How Student Loan Interest Is Calculated — Day by Day

The Daily Interest Formula

Student loan interest in the UK is calculated on a daily basis. The Student Loans Company (SLC) uses the following formula every single day to determine how much interest to add to your outstanding balance:

Daily interest = Outstanding balance × (Annual interest rate ÷ 365)

This is not an approximation — it is literally how the SLC's systems work. Each day, the system takes your balance at the start of that day, multiplies it by the daily rate (the annual rate divided by 365), and adds the result to your balance. The next day, the calculation runs again, but now the balance includes the previous day's interest. This is daily compounding in action.

Let us make this concrete with numbers. Suppose your outstanding balance is £50,000 and your annual interest rate is 6.2% (the current maximum for Plan 2 higher earners). The daily interest would be:

£50,000 × (0.062 ÷ 365) = £50,000 × 0.00017 = £8.49 per day

That means roughly £8.49 of interest is added to your balance every single day. Over a month, that accumulates to around £255 in interest. Over a year, it comes to approximately £3,100 — but actually slightly more than that because of compounding, as we will see.

Understanding the Compound Effect

The critical feature of daily compounding is that each day's interest calculation uses the balance including all previously accrued interest. This means you effectively pay interest on interest. The difference between simple interest and compound interest may seem small on a daily basis, but it accumulates over months and years.

Consider a simplified example over just 30 days with a £50,000 balance at 6.2% annual interest:

Day-by-Day Worked Example

Day 1: Balance = £50,000.00. Daily interest = £50,000.00 × 0.00017 = £8.49. New balance = £50,008.49.

Day 2: Balance = £50,008.49. Daily interest = £50,008.49 × 0.00017 = £8.49. New balance = £50,016.99.

Day 3: Balance = £50,016.99. Daily interest = £50,016.99 × 0.00017 = £8.49. New balance = £50,025.49.

Day 5: Balance = £50,033.99. Daily interest = £8.50. New balance = £50,042.48.

Day 10: Balance = £50,084.98. Daily interest = £8.50. New balance = £50,093.48.

Day 15: Balance = £50,127.50. Daily interest = £8.51. New balance = £50,136.01.

Day 20: Balance = £50,170.04. Daily interest = £8.52. New balance = £50,178.56.

Day 25: Balance = £50,212.60. Daily interest = £8.53. New balance = £50,221.12.

Day 30: Balance = £50,255.17. Daily interest = £8.53. New balance = £50,263.70.

After 30 days, the balance has grown from £50,000.00 to £50,255.17 — an increase of £255.17. If interest were calculated as simple interest (no compounding), the increase would have been exactly £254.79 (30 × £8.49). The compounding effect adds an extra £0.38 over just one month. That might seem trivial, but over a full year the compound effect becomes more significant, and over 25 or 30 years it meaningfully increases the total interest charged.

Using the compound interest formula for a full year: £50,000 × (1 + 0.062/365)^365 = £50,000 × 1.06395 = £53,197. Simple interest would give £50,000 × 1.062 = £53,100. The compounding adds approximately £97 extra interest over a year on a £50,000 balance. On higher balances and over longer periods, this difference is more pronounced.

How Repayments Reduce the Daily Calculation

When a repayment is received by the SLC, it reduces the outstanding balance on the date it is applied. From that point forward, the daily interest calculation uses the lower balance. This is why repayments save you money on interest — not just pound for pound on the principal, but also by reducing all future daily interest accruals.

For PAYE employees, repayments are deducted from each pay period (typically monthly) by your employer, passed to HMRC, and then forwarded to the SLC. There is a processing delay — your employer deducts the repayment from your January salary, but it may not reach the SLC and be applied to your balance until February or even March. During that delay, interest continues to accrue on the higher balance.

Suppose your balance is £50,000 on 1 January and you make a repayment of £150 that is credited on 1 February. For the 31 days in January, interest accrues on £50,000. From 1 February, interest accrues on £49,850. The daily interest drops from £8.49 to £8.47. That two-pence daily saving compounds over the following months and years, gradually widening the gap between your actual balance and what it would have been without the repayment. Over the 30-year life of a Plan 2 loan, even small monthly repayments produce meaningful cumulative interest savings through this mechanism.

Interest During Your Course

One of the most important and least intuitive aspects of student loan interest is that it begins accruing from the day each loan instalment is disbursed — not from the day you graduate. For a student starting a three-year undergraduate degree in September 2025, the timeline looks like this:

• October 2025: The first tuition fee instalment (approximately £9,535) and first maintenance loan instalment are paid out. Interest begins accruing on this amount immediately.
• January 2026: The second maintenance instalment is paid. Interest now accrues on the cumulative total of all disbursements so far.
• April 2026: The third maintenance instalment is paid. The balance grows further.
• October 2026 onwards: Second-year disbursements begin. Interest is now accruing on first-year plus second-year borrowing.

By graduation after three years, the first year's borrowing has accumulated approximately three full years of interest, the second year's borrowing has two years, and the final year's has one year. For a Plan 2 borrower, the interest rate during study is the maximum rate (RPI + 3%), currently around 6.2%.

Let us estimate the impact. If total borrowing over three years is £55,000 (tuition and maintenance combined), the graduated average period of interest accrual is roughly two years at 6.2%. Total interest during study is approximately £55,000 × 0.062 × 2 = £6,820 (simplified). This means a student who borrowed £55,000 may owe around £62,000 by graduation. The actual figure depends on the precise disbursement dates and rates, but this gives a reasonable approximation.

The RPI Link — Why Rates Change

Student loan interest rates in the UK are linked to the Retail Prices Index (RPI), a measure of inflation published monthly by the Office for National Statistics. The government uses the March RPI figure each year to set student loan interest rates for the following academic year (September to August). This means the interest rate changes once per year, in September, based on the previous March's inflation data.

