Debt has a worse reputation than it fully deserves. Used carelessly, borrowing is one of the fastest ways to wreck a financial life; used deliberately, it is how most people buy a home, gain an education, or start a business they could never fund from savings alone. The skill is not avoiding all debt — it is telling the two kinds apart before you sign.
A useful model divides borrowing into debt that tends to make you wealthier or more capable over time, and debt that simply pulls tomorrow's spending into today at a price. The test for sorting them is refreshingly simple.
The test: what is it buying?
Ask what the borrowed money is buying. If it funds something likely to grow in value or earning power — a home in a stable market, education that raises your income, capital for a viable business — it can be "good" debt, provided the terms are sane. The borrowing is a lever letting an asset work for you sooner than you could otherwise afford.
If instead the money buys something that loses value the moment you own it and leaves nothing behind — a holiday financed for months, a depreciating gadget at high interest, everyday spending you cannot cover from income — it is "bad" debt. It borrows from your future to inflate your present, and the interest is the price of that trade.
The interest rate changes everything
Even good purposes become bad debt at a bad rate. A mortgage at a reasonable rate is a tool; the same home financed at a punishing rate can sink you. High-interest revolving debt — the kind you carry month to month — is the most corrosive form, because the interest compounds against you exactly as investing compounds for you, only faster.
A practical rule follows: if a debt's interest rate is higher than the return you could reliably earn by investing, paying that debt down is itself one of the best investments available — a guaranteed, risk-free return equal to the interest you stop paying.
A borrowing checklist
Before any loan, run four checks. Does it buy something that lasts or grows? Could you meet the repayment even if your income tightened? Is the interest rate reasonable versus alternatives? And have you read the fees and penalties, not just the headline rate? Debt that passes all four can be a genuine tool; debt that fails one deserves a hard second look.
The goal is not a debt-free life at any cost — it is borrowing on purpose, for things worth borrowing for, on terms you understand. That single discipline separates debt that builds wealth from debt that quietly consumes it.
Where the simple categories break down
The two-category framework is a good starting filter and it fails at the edges in ways worth knowing about, because the edges are where most real borrowing decisions actually sit. Education is the clearest example. It appears in every list of good debt, and whether it deserves that placement depends entirely on the specific course, the specific institution, the specific field and the amount borrowed. The same qualification at the same price can be an excellent investment for one person and a lasting burden for another.
Property has the same problem. A mortgage on a home you can comfortably afford in a stable market is the canonical good debt. The identical instrument, stretched to the limit of what a lender will permit, in a market that subsequently falls, is how people end up unable to move for a decade. The category did not change; the margin of safety did.
This suggests the framework should be applied to the specific transaction rather than the asset class. The useful question is not whether mortgages are good debt but whether this mortgage, at this size, at this rate, against this income, is a sensible position to hold for twenty years including the bad years.
The repayment stress test worth running
Every borrowing decision is made under a particular set of conditions, and the loan then persists through conditions that were not anticipated. The single most useful exercise before signing anything is to run the repayment against a deliberately unfavourable version of the future and see whether it still works.
Three variables cover most of it. What happens to this repayment if the interest rate rises substantially from here, which for variable-rate borrowing is a real and recurring event rather than a hypothetical. What happens if household income drops by a third, whether through job loss, a health event or one earner stepping back. And what happens if a large unplanned expense lands in the same year.
A loan that survives all three is genuinely affordable. A loan that survives none of them is not affordable, however comfortably it fits the current month. Most lending assessments test some version of this, but they test it against the lender's risk tolerance rather than yours, and the two are not the same thing. Doing your own version takes twenty minutes and occasionally changes the decision entirely.
Reading the terms that are not the headline rate
The advertised interest rate is the most visible term and frequently not the most consequential one. A handful of others determine how a loan behaves when circumstances change, which is when the terms start to matter.
