Should you overpay your mortgage every month, or put that extra money into investments? This question comes up constantly, and most of the advice you will find online is vague. "It depends on your risk tolerance." "Pay off debt first." "The stock market always wins long-term." The truth is that this is a maths problem, and it has a clear answer once you put your actual numbers into it.
The core comparison
Every extra dollar you put toward your mortgage earns you a guaranteed return equal to your mortgage interest rate — because you are avoiding paying that interest. If your mortgage rate is 6.5%, paying it down early gives you a guaranteed 6.5% return on that money, after any tax consideration.
Every dollar you invest instead goes into an uncertain return. A diversified stock market index fund has historically returned around 10% annually before inflation in the US and about 8–9% in the UK and Australia — but that is an average over decades, with significant volatility along the way. Some years return 25%. Some years return -30%.
The comparison is therefore: guaranteed mortgage rate vs. expected investment return. If your mortgage rate is higher than your expected investment return after tax, pay down the mortgage. If your expected investment return is higher, invest.
How tax changes the answer
In the US, mortgage interest on your primary home may be tax-deductible if you itemize, which reduces the effective cost of your mortgage. In the UK, mortgage interest on a primary residence is not deductible, but investments in an ISA grow completely tax-free — which improves the case for investing. In Canada, RRSP and TFSA contributions both offer tax advantages. In Australia, investments in super grow at a concessional 15% tax rate.
These tax wrappers change the effective return on investments significantly. A 7% investment return inside a UK ISA or Canadian TFSA, where all growth is tax-free, is genuinely better than a 7% mortgage interest saving on which you would have paid tax on the freed-up income anyway.
A worked example
You have a $300,000 mortgage at 6.5% with 20 years remaining. You have $500 per month of spare cash. Option A: put all $500 toward the mortgage every month. Option B: invest all $500 per month in a broad equity index fund.
Option A (mortgage overpayment) eliminates your mortgage in roughly 14 years instead of 20 — saving approximately $48,000 in interest. After that, you invest the freed-up payment for the remaining 6 years.
Option B (invest immediately) at 7% average annual return over 20 years gives approximately $260,000 at the end of the period, assuming tax-advantaged accounts. Your mortgage still exists at the end of 20 years, but you have a substantial investment portfolio to repay it with $140,000 left over.
In this example, investing wins — but only because 7% investment returns beat the 6.5% mortgage rate, and only if you are disciplined enough to actually invest every month for 20 years. If market returns average 5% instead of 7%, the mortgage overpayment wins comfortably.
The honest answer
If your mortgage rate is above 7%, lean toward paying it down — you are unlikely to reliably beat that guaranteed return after tax and fees. If your mortgage rate is below 5%, maximise tax-advantaged investments first. Between 5% and 7%, the answer depends on your specific tax situation, risk tolerance, and how certain you are about your investment returns.
There is also a psychological dimension. Some people sleep better knowing their home is paid off. That peace of mind is real and worth accounting for — but it is a personal preference, not a financial argument. Run the numbers, then factor in how you feel about debt.