Accrued interest is the interest that has built up on a loan, credit card, or investment since the last payment date but has not yet been paid or collected. The math is simple: principal multiplied by the annual interest rate, multiplied by the portion of the year that has passed. That last piece, the time fraction, is where mortgages, credit cards, and bonds each go their own way, which is why the same word shows up on your student loan statement, your bond trade confirmation, and the fine print of a credit card agreement.
The Basic Calculation
The formula is Principal × Rate × Time. Principal is the balance interest is being charged on. Rate is the annual interest rate. Time is a fraction of a year.
Under simple interest, the rate only ever applies to the original principal. Borrow $10,000 at 6% for 90 days and the accrued interest is $10,000 × 0.06 × (90/365) = $147.95. The balance doesn’t change inside the calculation.
Compound interest works differently. Interest that has already accrued gets added to the balance, and the next period’s interest is calculated on that larger number. Monthly compounding means month two’s interest is slightly higher than month one’s, and the gap widens from there. Most mortgages, credit cards, and savings accounts compound in some form, which is why long-term debt costs so much more than the stated rate suggests at first glance.
Why the Day Count Matters
The “time” fraction depends on the day-count convention written into the contract. Two show up most often:
- Actual/365 uses the real number of calendar days in the period over 365.
- 30/360 treats every month as 30 days and every year as 360.
A $100,000 loan at 5% accrues $13.70 per day under Actual/365 and $13.89 per day under 30/360. Small per-day differences, but they compound into real money over a full loan or bond term.
Accrued Interest on Mortgages and Auto Loans
On a standard fixed-rate mortgage, interest accrues monthly. Each payment covers the interest that has built up since the last one, and only what’s left reduces the principal. That ordering is why your early mortgage payments barely dent the balance. On a $300,000 loan at 7%, the first payment sends roughly $1,750 to interest and about $245 to principal. As the balance drops, the split slowly shifts.
Most mortgages come with a grace period of about 15 days before a late fee hits. The grace period protects you from penalties, but for a monthly-accrual mortgage, paying on the first versus the tenth doesn’t change the interest portion of that payment.
Auto loans typically accrue interest daily instead of monthly. Paying a few days early each month trims a small amount of interest over the life of the loan. It’s not dramatic on a five-year note, but it is real.
Student Loans and Capitalization
Federal student loans split into two types that treat accrued interest very differently. With Direct Subsidized Loans, the government covers the interest while you’re enrolled at least half-time, during the six-month grace period after leaving school, and during certain deferments. No interest accrues against you in those windows.1Federal Student Aid. Federal Interest Rates and Fees
Direct Unsubsidized Loans offer no such break. Interest starts the day the loan is disbursed, even if you’re still in school. Undergraduate Direct Loans for the 2025–2026 academic year carry a fixed rate of 6.39%.1Federal Student Aid. Federal Interest Rates and Fees On a $20,000 unsubsidized loan, four years of in-school accrual adds over $5,000 before you make a single payment.
That accrued interest doesn’t stay separate forever. When repayment begins, it gets added to your principal through a process called capitalization. For federal loans held by the Department of Education, capitalization occurs when a deferment ends on an unsubsidized loan, or when you exit an income-based repayment plan under certain conditions.2Nelnet. Interest Capitalization Once it capitalizes, you’re paying interest on a larger balance, and the total lifetime cost of the loan goes up. Making interest-only payments during school or deferment prevents the capitalization and is one of the cheapest ways to reduce what you ultimately owe.
Credit Card Interest
Credit cards usually accrue interest daily using the average daily balance method. The issuer tracks your balance each day of the billing cycle, averages those balances, and applies a daily periodic rate derived from your APR.3Consumer Financial Protection Bureau. How Does My Credit Card Company Calculate the Amount of Interest I Owe?
A grace period can keep all of that off your bill. If your card has one and you pay the statement balance in full by the due date, interest doesn’t accrue on new purchases during the next cycle. Carry a balance, and the grace period disappears: interest then accrues on new purchases from the transaction date. Issuers aren’t required to offer a grace period, but if they do, they must send your bill at least 21 days before the due date.4Consumer Financial Protection Bureau. What Is a Grace Period for a Credit Card?
Trailing Interest
Daily accrual produces a quirk that confuses people trying to zero out a card. Between the statement closing date and the day your payment posts, interest keeps building. So even when you pay the full statement balance, the next statement can show a small interest charge covering those in-between days. It’s not an error. Credit card companies call it trailing or residual interest, and one more full payment generally clears it.
Accrued Interest When Buying or Selling Bonds
Bonds pay coupon interest on a fixed schedule, usually every six months, but interest accrues to the holder every day between those dates. When a bond trades between coupon dates, the buyer and seller split that interest.
The seller has been earning interest since the last coupon. The buyer will receive the next full coupon payment. To square things, the buyer pays the seller the interest that accrued during the seller’s holding period, and recoups it when the next coupon lands. The quoted “clean price” excludes accrued interest; the “dirty price” adds it back and is what the buyer actually pays.
When a Payment Doesn’t Cover the Interest
Negative amortization happens when your payment is less than the interest that accrued during the period. The unpaid interest gets tacked onto the principal, so you owe more after making the payment than before it. Federal regulations define a negative amortization loan as one where a minimum payment covers only part of the accrued interest and the principal grows as a result.5Consumer Financial Protection Bureau. Regulation Z – 1026.18 Content of Disclosures
Certain adjustable-rate mortgages, payment-option loans, and some income-driven student loan repayment plans can produce this. The problem feeds itself: a bigger principal generates bigger monthly interest, which makes the next minimum payment even less adequate. Borrowers who don’t notice can end up owing more than they borrowed, sometimes more than their property is worth.
Tax Treatment
Federal tax law generally allows a deduction for interest paid or accrued during the tax year, but the rules depend heavily on the type of debt.6Office of the Law Revision Counsel. 26 USC 163 – Interest
Mortgage interest is deductible if you itemize, on up to $750,000 of mortgage debt ($375,000 for married filing separately). Higher limits apply to loans taken out before December 16, 2017.7Internal Revenue Service. Publication 936 (2025), Home Mortgage Interest Deduction Form 1098 from your lender shows what you paid.
Student loan interest is deductible up to $2,500 a year, even if you don’t itemize, with a phase-out at higher incomes.8Internal Revenue Service. Topic No. 456, Student Loan Interest Deduction Voluntary payments against accrued interest during school or deferment count toward that deduction in the year paid.
Bond accrued interest cuts both ways. If you sell a bond between coupon dates, the accrued interest the buyer pays you is taxable interest income. If you’re the buyer, the accrued interest you paid reduces your cost basis; when the next coupon arrives, you subtract that amount from the coupon and report only the net as interest income.9Internal Revenue Service. Publication 550 (2025), Investment Income and Expenses
Credit card interest and most other personal interest is not deductible. That makes carrying a balance even more expensive than the APR suggests, because there’s no tax offset to soften it.