Spare money can go toward your loans or into investments, and the right call depends on your interest rate, your time horizon, and how much you value certainty. Here is a framework, not advice.
Paying down a loan early earns you a guaranteed, risk-free return equal to the loan's interest rate. Clear a balance charging 7% and you have, with certainty, avoided 7% of interest you would otherwise have paid. Investing instead offers a higher expected return over the long run, but that return is uncertain: markets fall as well as rise, and the timing is out of your hands. The whole decision comes down to comparing a sure thing against a probable but bumpy one.
Because of that, the rule of thumb most people use is to compare your loan rate with what you could reasonably expect to earn after tax by investing. If your debt costs more than your likely investment return, paying it down is both safer and probably more lucrative. If your debt is very cheap, investing has more room to come out ahead over a long enough horizon. The wider the gap between the two rates, the clearer the call.
High-interest debt is the easy case. Credit cards in the low-to-mid twenties, and many personal loans in the teens, charge more than almost any investment can reliably return, so clearing them is close to a guaranteed double-digit return with zero risk. Almost every framework agrees that high-rate consumer debt should be cleared before serious investing begins, because few investments beat the certainty of cancelling a 22% interest charge.
Paying down debt also wins when certainty itself has value to you. If a guaranteed result lets you sleep at night, reduces the risk of missing a payment, or frees up cash flow you will need soon, that peace of mind is worth something real that a spreadsheet does not capture. The closer you are to needing the money, the more the guaranteed return of debt payoff beats the volatility of investing, because a short horizon gives markets no time to recover from a dip.
Low-rate, long-term debt is the case where investing often comes out ahead. A fixed-rate mortgage at a low rate is cheap money, and over a long horizon a diversified portfolio has historically returned more than that, so directing spare cash to investments rather than overpaying the mortgage can build more wealth, provided you can tolerate the ups and downs along the way. Time is the crucial ingredient: the longer your money can stay invested, the more the odds favour it.
Tax-advantaged accounts tilt the maths further toward investing. Money put into an employer retirement plan that comes with a matching contribution earns an immediate return from the match that almost no debt payoff can rival, which is why capturing a full employer match usually comes before extra debt payments. Accounts that shelter growth from tax also raise the effective return on investing. None of this is a recommendation; it just explains why the answer is not always to clear the debt first.
Before you optimise between debt and investing, two foundations usually come first. An emergency fund of a few months of essential expenses keeps an unexpected bill from pushing you onto high-rate borrowing, which protects every other decision you make. And any debt that is in arrears, or any minimum payment you are at risk of missing, takes priority over both extra payments and investing, because the cost of default dwarfs the difference between the two strategies.
Once those are in place, you do not have to choose all or nothing. Many people split spare money, sending some to extra debt payments and some to investments, which hedges the uncertainty and keeps both habits going. To see the guaranteed side of the ledger clearly, run your loan through the calculators here: knowing exactly how much interest an extra payment would save, and how many months it would shave off, makes the comparison with an uncertain investment return far easier. This is general information, not financial advice; for your own situation, a qualified professional can help.