Guaranteed interest saved versus expected market returns — and your break-even rate
Paying extra on a student loan earns your APR, guaranteed. Investing earns an expected return that isn't. On the classic case — $30,000 at 6.52% (the 2026-27 federal undergraduate rate), minimum payment $340.95, plus $250 a month to deploy — investing at 7% beats prepaying by $969 over the ten-year horizon, but at 6% prepaying wins by $262 and at 5% by $1,373. The break-even return is 6.22%, just under the loan's own rate, because interest savings arrive early and compound. Prepaying also ends the loan at month 60 instead of 120 and cuts total interest from $10,914 to $5,196 — $5,718 saved. Rules of thumb: emergency fund first, employer match second (and check whether your plan matches loan payments under SECURE 2.0), then compare your APR against your realistic expected return — federal loans at 6-7% are a coin flip worth running both ways; private loans above 8% almost always favor payoff.
| Expected return | Pay off first — wealth at yr 10 | Invest instead — wealth at yr 10 | Winner |
|---|---|---|---|
| 5% | $40,356 | $38,982 | Pay off by $1,373 |
| 6% | $41,437 | $41,175 | Pay off by $262 |
| 7% (long-run default) | $42,555 | $43,524 | Invest by $969 |
| 8% | $43,711 | $46,041 | Invest by $2,331 |
$30,000 balance at 6.52% APR, $340.95 minimum, $250/month deployed either way, wealth measured at the month the minimum-only loan would finally die (120). Pay-off-first invests the freed-up $590.95/month for the remaining five years. Break-even return for these inputs: 6.22%.
| Your loan APR | Usual winner for $250/mo extra | Why |
|---|---|---|
| 8%+ | Pay off | Beating 8% after volatility, consistently, is a bar most investors miss |
| 6% – 8% | Genuine toss-up | Expected market return clears the APR, but barely — federal protections tip it toward investing the minimum and prepaying selectively |
| Below 5% | Invest | Old subsidized-era or refinanced fixed rates below typical expected returns |
| Employer match available | Capture it first | A 50-100% instant return beats any loan and any market |
The question is always the same shape: you have money beyond the minimum payment. Route it to the loan (guaranteed savings at your APR) or to the market (expected, variable returns). The calculator runs both futures to the same finish line and reports the gap.
Both strategies start from your balance, APR, minimum, and the extra monthly amount. The horizon is the month the minimum-only loan reaches zero. Pay off first: extra goes to the loan until it dies (month T), then the full payment-plus-extra is invested monthly until the horizon. Invest instead: minimums run the whole horizon while the extra is invested every month. Wealth is compared at the horizon; interest saved is the difference in total interest paid. The break-even return is the market rate at which the two endings tie — solved by bisection, and it's the number to memorize: invest only if your realistic expectation beats it by enough to pay for the risk.
$30,000 at 6.52% — the 2026-27 federal undergraduate rate — with a $340.95 minimum (the 10-year standard) and $250 a month extra. Pay off first: the loan dies at month 60, total interest $5,196, then $590.95 a month compounds at 7% for the next 60 months to $42,555 at the horizon. Invest instead: minimums run all 120 months (interest $10,914), the $250 a month compounds to $43,524. Investing wins by $969 — but the same inputs at a 6% return flip the answer by $262, and at 5% prepaying wins by $1,373. The break-even is 6.22%: a quarter-point below the loan's rate. Near the line, the guaranteed side deserves the tiebreaker.
Math isn't the whole decision. Federal loans bundle options private loans don't: RAP's income-based payments if your income drops, deferment and forbearance, PSLF, and discharge programs. Extra principal paid into a federal loan never comes back if you later need that flexibility. That's why many borrowers with federal loans at moderate rates split the difference: keep paying the minimum (preserving options), invest the extra, and direct any aggressive prepayment at private or high-rate debt first. If you're considering refinancing federal loans down to make this whole question cheaper, the refinance calculator prices it — remembering that refinancing makes you a private borrower permanently.
Educational model, not investment advice. Investment growth compounds monthly at the stated return with no taxes or fees modeled; real-world results will differ.
Compare a guaranteed return against an expected one. Extra loan payments save interest at exactly your APR — risk-free. Investments must beat that APR after taxes and volatility to win. On $30,000 at 6.52% with $250/month to deploy, investing wins at a 7% return by $969 over ten years, but paying off wins at 6% by $262 and at 5% by $1,373. The break-even return is 6.22% — below the loan's own rate, because interest savings arrive earlier and compound. If your expected return clears your APR by a point or two and you can stomach the volatility, investing usually edges ahead; if the margin is thin, the guaranteed payoff is the better sleep.
Yes — the match comes first, always. A 50% to 100% instant return beats any loan APR and any market expectation. And under SECURE 2.0, if your plan adopted the student loan match, your loan payments themselves can earn the match — meaning you may be able to pay loans and capture the match with the same dollars.
Yes. Federal loans carry protections private loans don't: the RAP income-based plan, deferment and forbearance, discharge options, and PSLF eligibility. Prepaying a federal loan is a one-way door — that money is gone if you later need flexible payments. A common split: keep federal minimums (cheap, flexible money), and direct extra payments at private loans or investing. Refinancing federal loans to a private lower rate permanently gives up those protections.
On the default example — $30,000 at 6.52% with a $340.95 minimum — adding $250 a month pays the loan off in 60 months instead of 120 and cuts total interest from $10,914 to $5,196: $5,718 saved and five years of payments deleted. The earlier payoff matters as much as the dollar figure: those freed-up payments can then compound for the second five years.
Three to six months of expenses in cash come before both — an emergency on top of debt forces new borrowing at worse rates. Then employer match, then high-APR debt (most private student loans and anything above roughly 8%), then it's a genuine choice between moderate-rate loans and the market.
Yes. Interest you never accrue is indistinguishable from interest earned, and unlike market returns it can't have a bad decade. That's why the fair comparison is your loan APR against your expected investment return — with the loan side carrying zero variance and the invest side carrying plenty. Investors usually demand a premium of one or two points to compensate, which is exactly what the break-even calculation shows.
Educational calculator only — not investment, tax, or financial advice. Market returns are variable; the model excludes taxes, fees, and sequence risk.