The mathematical argument
If your mortgage carries a 2.5% rate and your expected long-term net investment return is 6%, that 3.5-point spread compounds significantly in favor of investing over decades, thanks to compound interest working on that gap year after year.
But there are important nuances. Investment returns are not guaranteed; the interest you save by prepaying your mortgage is guaranteed with total certainty. Investing means accepting volatility and potential temporary losses, while prepaying is a guaranteed return equal to your mortgage rate.
A numerical example: comparing both paths
Example. Imagine you have €20,000 and a mortgage at 2.8% with 20 years remaining. If you use it to prepay and shorten the term, your savings on future interest are equivalent to that €20,000 earning a guaranteed 2.8% annually. If instead you invest it in a global ETF with an expected 6% annual return over those same 20 years, the capital could grow to around €64,000 before taxes, compared to a noticeably smaller interest saving from prepayment.
The gap looks clearly in favor of investing, but only if you hold the investment for the full horizon without selling during a sharp market downturn. That's where the second decisive factor comes in: your own risk tolerance, not just the pure math.
The liquidity factor, the most overlooked one
Money used to prepay a mortgage disappears from your account and can't be easily recovered if an emergency comes up. Invested money can be sold, though potentially at a loss if the market happens to be down at that moment. Without an emergency fund, aggressive prepayment can leave you exposed to any unexpected event.
Rule of thumb: never prepay mortgage principal unless you have at least six months of expenses covered in liquid savings. And never invest money you might need within five years, precisely because of short-term market volatility.
An often-overlooked middle path: if your liquidity would come from selling a portfolio with large unrealized gains, selling it only to fund a mortgage prepayment can generate a capital gains tax bill large enough to erase the mathematical benefit. A portfolio-backed pledge loan (borrowing against your securities without a disposal) is an alternative: it gives you cash to deploy toward either prepayment or investing without realizing the gain in the current tax year.
A practical decision framework
There's no single answer that fits everyone. But you can use this framework as a starting point: if your mortgage rate is high (above 3.5%) and being in debt causes you real stress, prioritize prepaying. If your rate is low (below 2.5%), you have a solid emergency buffer, and your investment horizon is 10+ years, investing usually wins mathematically.
What matters is that the decision is part of an ongoing system, not a one-off impulse. Connect it to your net worth, your cash flow, and your scenarios in the mortgage simulator before making an irreversible move.
Apply This Knowledge to Your Real Finances
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Frequently Asked Questions
Shortening the loan term usually generates greater cumulative interest savings, since it cuts the total time the debt accrues interest. Lowering the monthly payment provides more immediate month-to-month flexibility. The right choice depends on whether you prioritize total savings or monthly breathing room.
Yes, and many people combine both strategies. For example, prepaying up to any tax-deductible limit that applies to you and putting the rest of your savings capacity into investing. It's a balanced, common approach in practice.
With a variable rate, the spread against your expected investment return can shift over time if benchmark rates rise. It's worth revisiting this decision periodically rather than making it once and forgetting about it, especially in a rising-rate environment.
Yes. By reducing your outstanding mortgage balance, you directly increase your net worth, exactly as if that money had grown in an investment. The difference lies in the expected return of each path, not in whether it 'counts' as progress.
If your situation includes other relevant factors (large unrealized gains, multiple properties, complex tax planning), an advisor can help fine-tune the decision. For most standard cases, the framework of interest rate, taxation, and liquidity we've described is enough to decide with confidence.