Pay extra on student loans vs. invest calculator

How this calculator works

You enter loan balance, fixed rate, repayment term, extra monthly cash available, expected investment return, and how many years to model. Path A pays the minimum plus the full extra amount until the loan is paid off, then invests the entire monthly cash flow for the remaining months. Path B pays only the minimum on the loan and invests the extra amount every month from the start. Net worth at each month equals investment portfolio value minus remaining loan balance. The calculator reports final net worth under each path, the dollar difference, which path wins, and a break-even investment return found by binary search. All math runs locally in your browser.

This is a simplified wealth comparison — it does not model taxes on investment gains, employer matches, or changing loan payments under income-driven repayment. Use it when you have a fixed-rate loan and a steady surplus. For PSLF or IDR forgiveness planning, the plan comparison calculator is usually the better starting point.

What affects the result

Loan interest rate is the guaranteed return for paying extra — every dollar of principal retired saves that rate. Expected investment return is uncertain but compounds on Path B from day one. A higher loan rate tilts toward Path A; a higher expected return and longer horizon tilt toward Path B. Extra monthly amount scales both paths proportionally. Model length matters: Path A may still be paying the loan early in the horizon while Path B compounds investments immediately. Shorter horizons favor paying debt; longer horizons give investments more time to catch up after the loan is gone on Path A.

Real-world examples

Example 1 — 6.53% loan, 7% expected return, 15 years: With $200 extra on a $28,000 balance, Path A often wins because early guaranteed savings at 6.53% compound against a return only modestly higher — and Path B carries loan balance longer.

Example 2 — 3% federal loan, 8% return: Low-rate loans frequently favor minimum payments plus investing if you tolerate market risk and have a long horizon.

Example 3 — High extra, short horizon: $500 extra over five years on a mid-rate loan can eliminate debt quickly on Path A, producing a clean slate before investing — strong for risk-averse borrowers.

Example 4 — PSLF track: A borrower pursuing forgiveness should not use extra payments at all — minimum payments maximize forgiven balance. Use the plan comparison calculator instead of this tool for PSLF planning.

Example 5 — Employer match priority: Jordan has $300 monthly surplus and a 50% 401(k) match up to 6% of salary. Contributing enough to capture the match before either path here often dominates both modeled options — the match is an immediate return this calculator does not include.

Example 6 — Rising income: A borrower on Standard repayment expects a large raise in three years. Path B keeps investments growing during lower-payment years; Path A eliminates debt before the raise. Income changes are not modeled — adjust your horizon if your payment plan may change.

Common mistakes

Comparing loan rate to average historical stock returns without context.Markets are volatile; loan savings are guaranteed. Risk tolerance matters as much as averages.

Investing extra before an emergency fund exists. This calculator assumes you deploy all extra cash. Keep cash reserves before aggressive investing or prepayment.

Skipping employer match to pay loans. A 50–100% immediate match often beats both paths modeled here.

Paying extra while pursuing PSLF. Extra principal does not reduce forgiven amount under PSLF.

Using pre-tax return assumptions for taxable brokerage investing. Tax drag lowers real investment returns; the break-even return may be lower than you think.

Too short a model horizon.Five years may not show Path A's post-payoff investing phase fully if the loan term is ten years.

When to use this calculator

Use this when you have a fixed amount of monthly surplus and debate between accelerating student loan payoff and funding a brokerage or retirement account. It is most useful for borrowers not on PSLF or IDR forgiveness tracks, with fixed-rate loans above 4–5%, and a clear investment plan. Pair with the tax deduction calculator if you claim student loan interest, and read the taxes guide for annual checklist items.

Limitations

Investment returns are hypothetical and constant — real markets fluctuate. Taxes on dividends, capital gains, and the student loan interest deduction are not modeled. Employer 401(k) matches, HSA contributions, and other priorities are excluded. IDR payments that change annually are not supported — enter a fixed minimum only. Prepayment penalties on private loans are not included. Break-even return is a mathematical crossover point, not a recommendation to invest. Inflation and changing income over the model horizon are not reflected.

Sources

1. Federal Student Aid — Repayment
2. IRS Publication 970 — Tax benefits for education
3. Federal Student Aid — PSLF