If you have extra money each month and you also have debt, you face one of the most common questions in personal finance: should you pay off the debt first, or invest the money? The answer feels emotional — debt feels bad, investing feels smart — but it is actually a maths problem, and the maths has a clear answer. It comes down to comparing two numbers: the interest rate on your debt and the return you expect from investing. Here is the simple rule that settles it, and the exceptions that prove it.

Paying off debt gives you a guaranteed return equal to the interest rate on that debt. If your credit card charges 22 percent, paying it off is a guaranteed 22 percent return — better than any investment in existence. If your mortgage charges 4 percent, paying it off is a guaranteed 4 percent return, which is lower than the stock market's long-run average of around 8 to 10 percent. The rule is simple: if the debt's interest rate is higher than the return you expect from investing, pay the debt first. If it is lower, investing usually wins — with one important exception, which is the employer 401(k) match, because that is a guaranteed 50 to 100 percent return that beats any debt payoff.

Here is the order most financial planners recommend. First, capture the full employer 401(k) match — it is free money and beats everything. Second, pay off high-interest debt, meaning anything above 8 percent: credit cards, personal loans, and payday loans. Third, build a small emergency fund of $1,000 to one month of expenses so a surprise bill does not push you back into debt. Fourth, split your extra money between investing and paying off medium-interest debt in the 5 to 8 percent range, such as car loans. Fifth, once high and medium-interest debt is gone, invest aggressively while making minimum payments on low-interest debt below 5 percent, such as a mortgage or student loan. This order balances guaranteed returns with long-term growth.

The maths says that paying off a 4 percent mortgage is worse than investing at 8 percent — but the maths ignores how debt makes you feel. Some people sleep better with zero debt, even if it costs them a little in expected returns, and that peace of mind has real value. Being debt-free also reduces your monthly obligations, which gives you flexibility if you lose your job or want to change careers. If the emotional weight of debt is affecting your sleep or your decisions, paying it off faster is a reasonable choice even when the maths favours investing. The best financial plan is the one you can actually stick to, and for some people, that means becoming debt-free first. Just make sure you are not paying off low-interest debt while ignoring a 401(k) match — that is the one mistake that is never worth it.

You do not have to choose one or the other. A popular approach is to split your extra money — for example, 50 percent to debt and 50 percent to investing — so you make progress on both fronts and stay motivated. Another approach is to set a threshold: pay off all debt above 6 percent first, then invest while making minimum payments on anything below that. If you have multiple debts, use the avalanche method — pay the highest-interest debt first while making minimum payments on the rest — because it saves the most money. The snowball method, paying the smallest balance first, costs slightly more in interest but gives quick wins that keep some people motivated. Either method works if you stick with it.

Start by listing all your debts with their interest rates and balances, and all your investment options with their expected returns. Apply the priority order: match first, high-interest debt second, emergency fund third, then split between investing and medium-interest debt. Automate both the extra debt payments and the investment contributions so the decision happens once, not every month. And remember that this is a good problem to have — if you have extra money to allocate, you are already ahead of most people. Make the decision, automate it, and move on to the next financial goal.