Loan interest explained: why the same rate can cost double

The distinction that matters most

Before comparing any two loans, you need to know whether each rate is flat or reducing balance. Nothing else you compare means anything until you do.

Reducing balance means interest is charged on what you currently owe. As you repay, the balance falls, and the interest charged each period falls with it. This is how most mortgages and standard personal loans work.

Flat rate means interest is charged on the original amount for the entire term, regardless of how much you have repaid. You could be one payment from clearing the loan and still be charged interest as though you owed the whole thing.

The consequence is large. Because you owe on average roughly half the original amount over the life of the loan, a flat rate produces something in the region of double the effective cost of the same number quoted as reducing balance. A "6 percent" flat-rate loan is not competitive with a 6 percent reducing-balance loan — it is roughly comparable to a much higher one.

How to tell which you are being offered

It is not always stated prominently, and the language varies by market. Practical ways to find out:

1. Ask directly: "is this rate flat or reducing balance?" A lender who cannot answer clearly is telling you something.
2. Check whether an APR or equivalent standardised rate is disclosed alongside the headline number. A large gap between the two is a strong signal of a flat rate or heavy fees.
3. Do the arithmetic yourself. Multiply the payment by the number of instalments to get total payable, subtract the principal, and you have total interest. Compare that against the principal and the term.
4. Look for the phrase "interest on the original amount" or similar in the terms.

That third one is the reliable check because it does not depend on anyone's terminology. Total payable minus principal is total interest, and that number cannot be dressed up.

Nominal rate versus APR

A second layer of confusion: even among reducing-balance loans, the quoted rate may exclude costs you will definitely pay.

A nominal or headline rate is the interest rate alone. An APR — or the equivalent standardised measure in your jurisdiction — is designed to include mandatory fees and express the total cost as an annualised percentage, which makes offers comparable.

Things that often sit outside the headline rate but inside the real cost: processing or origination fees, mandatory insurance, documentation charges, and account maintenance fees. A processing fee deducted from the disbursement is particularly worth noticing, because you receive less than the principal you are paying interest on.

Exactly what must be included in a standardised rate varies by country. The general rule holds regardless: compare standardised rates where they exist, and compare total payable where they do not.

Compounding frequency

How often interest is calculated affects the real cost, though usually less dramatically than flat-versus-reducing.

Interest compounded daily costs more than the same nominal rate compounded monthly, which costs more than annually. For most instalment loans the effect is modest. For revolving credit — particularly credit cards, where daily compounding is common — it is meaningful, and it is one reason card debt grows faster than people expect.

The practical takeaway is not to compute compounding yourself, but to treat it as another reason the headline rate is insufficient. Standardised rates account for it; headline rates may not.

Simple interest and where it appears

Simple interest is charged on the principal only, with no interest charged on accumulated interest. It appears on some short-term products and in informal lending.

For a single-payment short-term loan this is genuinely simpler and often cheaper than a compounding equivalent. For anything longer, the distinction usually matters less than flat-versus-reducing does.

Where simple interest is worth knowing about is informal loans between people. If you lend money to a family member with an agreed simple interest arrangement, that is a perfectly reasonable structure and much easier to compute and verify than anything compounding.

Comparing two offers properly

A procedure that works regardless of how the offers are presented:

1. For each offer, get the payment amount and the number of payments. Multiply them. That is total payable.
2. Add any fee deducted up front or paid separately, since it is real money you spend.
3. Subtract the amount you actually receive. That is the true cost of borrowing.
4. Compare those costs directly. If terms differ in length, also compare cost per unit of time borrowed.
5. Check prepayment terms. A slightly more expensive loan you can clear early without penalty may cost less in practice.

This works because it never relies on anyone's rate terminology. Money out minus money in is not something marketing can reframe.

Why your ledger should record the split

Once a loan is running, the thing to track is how much of each payment goes to interest — because that is the number that tells you what the loan is actually costing you as it runs.

expenie asks you to enter the principal, interest, and charges split when you record a payment rather than generating a schedule. With flat-rate loans, bundled insurance, and lenders whose allocation differs from textbook amortisation, a generated schedule would look authoritative and be steadily wrong.

Entering the real split has a second benefit: it makes the cost visible every single month. A loan where two thirds of each payment is interest is a very different object from one where two thirds is principal, and seeing that repeatedly changes how you think about extra payments and refinancing.

The derived figures follow from it. Still-to-pay minus remaining principal gives interest left, which is the clearest single measure of what the debt will cost you from here.