How Student Loan Interest Accrues: Subsidized vs. Unsubsidized
The way interest accrues on your student loans is a primary driver of your overall cost. Federal student loans fall into two categories: subsidized and unsubsidized. For subsidized loans, the government pays the interest while you are enrolled at least half-time, during the grace period after leaving school, and during approved deferment periods. That means no interest accrues during these windows, so the balance you begin repaying is the amount you originally borrowed.
Unsubsidized federal loans, on the other hand, begin accruing interest as soon as the loan is disbursed, even while you are still in school. If you do not make interest payments during that time, the interest will be added to your principal balance later, which is called interest capitalization. This can increase the total amount you owe and, as a result, your monthly payment compared with what a standard loan calculator might suggest.
Private student loans also vary: some may have fixed rates, while others may have variable rates that depend on your credit score and, often, a cosigner for borrowers with limited credit history.
Sources: SmartAsset, Sallie Mae
The Standard Student Loan Payment Formula and Example
Most student loan payments are calculated using the same amortization formula used for fixed-rate loans: a fixed monthly payment is set so that the loan is fully paid off by the end of the term. The formula is M = P × [r(1+r)^n] / [(1+r)^n – 1], where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the number of monthly payments. For federal loans, the standard repayment plan uses a term of 10 years.
For a $30,000 undergraduate loan at 6% interest, the standard monthly payment is $333 and the total paid over 10 years is $39,967. For a $60,000 combined undergraduate and graduate loan at the same 6% interest, the standard monthly payment is $666, and the total paid is $79,935.
These figures assume that interest is fixed and that no additional fees are included. The standard plan's predictable payment can help you compare other repayment options.
| Loan amount | Monthly payment | Total over 10 years |
|---|---|---|
| $30,000 | $333 | $39,967 |
| $60,000 | $666 | $79,935 |
Sources: The Institute of Student Loan Advisors, Oregon State University Office of Financial Aid (reproducing Federal Student Aid information), FinAid
Grace Periods and Interest Capitalization: How They Affect the Total
When you leave school, you typically enter a grace period before you must begin repayment. For subsidized federal loans, no interest accrues during that time. For unsubsidized loans, interest continues to accrue. If you don't pay that accrued interest before repayment begins, it is added to your principal balance—this is called interest capitalization.
Interest capitalization can have a meaningful effect on your total cost. Because the unpaid interest becomes part of the principal, future interest is calculated on a larger amount. Even a modest amount capitalized early in the loan can increase the total interest you pay over the life of the loan. When using a student loan calculator, be aware that some may not automatically include in-school interest accrual or capitalization, so results may understate your total cost.
To estimate your payments accurately, you may need to manually account for any interest that has accumulated before you start repayment. Knowing whether your loans are subsidized or unsubsidized is key to that calculation.
Sources: SmartAsset, Sallie Mae
Income-Driven Repayment Plans: How They Change the Calculation
Income-driven repayment (IDR) plans base your monthly payment on your discretionary income rather than on the total amount you owe. Discretionary income is defined as your adjusted gross income minus 150% of the federal poverty guideline for your family size. Payments are calculated as a percentage of that discretionary income.
The main IDR plans include REPAYE, PAYE, IBR, and ICR, each with its own formula and repayment period. For example, both REPAYE and PAYE generally require a payment of 10% of discretionary income, but PAYE payments are capped at the 10-year Standard Repayment Plan amount. IBR payments are 10% of discretionary income for new borrowers (on or after July 1, 2014) and 15% for other borrowers, also capped at the standard plan amount. ICR payments are the lesser of 20% of discretionary income or a fixed payment over 12 years, adjusted for income.
IDR plans also come with longer repayment periods: 20 years for many plans, 25 years for others, and any remaining balance is forgiven after that time. Borrowers must recertify their income and family size each year, so payments can change as your financial situation changes. Private student loans are not eligible for federal IDR plans.
| Plan | Payment formula | Repayment period |
|---|---|---|
| REPAYE | 10% of discretionary income | 20 years (undergrad only) or 25 years (if any grad loans) |
| PAYE | 10% of discretionary income, capped at 10-year Standard amount | 20 years |
| IBR (new borrowers) | 10% of discretionary income, capped at 10-year Standard amount | 20 years |
| IBR (non-new borrowers) | 15% of discretionary income, capped at 10-year Standard amount | 25 years |
| ICR | Lesser of 20% of discretionary income or 12-year fixed payment adjusted for income | 25 years |
Sources: Oregon State University Office of Financial Aid (reproducing Federal Student Aid information), Student Loan Planner, The Institute of Student Loan Advisors
Steps to Estimate Your Monthly Payment and Total Cost
To estimate your student loan payment on your own, you can use the standard amortization formula: M = P × [r(1+r)^n] / [(1+r)^n – 1], where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the number of monthly payments.
