# Interest-free instalments: why they wreck your month even when they cost nothing

> The problem with instalments isn't the interest: today's purchase lands on a statement months from now. How to log them so the statement never shocks you.

_Debt · Published 2026-09-02_

Buying in interest-free instalments looks free, and in interest terms it is. The
problem is somewhere else, and it's one of personal bookkeeping: **you make the
purchase today and you see it on your statement for the next six months**. If you
log the expense on the day you buy, that month is inflated and the next five look
light. If you don't log it, it only shows up on the statement, and by then it's
too late.

Neither way lets you know how much you have left this month, which is the only
question that matters.

## Why they disrupt your month even when they cost nothing

Three reasons, and none of them is the rate:

**They stack up invisibly.** One purchase in six instalments is easy to hold in
your head. Four purchases in six instalments, made in different months, is not: by
month four you're paying pieces of four purchases you no longer remember.

**They commit months that don't exist yet.** March's income is already partly
spoken for, and that part was decided in October.

**They change the buying decision.** "It's 12 payments of 20" gets compared
against your month; "it's 240" gets compared against what the thing is worth. Same
price, different feeling — and the effect is measurable: instalments lower
resistance to buying.

## The way that works: log the instalment, not the purchase

When you buy something in N instalments, log it as a **recurring expense lasting N
months** at the instalment amount, starting on the statement where it actually
appears.

That way:

- Your month shows what you're paying this month, which is what you need to know.
- The coming months already have the instalment inside them before they arrive.
- When the last instalment is paid, the expense disappears on its own.

The second point is the important one. With instalments logged forward, the
question "can I buy this in instalments?" stops being "does the payment fit this
month?" and becomes "does it fit across the six months that already have
instalments in them?" — which is the correct question.

## The number to watch: how much is already committed

Add up every instalment you'll pay next month, before buying anything new. That's
your **card commitment**, and it's money you no longer have.

A practical rule that works: **if total instalments exceed 20% of your monthly
income, no new instalment plans until it comes down.** It isn't a sacred number;
it's a brake that exists before the temptation, which is when brakes are useful.

## When interest-free instalments are genuinely worth it

They aren't a trap. Where prices are rising, paying in fixed interest-free
instalments for something you were going to buy anyway is literally paying less in
real terms. Two conditions:

1. **You were going to buy it anyway.** If the instalment is what convinced you,
   it wasn't a purchase — it was financing.
2. **The instalment fits in the month alongside all your other instalments.** Not
   on its own.

If both hold, go ahead. If the first one fails, 0% interest doesn't fix having
bought something you didn't need.

## What not to do

**Paying the card minimum.** That's where the interest shows up, and it's the most
expensive you'll ever pay. If you're paying minimums, the issue isn't instalments
any more, it's debt — covered in
[how to get out of debt](/en/blog/how-to-get-out-of-debt).

**Refinancing a statement to "relieve" one month.** It converts interest-free
instalments into debt with interest.

**Treating foreign-currency instalments as fixed.** If the purchase was in another
currency, each instalment can land at a different number — the reason is in
[tracking two currencies](/en/blog/tracking-expenses-in-two-currencies).

## In one line

Log the instalment, not the purchase, and put it forward into the months where it
will appear. Check how much of next month is already committed before taking on
new instalments, and use them only for things you were going to buy anyway.

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- iPhone (App Store): https://apps.apple.com/app/id6758053806
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