For different loan plans, the RPI link works differently:

Plan 1 (Pre-2012 England/Wales, Northern Ireland)

Interest is the lower of RPI or the Bank of England base rate plus 1%. In 2026/27, this is approximately 3.2%. The "lower of" mechanism provides a natural cap, meaning Plan 1 interest rarely exceeds 4% or so.

Plan 2 (Post-2012 England/Wales)

Interest is on a sliding scale linked to income:

• Income below £29,385: interest = RPI (approximately 3.2%).
• Income between £29,385 and £52,885: interest slides from RPI to RPI + 3%.
• Income above £52,885: interest = RPI + 3% (approximately 6.2%).
• While studying: interest = RPI + 3% (the maximum rate).

The income-linked sliding scale for Plan 2 is a unique feature. It means that higher earners face a steeper effective interest rate, while lower earners have their interest burden somewhat reduced. The logic is that higher earners are more likely to repay in full and therefore benefit more from a lower interest rate, whereas lower earners are more likely to have their balance written off, making the interest rate less consequential for them.

Plan 4 (Scotland)

Interest works the same way as Plan 1: the lower of RPI or base rate + 1%. Currently approximately 3.2%.

Plan 5 (Post-2023 England)

Interest is capped at RPI only, with no RPI + 3% element regardless of income. This is a significant change from Plan 2 and means Plan 5 borrowers face substantially lower interest rates. At an RPI of 3.2%, a Plan 5 balance of £55,000 accrues about £1,760 per year in interest, compared with up to £4,015 for the same balance on Plan 2 at 6.2%.

Postgraduate Loan

Interest is RPI + 3% for all borrowers (approximately 6.2%), regardless of income. There is no sliding scale — the highest rate applies to everyone.

The Cap Mechanism

The government retains the power to cap student loan interest rates if RPI produces unusually high figures. This was exercised in 2022/23 when RPI briefly exceeded 13%, which would have resulted in Plan 2 interest rates above 16% (RPI + 3%). The government intervened and capped rates at a lower level to avoid politically unacceptable headline figures.

The cap mechanism is not written into the original loan legislation as a permanent feature — it is a discretionary measure that the government applies on an ad hoc basis. This means there is no guaranteed maximum rate, and future governments could choose not to apply a cap. However, in practice, political pressure makes it extremely unlikely that rates would ever be allowed to exceed levels that the public considers reasonable. The cap is effectively a political safety valve rather than a contractual protection.

Why Most People's Balance Goes Up After Graduation

A common source of frustration among graduates is watching their balance increase in the first years after they start working, even though they are making regular repayments. The reason is straightforward arithmetic: if annual interest exceeds annual repayments, the balance grows.

Consider a graduate with a £55,000 Plan 2 balance earning £32,000. Their annual repayment is 9% of (£32,000 − £29,385) = 9% of £2,615 = £235 per year. Meanwhile, at a blended interest rate of around 3.54% (for this income level), annual interest is £55,000 × 0.0354 = £1,947. The balance therefore grows by £1,947 − £235 = £1,712 in that year. The graduate is faithfully making their repayments, but the balance increases from £55,000 to £56,712.

This pattern continues until the graduate's salary (and therefore repayments) rises enough to exceed annual interest charges. For someone on Plan 2 with a typical starting salary, this crossover point often does not arrive until the salary reaches £45,000 to £55,000 or more, depending on the balance and the prevailing interest rate. For many graduates, particularly those in lower-paid professions, the crossover may never arrive, and the balance continues to grow until it is written off after 30 years.

This is not a design flaw — it is an intentional feature of the income-contingent system. The loans are designed so that only higher earners repay in full, while moderate earners make affordable repayments and have the rest written off. The balance figure is therefore misleading if treated as a traditional debt. What matters is not the balance but the monthly deduction from your salary and the total you will pay over the life of the loan. Use our student loan repayment calculator to see your projected total repayment. For current balance information, see our guide on how to check your student loan balance.

Voluntary Overpayments and Interest

Making voluntary overpayments reduces your balance and therefore reduces future daily interest charges. However, whether overpayments save you money overall depends on whether you would have repaid the loan in full during the repayment term anyway. If your balance is going to be written off at the end of the term, then any voluntary overpayments are essentially wasted — you are reducing a balance that would have been forgiven, rather than saving money.

Overpayments only make financial sense if you are on track to repay in full. In that case, reducing the balance early saves compound interest over the remaining years. The people most likely to benefit from overpayments are Plan 1 borrowers with relatively small balances and high salaries — they were always going to repay in full, and overpaying saves them interest. For most Plan 2 borrowers with balances above £40,000 and salaries below £50,000, overpayments are unlikely to be beneficial.

Before making voluntary overpayments, model your scenario carefully. Our calculator shows whether you are projected to repay in full or have a portion written off. For teachers, our teacher-specific guide covers how the Teachers' Pension Scheme affects the calculation. For PhD graduates, see our guide on student loans during a PhD. And for a clear picture of total borrowing amounts, read how much student loan you can get.

Key Takeaways

Student loan interest is calculated daily, compounds on itself, and begins accruing from the first disbursement — not from graduation. The rate is linked to RPI and varies by plan type and, for Plan 2, by income. Most graduates see their balance rise in the early years of employment because repayments are smaller than interest charges. This is normal and does not mean the system is broken; it is an inherent feature of an income-contingent loan designed to be affordable for borrowers at every salary level. The balance is written off after 25 to 40 years, and for the majority of borrowers, the relevant figure is not the balance but the total repayments made over their career.