What an extra $50, $100 or $200 a month does to a $30,000 student loan

By DecisionSheet · Updated October 10, 2026 · 2026 tax figures

Paying more than the minimum on a student loan does two things: the loan ends sooner, and less interest is charged over its life. DecisionSheet's student loan payoff vs. invest calculator measures both. This page runs it on its default loan, $30,000 over 10 years at 6.52%, with $50, $100 and $200 a month on top of the payment, then shows what the interest deduction, a higher rate, a longer term and a late start do to the figures.

The example

• The loan. $30,000, fixed at 6.52%, on the 10-year standard repayment plan. 6.52% is the rate on federal Direct Loans for undergraduates first disbursed between July 1, 2026 and June 30, 2027; graduate Unsubsidized loans are 8.07% (Federal Student Aid, GENERAL-26-33).
• The payment. $340.95 a month. Paid on schedule for 120 months, the loan costs $10,914 in interest.
• The extra. $50, $100 or $200 a month, every month from the first, applied to principal.

The table has the calculator's interest deduction switched off, so it shows the loan's full rate; the deduction gets its own section. The calculator's expected market return is left at its default and plays no part in these figures: interest saved and months cut depend on the loan alone.

Three levels

Open each level: $50 · $100 · $200. The three differ only in the extra; the loan and every other input are the calculator's defaults.

An extra $100 a month, about 31% on top of the payment in money terms, removes 31% of the interest and ends the loan 2 years 10 months early. Both measures rise with the extra, but not in proportion: $200 is 4 times $50 and saves 2.6 times the interest. Per dollar of monthly extra, the saving is $40 at $50, $34 at $100 and $26 at $200.

The reason is the same as on a mortgage. Each month the loan charges one-twelfth of 6.52% on the balance outstanding, and the fixed payment covers that interest first. Every dollar of extra goes to principal, and is then absent from every interest charge that follows. The first dollars of extra avoid the most interest; later ones, with less loan left, avoid less, which is why the per-dollar figure falls as the extra rises.

The interest deduction takes part of it back

Interest on a qualified student loan is deductible up to $2,500 a year, as an adjustment to income, whether or not you itemize (IRS Topic 456). For 2026 it phases out between $85,000 and $100,000 of modified adjusted gross income for a single filer, and $175,000 to $205,000 on a joint return (Rev. Proc. 2025-32, section 4.29).

Interest you avoid is interest you no longer deduct. Take a single filer with $75,000 of modified AGI, below the phase-out, in the 22% bracket. This loan's first year of interest, $1,891, is under the $2,500 cap and every later year's is smaller, so all of it is deductible, and the calculator puts the loan's effective rate at 5.09%. With the deduction switched on, $100 a month still saves $3,359 of interest and 2 years 10 months, but it gives up $739 of tax savings along the way, 22% of the interest it avoided: $2,401 of tax saved on the standard schedule becomes $1,662 with the extra. After tax, the $100 saves about $2,620. Open this scenario.

Above the phase-out, or on interest past the $2,500 cap, there is nothing to give back and the table's full figures stand.

What changes the answer

The rate. The same $100 on the same balance at the graduate rate of 8.07% saves $4,343 and 2 years 11 months, against $3,359 and 2 years 10 months at 6.52%: more interest to avoid, so more avoided. Open it.

The term. A longer loan has far more interest in it. The same $30,000 at 6.52% over 20 years has a payment of $224.03 and costs $23,766 in interest on schedule; $100 extra cuts 9 years 2 months and $11,951, more than twice the 10-year loan's saving, and 50% of the total. Open it. A borrower on an extended or consolidated schedule has the most to gain from an extra payment.

When you start. Extra payments begun 3 years in meet a balance of $22,945 with 7 years to run; the calculator reproduces the same $340.95 payment from those inputs, within a cent. $100 a month from there saves $1,598 and 1 year 10 months, under half the $3,359 it saves from day one. Open it.

Make sure the extra goes to principal

None of the above happens if the servicer treats the extra as an early payment of next month's bill. The CFPB notes that a lender may "credit" an overpayment against a future payment rather than apply it to the balance, called paid-ahead status, which is most common with federal loans, and that you can ask the servicer to put the money toward the balance instead (CFPB). Paid ahead, the extra lowers the next bill while the balance keeps charging the same interest; applied to principal, it does what the table says. Set a standing instruction with the servicer and check the first statement after it.

There is no penalty for paying early: the CFPB says that "generally, there are no penalties involved in paying off your student loans early" (CFPB), though a private loan's terms are worth reading. A borrower working toward Public Service Loan Forgiveness or forgiveness under an income-driven plan should not prepay at all: an extra dollar paid only shrinks what would have been forgiven.

Prepay or invest the extra?

A dollar of extra principal earns the loan's rate, compounded monthly, with certainty: 6.72% on this loan, which the calculator reports as the break-even market return. It is the same at $50, $100 and $200; the size of the extra changes how much is saved, not the rate it earns. Investing comes out ahead only if it returns more than that, and an employer match comes before either. Pay off student loans early or invest? runs that comparison at several rates and returns.

Assumptions and limits

• Fixed-rate loan, interest charged monthly at one-twelfth of the annual rate, on the standard 10-year schedule from the first payment. No grace period, capitalized interest, variable rate or income-driven plan.
• The extra starts in month one of each scenario and continues until the loan is paid off. The late start is the same loan picked up at its balance after 3 years.
• The table leaves the student loan interest deduction off. With it on, the calculator adds each year's tax saving at the end of the year, holding income and the tax rate constant; in practice it arrives with the tax return or through lower withholding.
• Every extra dollar is credited to principal in the month it is paid. Paid-ahead status, above, is the way that assumption fails.
• No prepayment penalty, no fees, nominal dollars: interest saved is summed over the loan's life with no adjustment for inflation or for when the saving occurs.
• One loan. A borrower with several should check how the servicer allocates an overpayment among them; applied to the highest-rate loan, it saves the most.

Method and sources

The model is calculateStudentLoan in the student loan payoff vs. invest calculator. The monthly payment is the standard fixed-rate amortization formula; the loan is run month by month on schedule and again with the extra applied to principal, and interest saved and time cut are the differences between the two runs. With the deduction on, each year's tax saving is the lesser of that year's interest and $2,500, times the share left after the phase-out, times the marginal rate, and the tax savings given up is the difference between the two runs' totals. Every figure above comes from running the model on the inputs in the scenario links; the deduction limits are read from the site's tax constants for 2026.

• Federal Student Aid, GENERAL-26-33, June 4, 2026: Direct Loan rates of 6.52% (undergraduate) and 8.07% (graduate or professional) for loans first disbursed between July 1, 2026 and June 30, 2027.
• IRS Topic 456, Student loan interest deduction: up to $2,500 a year, as an adjustment to income.
• Rev. Proc. 2025-32, section 4.29: the 2026 phase-out ranges.
• CFPB: Can I make additional payments on my student loan?: paid-ahead status, and asking the servicer to apply the money to the balance.
• CFPB: Can I pay off my student loan in full at any time?: no penalty for paying early.

Open this scenario in the calculator

All figures on this page come from the Student Loan Payoff vs. Invest calculator. Change any input there and the numbers update.