How we calculate

What it does each month

• Interest = balance × annual rate ÷ 12. Servicers charge interest daily, so real monthly amounts can differ by cents or a few dollars.
• RAP: the payment is reset each year from that year’s income. Payment goes to interest first. If it doesn’t cover the interest, the rest is waived. If principal fell by less than $50, the government match is added. Any remaining balance is forgiven after 360 payments (120 with PSLF).
• IBR: the payment is reset each year from income and the poverty guideline for your family size, capped at the 10-year standard payment on your starting balance. Unpaid interest builds up without compounding and is added to principal when the payment reaches the cap. Any remaining balance is forgiven after 240 or 300 payments (120 with PSLF).
• Standard and Tiered Standard: a fixed payment, at least $50, that pays the loan off in 10 years or in the Tiered Standard term for your balance.

Assumptions

• Income grows 3% a year and poverty guidelines 2.5% a year, unless you change them. RAP’s income bands stay fixed because the law sets them in dollars.
• Household, filing status and dependents stay the same.
• Every payment is on time and in full. No deferment or forbearance.
• All loans are unsubsidized Direct Loans just entering repayment, with no earlier progress toward forgiveness.
• No income tax on forgiven balances is included.
• If both spouses have loans, the split is fixed at the starting balances.
• Not modeled: parent PLUS, FFEL and Perkins loans, consolidation terms, PAYE and ICR, Graduated and Extended plans.

Checked against StudentAid.gov

On Oct. 17, 2026 we entered three cases into StudentAid.gov’s Repayment Calculator (without logging in) and into ours.

Case	Plan	StudentAid.gov	This site
Single, AGI $50,000, $40,000 at 6.5%	RAP	$167/mo · $70,350	$166.67/mo · $70,350
	IBR	$217/mo · $73,917	$217.17/mo · $73,198
	Standard	$454/mo · $54,503	$454.19/mo · $54,503
Single, 1 dependent, AGI $42,000, $60,000 at 6.52%	RAP	$90/mo · $113,467	$90.00/mo · $113,467
	IBR	$80/mo · $30,912	$79.50/mo · $29,938
	Standard	$682/mo · $81,828	$681.90/mo · $81,828
Single, AGI $75,000, $30,000 at 6.52%, loan from Sept. 2026	RAP	$438/mo · $36,831	$437.50/mo · $36,831
	Tiered Standard	$262/mo · $47,099	$261.66/mo · $47,099

RAP and the standard plans match to the dollar. IBR’s monthly payment matches; its long-run total differs by 1–3% because StudentAid.gov treats hand-entered loans as subsidized (three years of unpaid interest waived) and we treat them as unsubsidized. Both calculators showed the same plans as available in each case.

The interest deduction calculator

It follows IRS Publication 970, Worksheet 4-1: interest paid up to $2,500, minus the share set by how far your MAGI is into the phase-out band, with the ratio rounded to three decimal places. The tax-saved figure for 2026 is the federal income tax on (MAGI − standard deduction) minus the tax after the deduction, using the 2026 tax tables in IRS Rev. Proc. 2025-32. It assumes the standard deduction, ordinary income only and no credits, and leaves out state tax. Thirteen tests check it against worked examples, including a Publication 970 example.

The other calculators

• Student loan, payoff and refinance calculators amortize a fixed payment month by month (interest = balance × rate ÷ 12). Extra payments go straight to principal after that month’s interest. The refinance page holds the offered rate fixed and lists no lenders.
• PSLF calculator counts payments left (120 minus counted and bought-back months) and runs RAP or IBR for those months with the same engine as the RAP calculator.
• Parent PLUS calculator uses the 2026 ICR income percentage factors and interpolation from the June 9, 2026 Federal Register notice; it reproduces the notice’s worked examples ($105.23 and $266.90). Below the lowest AGI in the table we use the lowest factor, which the notice doesn’t address.
• Garnishment calculator takes the lesser of 15% of disposable pay and the pay above 30 × $7.25 per week (a month counted as 4⅓ weeks), and the Social Security offset as the lesser of 15% and the amount above $750; it reproduces the three examples in 31 CFR 285.4.

Tests

Forty-five automated tests in all. Twenty check the repayment engine against worked examples: the RAP band edges in StudentAid.gov’s table, the $10 floor, the dependent reduction, the interest waiver and principal match month by month, IBR’s $0 and $10 rules, the 10-year payment, Tiered Standard terms, plan eligibility, and the three cases above.

Every rule and its source: the rules.