The difference is the interest basis — the rule governing how that percentage is applied. It is the single most important thing to understand about the price of credit, and the thing borrowers are least often told. This guide explains the difference, and gives you the one question that makes it irrelevant.
Flat interest
Flat interest applies the percentage once, to the amount you originally borrowed, for the whole term. It does not care that you have been steadily repaying.
Borrow $1,000 at 15% flat and the interest is $150, whether you repay evenly across the term or in one payment at the end. The charge is fixed at the start.
Flat rates are simple to understand and simple to calculate, which is why smaller lenders often use them. They also mean you pay interest on money you have already given back.
Reducing balance interest
Reducing balance applies the rate to what is still outstanding. As you repay, the balance falls, so each period’s interest charge falls with it.
The same 15% on a reducing balance costs less in total than 15% flat over the same term, because the later months are charged on a smaller sum. This is how most bank lending works.
It is more complex to calculate — which is precisely why the schedule matters more than the percentage.
Where APR fits in
APR exists precisely to solve this problem. It expresses the total annual cost of a loan — interest plus compulsory fees — as one standardised figure, so a three-month loan and a twelve-month loan can be compared honestly.
It has a known distortion on very short loans: annualising a small charge over a short period produces a large-looking number. A modest fee on a one-month loan can read as a triple-digit APR without the loan being unreasonable. Use APR to compare lenders, not to judge a single loan in isolation.
The question that makes all of this unnecessary
You do not need to master interest mathematics to protect yourself. You need one document.
"Can I see the full repayment schedule before I sign?" A schedule lists every instalment, its exact amount and its due date. Add them up and you have the total repayable — a real number, requiring no interpretation, that no percentage can hide behind.
A lender who cannot or will not produce one before you commit has told you something more useful than any rate.
The term lever most borrowers miss
Whatever the basis, term does more to the total cost than most borrowers realise. A longer term produces a smaller instalment and a larger total.
The instinct is to take the longest term because the monthly figure looks manageable. The better discipline is the shortest term you can comfortably service — then test it against whether you could still pay if income arrived a month late.