Mortgage interest accrual is the daily or monthly buildup of interest charges on your outstanding loan balance, and federal law requires your lender to disclose exactly how those charges are calculated on the Loan Estimate you receive within three business days of applying and on the Closing Disclosure you receive at least three business days before closing. How much accrues, when it shows up as a line item, and how your payment timing affects it all depend on which accrual method your loan uses.
How the Interest Charge Is Calculated
Most U.S. home loans use monthly accrual. The lender divides the annual interest rate by 12 to get a monthly rate, then multiplies that rate by your current principal balance. On a $300,000 loan at 6%, the monthly interest charge is $1,500 (6% ÷ 12 = 0.5%, and 0.5% × $300,000 = $1,500). The number of days in the month doesn’t affect this calculation.
A less common structure called a simple interest mortgage works differently. The lender divides the annual rate by the number of days in a year and charges interest for each day individually. That per diem figure is multiplied by the days between payments. On the same $300,000 loan at 6%, the daily charge is about $49.32 using a 365-day year. Some lenders divide by 360 instead, a convention sometimes called the banker’s year, which bumps the daily charge to $50.00. That 68-cent daily gap adds up over decades.
Whichever method your loan uses, interest is always proportional to the balance still owed. As principal shrinks, so does the dollar amount of interest. The difference between the two methods surfaces most when payments arrive early or late: simple interest loans reward paying ahead and penalize dragging your feet, while monthly-accrual loans don’t care whether you pay on the 1st or the 15th (for interest purposes).
Why Your Early Payments Are Mostly Interest
A fixed-rate mortgage keeps your monthly payment identical for the entire term, but the split between interest and principal shifts dramatically. In the first few years, most of each payment covers interest because the balance is still close to the original loan amount. Very little reduces what you owe. By the final years, that ratio flips and nearly the whole payment chips away at principal.
This front-loading is a consequence of the math, not a lender trick. If you owe $300,000 and your monthly interest is $1,500, and your total payment is $1,799, only $299 reduces the balance in month one. The next month you owe $299 less, so the interest charge drops by about $1.50 and $300.50 goes to principal. That snowball accelerates slowly at first, then picks up speed in the second half of the term. Homeowners who sell or refinance within the first decade often find they’ve barely dented the principal despite years of on-time payments.
Per Diem Interest at Closing and Payoff
Even on standard monthly-accrual mortgages, per diem interest becomes a real line item at two specific moments: the day you close and the day you pay off the loan.
Odd-Days Interest at Closing
Mortgage billing cycles start on the first of the month, so a mid-month closing creates a gap. If you close on March 15, your first regular payment isn’t due until May 1 (covering April’s interest). That leaves the last 16 days of March unaccounted for. The lender collects interest for those days at the closing table as a prepaid charge. This amount appears on Page 2, Section F of your Closing Disclosure.1Consumer Financial Protection Bureau. What Are Prepaid Interest Charges?
Choosing a closing date near the end of the month minimizes this upfront cost because fewer odd days need coverage. Closing on the 28th means prepaying only two or three days of interest instead of half a month’s worth. That’s a real difference in cash needed at the table.
Payoff Statements
When you sell or refinance, the lender issues a payoff statement showing the exact amount needed to clear the debt. That figure includes your remaining principal plus daily interest accrued since your last payment. The statement is good only for a short window, because each additional day adds another per diem charge. If funds arrive a day or two after the quoted date, the title company or closing agent must account for the extra days. Payoff statements typically list the per diem rate so the parties can adjust on the fly.
The Disclosures Federal Law Requires
Two standardized documents carry the weight of the Truth in Lending Act’s disclosure requirements: the Loan Estimate and the Closing Disclosure.