Here are the steps to follow:
1. Identify your loan balance and interest rate. For federal loans, check your loan servicer's website or the National Student Loan Data System. For private loans, refer to your loan agreement.
2. Determine whether your loans are subsidized or unsubsidized, and whether any deferred interest will be capitalized. This affects your starting principal.
3. Choose a repayment term. Federal loans typically offer 10-year standard, extended, or income-driven options. Private loans range from 10 to 15 years.
4. Use the formula or an online calculator to compute your monthly payment. For IDR plans, calculate the plan-specific percentage of discretionary income (e.g., 10% for REPAYE, PAYE, and new IBR; 15% for non-new IBR; 20% for ICR, unless the ICR 12-year fixed amount is lower).
5. Multiply the monthly payment by the number of payments to get your total cost. Remember to include the effect of any interest capitalization and any fees.
6. Compare plans to see which fits your budget and total cost goals. Income-driven plans may lower monthly payments but can result in higher total interest over the long run.
- Identify your loan balance and interest rate.
- Determine if your loans are subsidized or unsubsidized, and if any interest capitalization will occur.
- Choose a repayment term (10-year standard, extended, or income-driven).
- Calculate your monthly payment using the standard amortization formula or your discretionary income for IDR plans.
- Multiply the monthly payment by the total number of payments to find the total cost over the loan term.
- Compare repayment options to balance monthly affordability and total cost.
Sources: FinAid, Oregon State University Office of Financial Aid (reproducing Federal Student Aid information), Sallie Mae, The Institute of Student Loan Advisors
Frequently asked questions
Why does my student loan payment differ from a standard loan calculator?
A standard loan calculator typically assumes a simple fixed-rate amortization with no interest accrual before repayment begins. For student loans, especially unsubsidized or private loans, interest may accrue while you're in school and during deferment, and that interest can be capitalized—added to your principal. This increases the amount you owe and, therefore, your monthly payment. Additionally, income-driven repayment plans calculate payments based on your income, not just the loan balance, which can also produce different results from a standard calculator.
Sources: Sallie Mae, SmartAsset, Oregon State University Office of Financial Aid (reproducing Federal Student Aid information)What is interest capitalization and how does it affect my loan?
Interest capitalization occurs when unpaid interest is added to the principal balance of your loan. This typically happens after a period of deferment or forbearance, or when you enter repayment after leaving school. Because the unpaid interest becomes part of the principal, future interest is calculated on the larger amount, increasing the total interest you pay over the life of the loan. This can raise your monthly payment compared with what you might expect if you only considered the original loan amount.
Sources: Sallie MaeHow do income-driven repayment plans affect my monthly payment?
Income-driven repayment (IDR) plans base your monthly payment on a percentage of your discretionary income, not on the total loan balance or the standard amortization schedule. Discretionary income is your adjusted gross income minus 150% of the federal poverty guideline for your family size. For most IDR plans, the payment is 10% of discretionary income, though the ICR plan uses 20% or a 12-year fixed payment, whichever is less. Payments can be lower than the standard plan, but you must recertify your income and family size each year, so payments may change. After 20 or 25 years, any remaining balance is forgiven.
Sources: Oregon State University Office of Financial Aid (reproducing Federal Student Aid information), Student Loan Planner, The Institute of Student Loan AdvisorsShould I use the standard 10-year plan or an extended plan?
The standard 10-year plan offers fixed monthly payments and ensures your loan is paid off in a decade. Extended plans, such as the Extended Fixed plan, spread payments over up to 25 years, which lowers the monthly payment but increases the total interest paid over time. The right choice depends on your budget and financial goals. If you can afford the higher standard payment, you'll save on interest and pay off the loan sooner. If you need lower monthly payments, an extended plan may be more manageable, but you'll pay more in total interest.
Sources: Oregon State University Office of Financial Aid (reproducing Federal Student Aid information), The Institute of Student Loan Advisors, Sallie MaeSources
- Income-Driven Repayment Plans for Federal Student Loans — Oregon State University Office of Financial Aid (reproducing Federal Student Aid information)
- Student Loan Calculator: Estimate Your Payments — Sallie Mae
- Student Loan Calculator - Estimate Your Loan Payment — SmartAsset
- Loan Payment Calculator — FinAid
- Student Loan Calculator — The Institute of Student Loan Advisors
- Income-Based Repayment Calculator (RAP + All Plans) - 2026 Update — Student Loan Planner