Daylila
How your money works

Lesson 5 of 13

The debt spiral

Explain why high-interest debt is compounding pointed in reverse — when a payment barely covers the interest, the balance hardly moves — compute roughly how long a minimum-only payment takes to clear a balance, and judge which debt to kill first (highest rate).

01 · Learn · the idea

A £2,000 balance sits on a credit card at 24% a year. You pay £45 every month — more than the minimum, you’re being careful. A year passes. You check the balance, expecting it to be well down. It reads about £1,940. Twelve payments, £540 handed over, and the debt has barely flinched. Sixty pounds gone in a year. Something is eating almost everything you pay, and it isn’t the shop you bought from. It’s the interest — and it’s working exactly the way compounding works, only pointed at you.

The engine in reverse

In the last module you met compounding: gains that earn gains, the snowball that grows faster the bigger it gets. That same engine doesn’t care who it works for. Lend money and the interest builds for you. Owe money and the interest builds for the lender — against you. Same machine, opposite direction.

24% a year is 2% a month. On a £2,000 balance, that first month the card adds 2% of £2,000 — £40 — before you’ve paid a penny. So the balance you actually carry isn’t £2,000. It’s £2,040, and then your payment lands.

Pay £45, and only £5 of it touches the debt. The other £40 just covers the rent on money you already owe. Next month the card charges 2% again, on a balance that’s hardly moved, and adds about £40 once more. You pay £45, £5 comes off. The debt shrinks by the price of a coffee while you hand over a meal.

When the payment barely beats the interest

This is the spiral, and it has a clean shape. Every month the balance gets multiplied by 1.02 — that’s the 2% interest landing — and then your payment is subtracted. If your payment is much bigger than the interest, the balance falls fast. If it’s barely bigger, almost nothing comes off, and the interest keeps reloading on a balance that won’t go down.

Watch what the size of the payment does to the same £2,000 debt.

  • £45 a month — only £5 above the interest. It takes more than nine years to clear. Over that time you pay roughly £2,993 in interest — more than the original debt. You buy the £2,000 thing twice and keep one.
  • £100 a month. Now you’re clearing £60 of real debt after the interest. Gone in 26 months — just over two years — for about £580 in interest.
  • £200 a month. Cleared in 12 months, about £254 in interest.

Look at the jump from £45 to £100. You didn’t double the pain — you slightly more than doubled the monthly payment, and the time collapsed from nine years to two, the interest from £2,993 to £580. A little more each month doesn’t help a little. It helps enormously, because every extra pound goes straight at the balance and shuts the compounding down faster.

The line where it never clears

There’s a floor, and it’s worth knowing exactly where it is. The interest on £2,000 at 2% a month is £40. Pay £40, and you cover the interest and nothing else — the balance ends each month right back where it started. Pay anything below £40, and the balance actually grows even as you pay. You are renting the debt forever, and the rent goes up.

This is why the minimum payment on a card is a trap dressed as a kindness. Minimums are often set as a small percentage of the balance — just above the interest line, by design. They keep you technically paying, legally fine, and financially stuck. The card issuer is happiest when you pay near the floor, because that’s when the interest keeps flowing for the longest.

Which debt to kill first

Most people don’t have one debt. They have a few — a card at 24%, a store card at 30%, a car loan at 9%. Money is tight, so you can put a bit extra toward one of them. Which?

The rule is simple and it falls straight out of everything above: attack the highest interest rate first. Pay the minimum on everything to stay current, then throw every spare pound at the debt with the steepest rate. That’s where compounding is running fastest against you, so that’s where each pound you kill does the most damage to the spiral. The 30% store card is bleeding you faster than the 9% car loan — clearing it saves more than clearing a bigger balance that charges less.

This is sometimes called the avalanche: knock down the steepest slope first. It clears your debts for the least total interest. People debate it against paying the smallest balance first for the motivation of an early win — and if you genuinely need that win to keep going, take it. But the maths is not in doubt. Highest rate first costs you the least.

On the whole

High-interest debt is the most urgent thing in personal money, and now you can see why. It is compounding at full speed in the wrong direction — the same force that quietly builds a pension over decades, turned around to dig a hole just as fast.

That also makes clearing it the surest thing you’ll ever do with money. Paying off a 24% debt is a guaranteed 24% return — no risk, no market, no luck, just the interest you stop paying. No investment on earth offers that safely. The engine is neutral; it only knows time and direction. The whole of money is learning which way you’ve pointed it — and a debt spiral is what it looks like when the most powerful force you have is working against the person who owns it.

02 · Try · the lab

03 · Check · quick quiz

1. A £2,000 card charges 2% a month, so £40 of interest lands before your payment. You pay £45 a month. Why does the balance barely move?

  • The card is faulty — £45 should clear £2,000 in about four years
  • Only £5 of your £45 actually touches the balance; the other £40 just covers the month's interest
  • Credit cards don't apply interest until you miss a payment
  • Because £45 is below the minimum the card requires
Answer

Only £5 of your £45 actually touches the balance; the other £40 just covers the month's interest — At £45 you beat the £40 interest by only £5, so just £5 comes off the debt each month. The rest is rent on money you already owe — that thin margin is what makes the spiral. Push the payment up and far more goes at the balance.

2. On the same £2,000 card, the monthly interest is £40. What happens if you pay exactly £40 a month, every month?

  • The balance never goes down — you cover the interest and nothing else, renting the debt forever
  • The debt clears slowly — it just takes many years
  • The balance falls by £40 a month, the same as the interest
  • The card waives the interest because you're keeping up
Answer

The balance never goes down — you cover the interest and nothing else, renting the debt forever — Paying exactly the interest leaves the balance back where it started each month. Pay below £40 and it actually grows. Minimum payments are often set just above this floor by design — enough to keep you paying, not enough to free you.

3. You have three debts and a little spare cash each month: a store card at 30%, a credit card at 24%, and a car loan at 9%. Where should the spare money go?

  • The car loan — it's usually the biggest balance, so clearing it saves the most
  • Split it evenly across all three so each shrinks a bit
  • Whichever debt has the smallest balance, to feel a quick win
  • The store card at 30% — pay the minimums on the rest and throw everything spare at the steepest rate
Answer

The store card at 30% — pay the minimums on the rest and throw everything spare at the steepest rate — Compounding runs fastest against you on the highest rate, so every spare pound does the most damage to the spiral there. Attack the steepest rate first (the avalanche) and you clear the debts for the least total interest — the rate matters more than the balance size.

4. On the £2,000 card, paying £45 a month takes about 9 years and costs roughly £2,993 in interest. Bumping to £100 a month clears it in about 2 years for roughly £580. Why does a bit more each month help so much?

  • Paying faster makes the card lower your interest rate as a reward
  • After the £40 interest, every extra pound goes straight at the balance — so it shrinks faster and there's less left to compound against
  • It doesn't really; the totals are about the same either way
  • £100 is below the level where interest applies
Answer

After the £40 interest, every extra pound goes straight at the balance — so it shrinks faster and there's less left to compound against — At £45 only £5 attacks the debt; at £100 it's £60 — twelve times as much hitting the balance. A smaller balance compounds less interest, which shrinks it faster still. That's why slightly more each month collapses both the time and the interest, not just a little.