The cost of borrowing is quoted in several ways that do not compare directly

The same borrowing can be described by several different numbers

A rate attached to a loan is the answer to a question, and different quoting conventions answer different questions. Some express the charge as a proportion of the original amount borrowed, applied for every year of the term. Some express it as a proportion of the balance still outstanding, which falls as the loan is repaid. Some are annual figures derived from a monthly charge. Some include fees and some do not.

These produce genuinely different numbers for identical borrowing, and the differences are not small. A figure calculated on the original amount will look much lower than one calculated on the declining balance for exactly the same loan, because the base it is applied to never shrinks.

Which conventions are permitted, and which must be disclosed, varies by country and changes over time. What does not vary is that the borrower has to know which question a quoted number is answering before comparing it with another one.

Why a declining balance changes everything

On a repayment loan the balance falls with every payment, so interest charged on the outstanding amount falls too. On a charge calculated against the original sum, it does not. The gap between the two conventions widens as the loan progresses, and across a full term it can be substantial.

A stripped-down illustration. Suppose 1,000 is borrowed for two years, repaid in equal monthly instalments, and the charge is 10% of the original amount per year. That is 200 of interest on a balance that averages roughly half of 1,000 over the term. Expressed against the amount actually outstanding, the effective cost is close to double the quoted figure.

The arithmetic is not a trick, and the convention exists for reasons of simplicity. But someone comparing that quote against a rate charged on the outstanding balance is comparing two things that are not the same measurement, and will pick wrong.

Compounding frequency is a second, quieter variable

A rate also depends on how often interest is added to the balance. Interest applied once a year, on a balance that has not itself accrued interest during that year, costs less than the same nominal rate applied monthly, because the monthly version starts charging interest on interest earlier.

The effect is modest at low rates over short periods and becomes significant at high rates over long ones. It is one reason that the same nominal figure attached to two products can produce different totals, and it is rarely the number a borrower is looking at.

This is also why standardised comparison figures exist in many jurisdictions: they are an attempt to reduce all these conventions to one number that can be compared across products. They are useful, imperfect, and their definitions differ, which is worth remembering when comparing across borders.

What the quoted rate leaves out

Fees are the largest omission when they are not included. An arrangement fee, an annual charge, a cost for a required additional product, or a penalty for repaying early each affects the total handed over without appearing in a simple interest figure.

Timing is another. A charge levied at the start behaves differently from the same amount spread over the term, because money paid earlier costs more in real terms than money paid later. And promotional structures — a cheaper period followed by a standard one — mean the advertised figure may describe only the first phase of the arrangement.

This is why the most reliable comparison for a household is not a rate at all. It is the total amount repayable over the full term, which absorbs every one of these variables into a single figure that means the same thing on both sides of the comparison.

A practical basis for comparing

Ask three questions of any borrowing and the confusion largely resolves. What is the total amount I will hand over across the whole arrangement? Over how long? And what happens if I repay early or miss a payment? Those cover cost, duration and the parts of the contract that only appear under stress.

Where a lender cannot or will not produce a total repayable, that is itself informative. Where two arrangements have different terms, the totals are not directly comparable either, and the sensible step is to work out what each costs per year of borrowing.

Rules on disclosure, permitted charges and standardised comparison figures differ enormously between countries and are revised regularly, so nothing here should be read as a description of what you are entitled to see. Anyone weighing borrowing of any size should have the actual documents reviewed by a regulated adviser rather than relying on a general explanation.