Student Loan Interest Calculator
Interest Accrues Daily, Not Monthly
Federal and most private student loans don't charge interest in a single monthly lump — they accrue it every single day against the outstanding principal, using a daily interest rate derived from the annual rate. That distinction matters most during in-school periods, grace periods, deferment, or forbearance, when no payment is being made and interest quietly builds day by day. This calculator applies the same daily-accrual math your servicer uses to show exactly how much interest a given stretch of time adds to a balance.
The Formula
Interest = Principal × Daily Rate × Days
This is simple interest, not compound — each day's charge is based on the principal alone, not on interest that has already accrued (unless and until that interest is capitalized into the principal, typically when a deferment or forbearance period ends).
Where This Calculation Matters
- Grace period estimates — unsubsidized loans accrue interest through the standard six-month grace period after graduation, and knowing that figure avoids a surprise when repayment starts.
- Timing a lump-sum payment — borrowers deciding whether to pay down a balance now or in three months can quantify exactly what waiting costs.
- Verifying a servicer's statement — running the same daily-rate formula against your own numbers is a quick way to confirm accrued interest shown on an account statement.
- Comparing accrual periods — short deferments and full-year forbearances accrue interest on very different scales, useful to see side by side.
Interest Accrued at Different Balances and Time Periods
| Principal | Annual rate | Period | Interest accrued |
|---|---|---|---|
| $10,000 | 5.0% | 30 days | $41.10 |
| $25,000 | 6.5% | 30 days | $133.56 |
| $25,000 | 6.5% | 365 days | $1,625.00 |
Over a full 365-day year, simple daily interest on a fixed balance equals principal × annual rate exactly, since the daily-rate division and the 365-day multiplication cancel out.
How to Use This Calculator
- Enter the principal balance in dollars.
- Enter the annual interest rate as a percentage.
- Enter the number of days interest should accrue over.
- Select Calculate to see the interest accrued and the resulting balance.
Related Calculations
If the accrued interest will be added to your principal at the end of a deferment period, see the Deferment Interest Calculator, or use the Student Loan Payoff Calculator to see how a larger starting balance affects your payoff timeline.