Student loan payments are calculated one of two ways. Federal loans on the standard, graduated, or extended plans use an amortization formula that turns your balance, interest rate, and repayment term into a fixed monthly amount. Federal loans on an income-driven plan ignore the balance almost entirely and instead charge a set percentage of your discretionary income. Private student loans use amortization only, with the interest rate your lender assigned at approval. Understanding how student loan payments are calculated starts with knowing which bucket your loan falls into, because the inputs are completely different.
Daily Interest Comes First
Every payment calculation sits on top of the way interest accrues. Federal student loans use simple daily interest. Your servicer multiplies your current principal balance by your annual interest rate and divides by 365.25 to get a daily charge. On a $30,000 loan at 6.39%, that’s about $5.25 per day.1Edfinancial Services. Payments, Interest, and Fees When your monthly payment posts, it covers the interest that has accumulated since your last payment first, and only what’s left goes toward principal.
Payment timing matters for that reason. A payment that posts a few days late means more accumulated interest to clear before any of your money reaches the balance. Federal Direct Loan rates are fixed for the life of each loan and set each May based on the 10-year Treasury auction, so the rate on your paperwork is the rate the formula will keep using.2Federal Student Aid Partners. Interest Rates for Direct Loans First Disbursed Between July 1, 2025 and June 30, 2026
Standard Federal Repayment: The Amortization Formula
The standard plan is the default, and it’s the simplest to calculate. Your servicer spreads the cost of the loan evenly over 120 monthly payments. The formula multiplies your monthly interest rate (annual rate divided by 12) by (1 plus the monthly rate) raised to the 120th power, then divides by (1 plus the monthly rate) raised to the 120th power minus 1. That result, multiplied by your principal, is your fixed monthly payment. If the math produces anything less than $50, the minimum is $50.3Federal Student Aid. Standard Repayment Plan
In practice, a $30,000 loan at 6.39% works out to roughly $339 per month. Early on, about $160 of each payment covers interest and $179 reduces the balance. By year eight, those proportions flip. Amortization front-loads the interest, which is why extra payments early in the loan save more than the same dollars paid later.
Graduated and Extended Plans
Graduated plans use the same amortization math but start with lower payments that step up every two years across a ten-year term.4eCFR. 34 CFR 685.208 – Fixed Payment Repayment Plans The initial amount is sometimes just enough to cover accruing interest. The final payments can run well above what the standard plan would charge, and total interest paid is higher because the balance stays larger for longer.
Extended plans stretch repayment to 15 to 30 years, depending on how much you owe. You need more than $30,000 in outstanding Direct Loans to qualify if you entered repayment on or after July 1, 2006, and $60,000 or more unlocks the full 30-year term.4eCFR. 34 CFR 685.208 – Fixed Payment Repayment Plans The monthly payment comes from the same amortization formula with more periods plugged in. Lower monthly bill, considerably more interest over the life of the loan.
Discretionary Income: The First Step for IDR Plans
Income-driven repayment plans don’t look at your balance to set the payment. They start with your Adjusted Gross Income from your most recent federal tax return and subtract a protected amount tied to the federal poverty guideline for your family size and state.
For 2026, the poverty guideline in the 48 contiguous states is $15,960 for a single-person household and $33,000 for a family of four.5U.S. Department of Health and Human Services. 2026 Poverty Guidelines – 48 Contiguous States Your plan determines which multiple of that number gets subtracted. IBR and PAYE protect 150% of the poverty guideline. The SAVE/REPAYE plan protected 225%, though that plan’s status is unsettled.6eCFR. 34 CFR 685.209 – Income-Driven Repayment Plans
A worked example: a single borrower earning $45,000 in 2026 on IBR. The poverty guideline is $15,960; 150% of that is $23,940. Subtracting leaves $21,060 in discretionary income. That $21,060 is what the plan’s percentage gets applied to. If the same borrower were on SAVE’s 225% threshold, the protected amount would be $35,910, leaving only $9,090 in discretionary income. The protection threshold is the biggest reason different IDR plans produce such different numbers.
Family size moves the result. The IDR definition generally includes you, your spouse, your dependent children, anyone else who lives with you and gets more than half their support from your household, and an unborn child expected during the year. Each additional person raises the protected amount and lowers your discretionary income.
What Each IDR Plan Charges
Once discretionary income is set, the plan applies its own percentage and divides by 12 for a monthly payment.
