Interest on a UK student loan can feel alarming once you notice your outstanding balance growing every year, even while you’re making regular repayments. In most cases this is completely normal and not a sign anything has gone wrong. Understanding how the interest rate is actually calculated explains why, and helps separate genuine financial concerns from the ordinary mechanics of the system.
This guide explains how student loan interest works for each repayment plan, why your balance can rise even as you repay, and what it actually means for you.
What Determines Your Interest Rate?
Student loan interest is based on RPI, the Retail Prices Index, a measure of inflation that tends to run slightly higher than the more commonly quoted CPI figure. Depending on your plan, an additional percentage may be added on top of RPI, and in some cases the rate also depends on your income once you start repaying.
Interest Rates by Plan
| Plan | While Studying | After Graduating |
|---|---|---|
| Plan 1 | Lower of RPI or Bank of England base rate plus 1% | Same formula throughout |
| Plan 2 | RPI plus 3% (capped) | Sliding scale from RPI only up to RPI plus 3%, based on income |
| Plan 4 | Lower of RPI or base rate plus 1% | Same formula throughout |
| Plan 5 | RPI only | RPI only, regardless of income |
| Postgraduate Loan | RPI plus 3% | Sliding scale, similar to Plan 2 |
Because RPI itself moves with inflation, the actual percentage rate changes each year on 1 September, so your rate today may be noticeably different from a figure you saw a year or two ago.
How the Plan 2 Sliding Scale Works
Plan 2 and Postgraduate Loan interest rates are not fixed once you graduate, they scale with your income between two thresholds:
- At or below the lower threshold, interest is charged at RPI only, the lowest rate available.
- Between the lower and upper thresholds, interest rises proportionally, calculated based on exactly where your income sits between the two figures.
- At or above the upper threshold, interest is charged at the maximum rate, RPI plus 3%, capped at a government-set ceiling.
This means two Plan 2 borrowers with different salaries can be charged noticeably different interest rates on the same outstanding balance.
Why Your Balance Can Grow Even While You Repay
For many graduates, particularly those on Plan 2 and Plan 5 in the early years of their career, the interest added each year exceeds the amount deducted through repayments. This means the outstanding balance can genuinely increase over time, even though repayments are being made consistently and correctly.
This is expected behaviour built into the system, not a sign of a problem. What actually matters is not the size of the headline balance, but whether you are likely to repay it in full before your plan’s write-off date. For most Plan 2 borrowers, current estimates suggest a majority will not repay in full before their loan is written off, at which point the remaining balance, however large, is simply cancelled.
Does Interest Actually Affect What You Repay Monthly?
No. This is one of the most important things to understand about UK student loans. Interest affects only your total outstanding balance, not your monthly repayment amount. What you repay each month is based purely on your income and your specific plan’s threshold and rate, calculated the same way regardless of how much interest has accrued.
Frequently Asked Questions
Why is my student loan balance going up instead of down?
This happens when the interest added each year is higher than your repayments, which is common in the early years of a graduate’s career, particularly on Plan 2 and Plan 5, and is not a sign of a problem in itself.
Does a higher interest rate mean I repay more each month?
No. Interest only affects your total outstanding balance, not your monthly repayment, which is calculated purely from your income above your plan’s threshold.
Why does Plan 5 have a different interest rate to Plan 2?
Plan 5 charges RPI only with no additional income-based margin, while Plan 2 can charge up to RPI plus 3% depending on income, though Plan 5 has a longer 40-year write-off period to offset this.
When does the interest rate change?
Student loan interest rates are reviewed and can change on 1 September each year, based on the latest published RPI figure.
Student loan interest can look concerning on paper, but for most borrowers it simply reflects how the system is designed to work rather than a genuine cause for concern. For the fuller picture on thresholds, write-off dates and whether overpaying makes sense, our guide to student loan repayment covers that in detail.
