Prepaying a loan vs. investing the difference

Get a bonus, a raise, or just a good month, and a familiar question shows up: pay down the loan faster, or put that money to work elsewhere instead. There’s a spreadsheet answer to this, and there’s a real-life answer, and they don’t always agree.

The spreadsheet answer

If a loan charges less interest than you could reasonably expect to earn elsewhere after tax, the arithmetic favors not prepaying — let the loan run its course at its lower rate, and put the extra money toward the higher-returning option instead. This is genuinely true for many home loans, where rates can sit well below long-term equity returns.

It flips completely for expensive debt. A loan or card balance running at 24–40% annually isn’t something any ordinary investment reliably beats, after tax, after risk. There, prepayment isn’t just reasonable — it’s close to a guaranteed, risk-free return equal to whatever rate you stop paying.

Where the spreadsheet stops being the whole story

Guaranteed peace of mind has a value the math doesn’t capture. Carrying debt affects how people feel about risk, spending, and their own financial position, even when the numbers say the debt is “cheap.” Someone who sleeps better with zero outstanding loans is optimizing for something real, even if a strict return comparison says otherwise.

The honest way to hold both of these truths: run the math first, so the decision is informed rather than a guess. Then decide, knowingly, whether the psychological value of being debt-free is worth leaving some return on the table. That’s a legitimate choice — it just shouldn’t be an accidental one.

The number to check first

Before deciding either way, it helps to see where a loan sits in the full picture — how much it’s costing monthly, how it compares with everything else owed, and how much room exists either way.

Dette’s Debt Score lays that out clearly, so the prepay-or-invest decision starts from real numbers instead of a gut feeling.