Mortgage interest is calculated each month by dividing your annual note rate by 12 to get a monthly rate, then multiplying that monthly rate by your current outstanding principal balance. On a $300,000 balance at 6%, the math is $300,000 × 0.005 = $1,500 in interest for that month. Because the lender runs this calculation fresh every billing cycle on whatever you still owe, the interest charge drops gradually as your balance falls, even though your total payment stays the same.
What You Need Before Running the Numbers
Two figures drive the whole calculation, and mixing either one up with a similar-sounding number is the most common reason borrowers get a different answer than their statement shows.
The first is your outstanding principal balance. That is what you owe right now, not what you originally borrowed. Your servicer is required to show the current balance on every periodic statement, along with how your payment is split between principal, interest, and escrow.1eCFR. 12 CFR 1026.41 – Periodic Statements for Residential Mortgage Loans
The second is your note rate, the annual interest rate written into your loan agreement. This is not your Annual Percentage Rate. The APR folds in origination fees, discount points, and other closing costs so you can compare loan offers side by side, but it plays no role in the monthly interest charge. Only the note rate goes into the formula.
One more thing to strip out before you check the math: most lenders collect property taxes and homeowners insurance through an escrow account and bundle it all into a single payment, often called PITI for principal, interest, taxes, and insurance.2Consumer Financial Protection Bureau. What Is PITI? The interest calculation only touches the principal-and-interest portion. If you are reconciling against your total payment, subtract the escrow piece first.
The Formula, Step by Step
Once you have the right inputs, the arithmetic is quick.
- Divide your annual note rate by 12 to get the monthly rate. A 6% rate becomes 0.5%, or 0.005 as a decimal.
- Multiply that monthly rate by your current outstanding principal. If you owe $300,000, then $300,000 × 0.005 = $1,500 in interest for the month.
- Subtract the interest from your total principal-and-interest payment to see how much reduces your balance. If the payment is $1,798.65, then $1,798.65 − $1,500 = $298.65 goes to principal.
One detail changes how you read your statement: your payment covers last month’s interest, not next month’s. Residential mortgages charge interest in arrears, so the payment due on the first of the month satisfies interest that accrued over the previous 30 days. That is also why your first regular mortgage payment is typically due a full month after closing.
Per Diem Interest at Closing
Interest between your closing date and the end of that month is handled separately, as “odd-days” or per diem interest. The formula is a variant of the monthly one:
- Multiply the loan amount by the annual interest rate.
- Divide by 365 (or 366 in a leap year) to get the daily cost.
- Multiply by the number of days remaining in the month.
On a $300,000 loan at 6%, the daily rate is about $49.32. Close on the 20th of a 30-day month and you owe roughly $493.20 in per diem interest, which shows up on your Closing Disclosure and is paid at settlement. Close earlier in the month and the upfront charge is larger, but your first regular payment arrives nearly two months later.
Why the Interest Portion Shrinks Each Month
On a fixed-rate loan the monthly principal-and-interest payment never changes, but the split inside that payment shifts dramatically. This is called amortization. Early on, interest consumes most of the payment because the balance is still large. As principal comes down, the monthly interest charge comes down with it, which means more of each payment attacks principal, which pulls the balance down faster still.
Take the same $300,000 loan at 6% over 30 years. In month one, $1,500 of the $1,798.65 payment is interest and $298.65 is principal. By month 180, the halfway point, roughly $900 goes to each. In the final years, nearly the entire payment is principal. Over the full term, total interest paid exceeds $347,000, more than you originally borrowed.
You can watch the shift year to year on IRS Form 1098, which your lender must send if you paid at least $600 in mortgage interest during the year.3Internal Revenue Service. Instructions for Form 1098 (12/2026) Box 1 should get smaller every year.
How the Formula Works on an Adjustable-Rate Mortgage
Adjustable-rate mortgages use the same monthly formula, but the rate itself resets after an initial fixed period, commonly 5, 7, or 10 years. The new rate is built from two pieces: an index that moves with the broader market, and a fixed margin the lender adds on top. Since mid-2023, the Secured Overnight Financing Rate (SOFR) has been the standard benchmark for new ARMs after LIBOR was retired.4Federal Reserve Board. Federal Reserve Board Adopts Final Rule That Implements Adjustable Interest Rate (LIBOR) Act The margin, often two to three percentage points, never changes.
At each adjustment date, the lender adds the current index value to your margin to set the new rate. If SOFR is at 4.3% and your margin is 2.5%, the new rate is 6.8%. Divide by 12, multiply by the remaining balance, and you have the new monthly interest charge. The lender must give you written notice at least 60 days and no more than 120 days before the first payment at the adjusted level is due.5eCFR. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events
Rate Caps Set the Worst-Case Payment
ARMs come with three built-in caps that limit how far the rate can move.6Consumer Financial Protection Bureau. What Are Rate Caps With an Adjustable-Rate Mortgage (ARM), and How Do They Work The initial adjustment cap limits how much the rate can change the first time it resets, often two or five points. The subsequent adjustment cap, typically one or two points, limits each later change. The lifetime cap, most commonly five points above the starting rate, limits the total movement over the life of the loan.
A 5/2/5 cap on a loan starting at 5% means the rate can never exceed 10%, no matter where SOFR goes. Running the monthly formula at the lifetime-cap rate against your expected balance tells you the highest payment you could ever face.
Using the Formula to Pay Less Interest
Because the lender recalculates interest on whatever balance remains, anything that shrinks the balance faster compounds savings over the remaining term. The earlier the dollar goes in, the more future interest it eliminates.
Extra Principal Payments
Small additions make a surprising difference. On a $250,000 loan at 5% over 30 years, adding $50 a month to the regular payment saves roughly $21,000 in total interest and finishes the loan more than two years early. The savings land up front because that $50 removes principal that would otherwise generate interest for decades.
Biweekly Payments
Pay half your monthly amount every two weeks and you make 26 half-payments a year, the equivalent of 13 full monthly payments instead of 12. The one extra payment per year can shave about five years off a 30-year loan and save tens of thousands in interest. Confirm with your servicer that the extra funds go straight to principal rather than sitting until the next due date.
Shorter Terms
A 15-year loan carries a higher monthly payment than a 30-year loan but a lower rate and half the time for interest to accrue. Borrowers who take the 15-year term often pay less than half the total interest they would on a 30-year loan at the same amount.
Check for a Prepayment Penalty First
Before sending extra money, confirm your loan does not charge for early payoff. Federal rules prohibit prepayment penalties on most residential mortgages originated after January 2014. Where a penalty is allowed, it can only apply during the first three years, capped at 2% of the prepaid balance in the first two years and 1% in the third, and the lender must also have offered you an alternative loan without one.7eCFR. 12 CFR 1026.43 – Minimum Standards for Transactions Secured by a Dwelling Loans originated before 2014 can still carry prepayment language in the promissory note, so read it before you accelerate.
What Happens If You Miss a Payment
Interest does not pause when a payment is late. The formula still runs on the outstanding balance each month, and a skipped payment keeps that balance higher, so the next month’s interest charge is calculated on a larger number. That is the compounding effect that makes falling behind so expensive.
Most mortgage contracts also authorize a late fee after a grace period, usually 15 days. For FHA-insured loans, the fee is capped at 4% of the overdue payment.8eCFR. 24 CFR 203.25 – Late Charge Conventional loan late fees generally sit in the same range. If the delinquency continues, the servicer must follow federal loss-mitigation procedures before starting foreclosure, but interest keeps accruing the whole time. Every day you stay behind, the balance the formula runs against is larger than it needs to be.