The Loan Estimate
Your lender must deliver the Loan Estimate no later than three business days after receiving your mortgage application.2eCFR. 12 CFR 1026.19 – Certain Mortgage and Variable-Rate Transactions It lays out projected interest charges based on your anticipated closing date, including estimated prepaid daily interest owed at the table. It also gives you two figures designed to show the true cost of borrowing beyond the stated rate:
- The Annual Percentage Rate (APR) reflects the total cost of credit, including interest and certain fees, expressed as a single annualized rate. Because it folds in costs beyond the base rate, the APR is almost always higher than the advertised interest rate.3Office of the Law Revision Counsel. 15 USC 1606 – Determination of Annual Percentage Rate
- The Total Interest Percentage (TIP) shows the total interest you’ll pay over the entire loan term as a percentage of the loan amount. On a 30-year fixed mortgage, this number can be startlingly high.4eCFR. 12 CFR 1026.38 – Content of Disclosures for Certain Mortgage Transactions
The Closing Disclosure
Before you finalize the loan, the lender must ensure you receive the Closing Disclosure at least three business days before consummation.2eCFR. 12 CFR 1026.19 – Certain Mortgage and Variable-Rate Transactions This document replaces the Loan Estimate’s projections with final numbers. The prepaid interest section shows your exact per diem rate and the number of days being charged, so you can verify the math yourself. If significant terms changed since the Loan Estimate, the three-day waiting period resets, giving you time to review the revised figures.
Rate Adjustment Notices for Adjustable-Rate Mortgages
If you have an adjustable-rate mortgage, the daily cost of your loan changes when the rate resets. Federal regulation requires your servicer to warn you in advance, with timelines that depend on whether it’s the first adjustment or a later one:
- For the first rate adjustment, the servicer must send notice at least 210 days, but no more than 240 days, before the first payment at the new rate is due. If the first adjusted payment falls within 210 days of closing, the notice must be provided at consummation.5Consumer Financial Protection Bureau. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events
- For later adjustments that change your payment, notice must arrive at least 60 days, but no more than 120 days, before the new payment amount is due.5Consumer Financial Protection Bureau. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events
These notices must state the new interest rate, the new payment amount, and the effective date. The extended lead time for the initial adjustment exists because the jump from an introductory rate to the fully indexed rate is often the largest change a borrower faces.
Payment Timing, Grace Periods, and Extra Payments
Mortgage interest is paid in arrears. The payment you make on June 1 covers interest that built up during May. That backward-looking structure is why there’s a prepaid interest charge at closing to bridge the gap before the first regular payment.
Grace Periods
Most mortgage contracts include a grace period, commonly 15 days, before a late fee kicks in. A payment due on the first isn’t late for fee purposes until the 16th. But that’s only about the fee. Interest keeps accruing on the outstanding balance during the grace period. On a standard monthly-accrual loan, paying on the 10th instead of the 1st doesn’t change your interest for that month. On a simple interest loan, those extra days do cost you.
Extra Principal Payments
Paying more than your required amount and directing the surplus to principal is one of the most effective ways to cut lifetime interest. Because interest is always calculated against the current balance, every dollar of extra principal lowers every future interest charge. A $200 extra payment in month one doesn’t just save the interest on $200 for one month. It saves the interest on that $200 for every remaining month of the loan, and the compounding effect over 25 or 30 years is substantial. When making extra payments, confirm with your servicer that the additional funds are applied to principal rather than held for the next scheduled payment.
What Happens to Accrual When You Fall Behind
When you stop paying, interest doesn’t stop. It keeps accruing on the full outstanding balance, and the amount you owe grows every day. On a $250,000 balance at 6%, roughly $41 in interest piles on daily. After six months of missed payments, that’s nearly $7,500 in interest alone on top of the missed payment amounts.
Some loan agreements allow the lender to charge a higher default interest rate after a borrower falls behind. Federal regulation doesn’t cap that rate directly but does require any post-default rate to stay within the maximum rate ceiling set in the original loan agreement.6Consumer Financial Protection Bureau. Comment for 1026.30 – Limitation on Rates Beyond that ceiling, state law governs.
If your payments aren’t large enough to cover the interest owed, the unpaid interest gets added to your principal balance, and you start paying interest on interest. This is called negative amortization, and it means your debt grows even while you’re paying.7Consumer Financial Protection Bureau. What Is Negative Amortization? Some loan structures allow this by design through minimum-payment options. Others fall into negative amortization only when borrowers enter forbearance or payment plans that don’t fully cover the monthly interest charge. Either way, catching up gets harder the longer it continues.