QuietCalcsIt comes down to one comparison
Every dollar you put toward debt earns you a guaranteed, tax-free return equal to the interest rate you're no longer paying. Every dollar you invest earns an uncertain market return. So the whole decision hangs on two numbers: your debt's interest rate versus the return you expect from investing.
When the debt rate is higher — credit cards at 20%+, most personal loans — paying it off is a risk-free win almost nothing beats. When it's lower — a subsidized student loan, a low fixed mortgage — investing the money has usually grown more wealth over long stretches, though never with a guarantee.
Why both sides spend the same
A fair comparison holds your monthly outlay constant. Both paths here spend your minimum payment plus the extra, every month:
- Pay off debt first: the whole budget attacks the loan until it's gone, then the freed-up cash gets invested for the rest of the time.
- Invest first: the debt gets only its minimum while the extra is invested from day one; once the minimum finishes the loan, that money joins the investing too.
Because the spending is identical, the ending net worth — investments minus any debt still owed — is a clean, honest comparison.
Do these two things first, whatever the math says
Before optimizing debt-versus-invest, two moves beat both: keep a small emergency fund so a surprise doesn't send you back to the credit card, and capture any employer retirement match — that's an instant 50–100% return no debt payoff or market can touch. This calculator assumes you've already handled those and are deciding what to do with the next dollar.
The certainty discount
Even when investing wins on paper, the two outcomes aren't equal in kind. Paying off debt is a sure thing; the investment return is an average that hides good years and brutal ones. Many people rationally choose the guaranteed win even when the expected value slightly favors investing — being debt-free has a real, if unquantifiable, payoff. If you'd lose sleep over it, weight the decision toward the debt. This tool gives you the numbers; the temperament is yours.
A couple of things it leaves out: taxes on investment gains (which trim the investing side a little) and the employer match above. Both are worth folding into your own judgment.