Overpay the loan or invest? A framework
Figures as of · Should I overpay at all? covers the write-off question — read that one first if you haven't.
The question this answers
Assume you've already worked out, via the write-off guide, that you're on track to clear your balance in full — so a voluntary overpayment genuinely saves interest rather than just reducing an amount that would have been forgiven anyway. Given that, is putting spare money toward the loan actually the best home for it, compared with a pension or investment account instead?
A known rate vs an uncertain one
An overpayment's "return" is the interest rate you stop paying on that pound from the moment you pay it. The calculator's "Overpayments" box estimates that directly: it re-runs your whole schedule with an extra £100 a month and reports the nominal rate of return on that extra money — and it shows nothing at all when the extra payments wouldn't reduce what you'd repay, which is the write-off case this guide assumes you've already ruled out. Investing instead has no guaranteed return; it depends on markets, and can go backwards over any given stretch.
Current headline rates by plan, for reference:
- Plan 1: 4.1%
- Plan 2: 6.0% (max)
- Plan 4: 4.1%
- Plan 5: 4.1%
- Postgraduate Loan: 6.0% (max)
Plan 2 interest is a maximum rather than a fixed figure: it varies with income between RPI and RPI + 3%, and moves with RPI and the cap from one year to the next. Postgraduate Loan interest is not income-based: it follows RPI + 3%, subject to the published cap. See how student loan interest works for which rate applies to you. The comparison that matters is between that rate and a realistic, not optimistic, expectation for whatever the money would otherwise go into.
A published range to compare it against
The Money Planner runs the same comparison for debt generally, treating anything above 7.5% as high-interest debt that published guidance commonly ranks above investing, because few people expect to reliably beat it after tax. On the other side, it uses a typical long-run return range of 5–7% a year for a diversified investment — not a promise, a commonly published rule of thumb. Plan 5 charges RPI with no income-based margin, subject to the government's market-rate cap, so its current rate — 4.1% — sits below that typical long-run range, which is the kind of comparison worth redoing on whatever the current published figures are, rather than treating either side as a fixed answer.
Who this favours
Overpaying tends to make more sense the closer you already are to clearing the balance, because every pound sent in is a pound you were going to have to find anyway — the only question is when. It tends to make less sense earlier in a large balance, both because more can change before you get there, and because compounding gives an early investment contribution longer to work.
Getting the write-off question wrong flips the answer
If it later turns out the balance won't clear before write-off after all, every earlier overpayment was a straightforward loss — the debt would have been cancelled regardless, and that money can't be recovered from the Student Loans Company once paid. Money that went into an investment account instead is still sitting there, whatever the loan did. This is why the write-off question has to come first, not this one.
Risk and access are part of the comparison
An investment can be sold if circumstances change; a loan overpayment cannot be undone once made. Investment returns also aren't guaranteed and can sit below the loan's rate for years at a time, even if they're expected to beat it over the long run. Neither side of this comparison is risk-free — one risk is "the debt gets written off anyway", the other is "the market underperforms the loan's rate".
The order published guidance tends to use
- Whether the balance will clear before write-off — if it won't, an overpayment has no return to compare.
- Any higher-interest debt, which published guidance ranks ahead of both options.
- An emergency fund, since an overpayment can't be pulled back out.
- Only then, the loan's rate against a realistic return elsewhere.
The same question shows up elsewhere
This isn't unique to student loans. A mortgage carries the same known-rate-versus-uncertain-return question — overpay the mortgage, or invest the spare money instead. See the mortgage overpayment calculator for that version of the same framework, and the Money Planner for a single view across every debt and goal you have at once, rather than one comparison at a time.
Model it
Use the student loan calculator's overpayment box for the loan side, and the investment calculator or pension calculator for the other side, to compare both on your own numbers.