The Measuring Stick That Doesn’t Measure Straight

There is a number buried inside every UK student loan that has never, in any government leaflet, had a spotlight turned on it. Not the interest rate itself – you might know that one, or at least have vaguely read it. The hidden thing is the tool used to calculate it: a piece of statistical machinery called the Retail Price Index, or RPI. It does not sound like something to get angry about. That is, genuinely, part of the point.

The Instrument That Got Demoted (But Kept Its Job)

RPI is one of several ways the UK measures inflation – how fast the prices of everyday things are rising. For decades it was the standard tool. Then, in 2019, the UK Statistics Authority formally declared it flawed. Their conclusion: the methodology had a known bias causing it to overstate inflation compared to more modern measures. It was stripped of its designation as a National Statistic. The government's own statisticians, in plain language, said: this one gets it wrong.

The replacement – CPIH, which includes housing costs and uses better averaging methods – tends to run about 0.5 to 1 percentage point lower than RPI in most years. That sounds like a rounding error. On a student loan balance of £50,000, compounding over decades, it is not.

Why are student loans still tied to RPI?

The current student loan structure was remodelled in 2012, when tuition fees rose to £9,000 a year. At that point, someone chose RPI as the inflation base for interest calculations. It was already a contested measure then; the arguments about its accuracy were not new. But it was embedded anyway, and it has stayed embedded ever since, surviving the formal downgrade in 2019 without so much as a tweak.

The interest rate on a Plan 2 student loan (the one most graduates from English universities since 2012 carry) is currently RPI plus up to three percentage points while you are studying, and RPI plus zero to three points after you graduate, depending on income. The RPI component is not a minor garnish – it is the foundation.

For a parallel example of policy quietly designed to work on people rather than explain itself to them, visit The Government That Gave Up Trying to Persuade You at https://savingourplanet.co.uk/the-government-that-gave-up-trying-to-persuade-you/.

Does the interest rate affect what most graduates repay?

For most graduates, this produces an odd phenomenon: the loan balance rises for years after they start repaying it. You send money in. The balance goes up. Not because of bad luck or missed payments – just because the interest, anchored to a measure the statisticians themselves called defective, outpaces what a typical salary can chip off.

The government's stock response is that most borrowers will never fully repay anyway (the debt is written off after 40 years on Plan 2), so the interest rate is largely notional. Which is true for some borrowers. But for higher earners who do repay in full, the RPI attachment costs them real money over the life of the loan – and the choice of instrument, made quietly in 2012 and never reversed, is what does the work.

The Reason It Matters That Nobody Told You

A flawed ruler, kept in use because it measures long, built into a debt that 1.9 million people are currently repaying. The interesting thing is not the injustice of it, though you can argue that too. The interesting thing is how unremarkable it seems – just a technical detail in a financial product you had no choice but to take, expressed in an acronym you were never asked to look up.

RPI is not a conspiracy. It is just an old, convenient tool that happens to benefit the lender, inherited from a previous design, never examined by most of the people it shapes.