Whether the rate is fixed or variable, and for how long, decides who bears the risk of rates moving. Early repayment charges determine whether you can clear the debt if your position improves, and some are steep enough to trap borrowers in expensive arrangements for years. The presence of a promotional rate that reverts to a much higher one on a specific date has caught out an enormous number of people who intended to refinance before it happened and did not.
Fees added to the principal rather than paid upfront quietly increase the amount you are borrowing and the interest you pay on it. Any charge expressed as a percentage of the balance behaves like an additional interest rate. None of this is hidden, exactly, but it is distributed across a document written to be complied with rather than read, and locating it requires deliberately looking rather than skimming.
The order in which to clear multiple debts
Someone holding several debts at once faces a sequencing decision, and there are two established approaches that reach different answers. Paying highest interest rate first minimises total interest paid and is arithmetically optimal. Paying smallest balance first clears individual debts faster, producing visible progress and a shorter list, which sustains motivation.
The evidence on which works better in practice is more mixed than the arithmetic suggests, because completion rates matter as much as efficiency. A strategy that is slightly suboptimal and gets finished beats an optimal one that gets abandoned in month eight. People who respond well to visible milestones frequently do better with the smaller-balance approach despite paying somewhat more in total.
There is a reasonable hybrid: clear anything small enough to be eliminated within a month or two, which removes clutter and reduces the number of payments to track, then switch to strict highest-rate order for the remainder. This captures most of the motivational benefit and most of the efficiency, and it avoids the situation where a genuinely expensive balance sits untouched for a year because it happened to be large.
Debt that is neither good nor bad but merely expensive to ignore
A third category exists that the two-way split does not accommodate well: borrowing that arose from circumstance rather than choice. Medical costs, a period of unemployment, a relationship ending, a family obligation. These were not decisions in any meaningful sense and applying a framework designed for evaluating choices produces nothing useful except guilt.
The relevant question for this category is not whether it should have happened but what it costs now and how quickly it can be dealt with. Debt of this kind frequently carries the worst terms available, because it was taken under pressure from whoever would lend, which means it usually deserves priority regardless of size.
It is worth separating this out explicitly because the moralised language around good and bad debt discourages people in this position from engaging with the numbers at all. The arithmetic does not care how a balance arose. A high rate compounds identically whether the borrowing was foolish or unavoidable, and the response is the same: understand the terms, prioritise by rate, and see whether refinancing at a lower rate is available.
When paying off early is not the right answer
The guidance to clear debt whose rate exceeds your expected investment return is sound and it has boundaries. A low-rate long-term loan taken years ago may sit well below both current rates and plausible returns, in which case accelerating repayment is a choice to accept a low guaranteed return over a higher uncertain one. That is defensible on peace-of-mind grounds and it is not the arithmetically strongest option.
There are also situations where liquidity matters more than the interest saved. Directing every spare pound at a mortgage while holding no emergency fund converts accessible money into home equity, which is among the least accessible assets there is. If a job loss then arrives, you have a smaller debt and no way to pay it, which is a worse position than a slightly larger debt and cash in an account.
The general principle is that debt reduction competes with other uses of the same money and should be assessed against them rather than treated as automatically virtuous. Expensive debt almost always wins that comparison. Cheap debt frequently does not, and treating all debt as equally urgent leads people to make genuinely worse decisions while feeling responsible.
Borrowing on purpose, in a sentence you can write down
A useful discipline before any significant borrowing is to write one sentence explaining what the debt is for, what it will cost in total rather than monthly, and what would have to happen for it to become a problem. If the sentence is difficult to write, that difficulty is information.
The total cost figure matters because monthly payments are designed to feel manageable and successfully obscure the aggregate. A loan presented as a comfortable monthly amount over a long term can cost a substantial multiple of the sum borrowed, and seeing that number written down changes how some decisions look. Lenders are required to disclose it and it is rarely the number anyone quotes.
None of this constitutes financial advice and the right answer varies enormously by circumstance. But the difference between debt that builds something and debt that quietly consumes a decade is usually visible in advance, in the terms and the arithmetic, to anyone who slows down long enough to look. The cost of looking is an hour. The cost of not looking is measured in years.