How Student Loan Interest Is Calculated, and Why Your Balance Can Grow Even While You're Repaying
Making monthly repayments doesn't always mean your student loan balance is shrinking, and understanding why comes down to how interest is added.
Why this catches people out
Many graduates check their student loan balance after a year or two of steady repayments and find it hasn’t gone down much, or has even gone up. This isn’t a mistake. It’s how the system is designed to work. Student loan interest is added continuously, and for a large chunk of borrowers, the amount charged in interest each year can outpace what’s being repaid through their salary. Understanding the mechanics helps you judge whether extra repayments would actually help you, or whether the loan is likely to be written off before it matters.
Interest builds daily, not annually
Interest on a student loan is calculated daily and applied to your outstanding balance, then compounded, so you’re effectively charged interest on interest that’s already been added. The daily rate is worked out from an annual rate set by government rules, and that annual rate is reviewed regularly and can change during the year depending on the plan type you’re on and, in some cases, your income.
Because the exact percentage moves and differs by plan (broadly, older Plan 1 loans use a lower, more stable rate linked to bank base rates or similar comparisons, while Plan 2 and Plan 5 loans use a rate linked to inflation and can vary further with income), you should never rely on a fixed figure you’ve seen quoted elsewhere. The Student Loans Company and gov.uk publish the current rates for each plan type, and these are the only figures worth trusting at any given time.
Why repayments don’t always beat interest
Your monthly repayment through PAYE is based on how much you earn above a repayment threshold, not on how much interest has accrued. If you’re on a lower or middle income, the interest added to your balance over a year can be larger than the total you’ve repaid, so your balance grows in cash terms even as you make every payment on time. This is most noticeable early in a career, when salaries and repayment amounts are lower but the loan balance (and therefore the interest charged on it) is at its highest.
As income rises, monthly repayments increase and can start to outpace interest, so the balance may plateau and then fall. For many borrowers, particularly on Plan 2, the loan is never fully repaid through this process and is instead written off after a set number of years from when the loan first became repayable. This is a deliberate feature of the system, not a shortfall.
Does the growing balance actually cost you anything?
This is the crucial question, and the honest answer is: it depends on your income trajectory. If you’re likely to clear the loan naturally through repayments before the write-off date, then a period of the balance rising doesn’t cost you anything extra, because what you eventually pay back is capped by your income and the loan’s term, not by the total balance shown on your statement. If you’re a high earner likely to repay the loan in full well before write-off, then the interest genuinely does matter, because a bigger balance early on means more total interest paid over the life of the loan, and voluntary overpayments could save you money.
This is why financial guidance on student loans tends to focus less on the headline balance and more on your realistic future income. Someone expecting steady, high earnings has a different calculation to make than someone whose income is likely to stay modest.
How to check your own position
Your annual statement from the Student Loans Company shows the interest rate applied, the amount of interest added over the year, and your total repayments. Comparing those two figures for a full year tells you immediately whether your balance is likely to be rising or falling in real terms. You can also log into your online account at any time to see the current balance and the interest rate currently being applied to your plan type.
Before making any decision about voluntary overpayments, it’s worth checking the current interest rates and repayment thresholds directly with the Student Loans Company or gov.uk, since these change and any figure printed here could be out of date by the time you read it. MoneyHelper also offers free, independent guidance on weighing up whether overpaying a student loan makes sense for your circumstances, which is different advice for almost every income level and plan type.
The bottom line
A rising balance on a student loan statement usually reflects daily compounding interest outpacing income-based repayments, not a fault in your payments. Whether that matters to you personally depends entirely on whether you expect to clear the loan before it’s written off, so the balance figure alone tells you far less than comparing it with your likely future earnings.