Payment allocation decides which balance you actually reduced

There is a short answer about payment allocation and a useful one, and they are not the same. What follows is the useful one.

One card, several debts

A card that has been used for a balance transfer and then for purchases holds at least two balances charged at different rates. Add a cash advance and there are three, each accruing separately while showing as one total. The statement usually breaks this down, and most people read only the headline balance.

Understanding which balance your payment touched is the whole of the problem.

The rule in many regimes

Regulators in a number of countries require issuers to allocate payments above the contractual minimum to the highest-rate balance first. That reform replaced an older practice of clearing the cheapest balance first, which trapped expensive debt indefinitely.

The useful part is this: whether it applies to you depends on where the card is issued, so check your own rules rather than assuming. Where it does apply, it only bites on the amount above the minimum.

Why the minimum is the loophole

If you pay exactly the minimum, there is nothing above it to allocate favourably. A promotional transfer balance at zero per cent alongside purchases at the standard rate therefore sees the purchases persist. The interest on those purchases accrues monthly and compounds while the promotional balance sits untouched.

For most people, this is the mechanism by which a transfer card quietly becomes expensive.

The practical rule

Do not spend on a card that carries a promotional balance, at all, until the promotion is cleared. Keep one card for the transfer and a separate one, cleared in full monthly, for spending. If purchases have already accumulated, pay enough above the minimum to clear them entirely and then stop using the card.

The alternative is paying the standard rate on the purchases for the whole promotional period.

Reading the statement

Look for the section listing each balance type, its rate and its amount, which most issuers are required to provide. Compare that against how much you paid and how each balance moved. A balance that has not fallen despite payments tells you exactly where the money went.

If the breakdown is not there, ask the issuer for it in writing.

Promotional expiry timing

When a promotional rate ends, the remaining balance moves to the standard rate, usually the highest on the card. From that point allocation rules work in your favour again, which is small comfort.

Diary the expiry date and plan to have the balance cleared or moved before it. Serial transferring works only while applications keep being accepted, and that is not a plan.

The takeaway

One card, one job. Mixing a promotional balance with new spending is how the promotion stops working.

Small and repeatable beats ambitious and abandoned, almost every time.