy a boat, the mortgage prepayment — which you can't easily un-do — is the better discipline tool.

When investing clearly wins

• Mortgage rate under 4%. Hard for any "guaranteed" return to compete.
• Long horizon (15+ years). Variance smooths out; expected return becomes more like realized return.
• TFSA room available. The 7% stays 7%.
• Disciplined investor. Auto-contribution set up, won't sell during drawdowns, won't raid the account.

The blended approach most planners actually use

Once you stop treating it as a binary, the answer most fee-only planners arrive at is: do both. Put enough extra against the mortgage to clear it by retirement (or by the time the kids hit university — whichever is sooner), and invest the remainder inside your TFSA, then RRSP.

For most clients in their 30s and 40s, that ends up looking like roughly 40% of extra cash flow against the mortgage and 60% into TFSA/RRSP — but the exact split depends on rate, horizon, and how much existing investment runway you have.

Renewal risk nobody talks about

Here's the part the calculators don't model well: your mortgage rate isn't fixed forever. Most Canadians are on 5-year fixed. If you locked at 5.49% today and renew at 7% in 2031, every assumption you made about the pay-down vs invest comparison changes. The pay-down case strengthens significantly.

On every long-term plan we run, we stress-test renewal at +2% over the current rate. If the invest-vs-pay-down decision is sensitive to that stress test, we know the answer is "lean pay-down." If it doesn't move much, the invest path is robust.

Want to run yours? Try the calculator, then book a free call if you want a second set of eyes on the assumptions.