The core trade-off
Every extra dollar has two competing homes. Put it toward debt and you earn a guaranteed, risk-free return equal to the debt's interest rate — paying off an 8% loan is exactly as good as a guaranteed 8% investment. Put it into the market instead and you earn an expected return that's usually higher over the long run, but uncertain and taxable. This calculator settles the argument by simulating both strategies month by month and comparing the net worth — investments minus any remaining debt — each leaves you with.
How to read the result
The math almost always follows one simple rule: compare the debt's interest rate to your expected after-tax investment return.
- High-rate debt (credit cards, ~15–25%): paying it off wins decisively. Almost nothing reliably beats a guaranteed 20% return, which is why clearing expensive debt is treated as an emergency.
- Low-rate debt (a ~3–4% mortgage or subsidised student loan): investing usually builds more wealth over long horizons, because expected market returns comfortably exceed the interest you'd save.
- The middle (~5–8%): it's close, and the "right" answer depends on your assumed return, taxes and time horizon — exactly what you can test above.
Why the guaranteed vs expected distinction matters
Paying down debt delivers a certain outcome; investing delivers a probable one. Two people with identical numbers can rightly choose differently based on risk tolerance. The debt payoff is the lower-stress, lower-variance option; investing has higher expected wealth but real downside risk and requires you to actually stay invested through crashes. The calculator shows the expected-value winner — you weigh the certainty premium yourself.
What the simple comparison leaves out
- Employer retirement match comes first. A 50–100% match beats paying off any debt — always capture it before either strategy. See the financial order of operations.
- Emergency fund first. Without cash reserves, a surprise expense lands on high-rate debt and undoes your progress.
- Taxes and behaviour. Investment returns may be taxed; debt payoff is tax-free. And guaranteed progress keeps some people motivated in a way volatile markets don't.
Pair this with our debt payoff and investment return calculators, and the good debt vs bad debt guide, to decide with the full picture.