- IBR for new borrowers (first loan on or after July 1, 2014): 10% of discretionary income, capped at what the standard 10-year plan would charge.
- IBR for older borrowers: 15% of discretionary income, with the same cap at the standard plan amount.
- PAYE: 10% of discretionary income, also capped at the standard plan amount.6eCFR. 34 CFR 685.209 – Income-Driven Repayment Plans
- ICR: the lesser of 20% of discretionary income divided by 12, or what you’d pay on a 12-year fixed plan adjusted for income. ICR is the only income-driven option for parent PLUS borrowers who consolidate.6eCFR. 34 CFR 685.209 – Income-Driven Repayment Plans
If the formula produces a monthly amount under $5 on IBR or PAYE, it rounds down to $0, and that $0 is a real qualifying payment that counts toward forgiveness. On ICR, an amount between $0 and $5 rounds up to $5.6eCFR. 34 CFR 685.209 – Income-Driven Repayment Plans If your income is below the protected threshold entirely, your payment is $0 on any of these plans.
These percentages are set by federal regulation. Your servicer doesn’t negotiate them, and your credit history has no effect. The only inputs you influence are which plan you choose, how you file your taxes, and your household size.
Where SAVE Stands
SAVE was designed to be the most generous IDR option, protecting 225% of the poverty line and charging just 5% of discretionary income for undergraduate loans and 10% for graduate loans, with a full subsidy of any interest the monthly payment didn’t cover.6eCFR. 34 CFR 685.209 – Income-Driven Repayment Plans As of early 2026, SAVE is effectively frozen. Federal courts enjoined key provisions, and in December 2025 the Department of Education announced a proposed settlement that would end the plan, deny pending applications, and move current enrollees onto other plans.7Federal Student Aid. IDR Plan Court Actions – Impact on Borrowers
Current SAVE enrollees have been placed in a general forbearance. Interest has been accruing since August 1, 2025, no payments are due during the forbearance, and those months do not count toward IDR forgiveness or Public Service Loan Forgiveness. Borrowers working toward PSLF who want qualifying payments to resume need to switch to an active IDR plan such as IBR or PAYE.7Federal Student Aid. IDR Plan Court Actions – Impact on Borrowers
How Marriage Changes the Inputs
Tax filing status feeds directly into the IDR formula. File jointly, and most IDR plans use your combined household income before the poverty-line subtraction. That generally raises the payment. If your spouse also carries federal student loans, the servicer prorates your payment based on your share of the couple’s combined federal loan debt.8Federal Student Aid. 4 Things to Know About Marriage and Student Loan Debt
Filing separately is a common workaround. On IBR and PAYE, only the borrower’s own income counts when the return is filed separately. The trade-off is losing tax benefits that come with joint filing, like education credits and the larger standard deduction. Run the numbers both ways before locking in a filing strategy.8Federal Student Aid. 4 Things to Know About Marriage and Student Loan Debt
Annual Recertification Changes Your Payment
IDR payments reset every year. You recertify your income and family size on your anniversary date, and the servicer runs the formula again with the new inputs. Income up, payment up. New child, payment down.
Miss the recertification deadline and the consequences are real. Your payment can be bumped to the standard plan amount, which is often several hundred dollars more a month. On IBR, a missed recertification also triggers interest capitalization: any unpaid interest gets rolled into the principal, and future interest accrues on that larger number. Set a calendar reminder ahead of the anniversary date.
How Private Student Loan Payments Work
Private lenders don’t use income-based formulas. The calculation depends on three inputs set when you signed the promissory note: the principal balance, the interest rate, and the repayment term. Your rate was based primarily on your credit score and debt-to-income ratio at application. Strong credit can produce rates near or below federal levels; thin credit or heavy existing debt can push rates well into double digits.
You typically chose between a fixed rate and a variable rate tied to a benchmark like SOFR or the Prime Rate. The monthly payment uses the same amortization formula federal standard plans use. For a fixed-rate loan, the payment stays constant. For a variable-rate loan, the servicer recalculates when the benchmark moves, and the monthly payment can rise or fall over time. Terms generally run 5 to 20 years, with shorter terms producing higher monthly payments and considerably less total interest.
Unlike federal loans, private lenders rarely offer forbearance, deferment, or income-driven options. If the calculated payment is unaffordable, the realistic choices are refinancing with another private lender or asking your current lender for a temporary hardship arrangement. Late fees and default-rate increases are governed by your individual loan contract rather than a single federal rule, so the original paperwork is where those specifics live.