Mortgage interest is calculated each month by multiplying your current principal balance by one-twelfth of your annual interest rate. On a $300,000 balance at 6%, that’s $300,000 × (0.06 ÷ 12), or $1,500 of interest for the month. If your full principal-and-interest payment is $1,799, the lender keeps $1,500 as interest and applies the remaining $299 to reducing the balance. Next month the same formula runs again on the slightly smaller balance of $299,701, so the interest charge drops a little and the principal payment grows a little. That single recalculation, repeated every month, is the entire mechanism.
The Three Numbers That Drive the Calculation
You need your current principal balance, your annual interest rate, and your payment frequency. The principal balance is the remaining loan debt on your most recent mortgage statement, not the payoff figure, which can include fees. The annual interest rate is the nominal rate on the promissory note you signed at closing. Payment frequency for residential mortgages is almost always monthly, meaning 12 payments per year.
Two things are easy to confuse with the inputs but don’t belong in this formula. Your interest rate is not your APR. The APR folds in origination fees, mortgage insurance, and other costs to describe the overall cost of borrowing, but the monthly interest charge itself uses only the nominal rate.1Consumer Financial Protection Bureau. 12 CFR 1026.18 Content of Disclosures And if your monthly payment includes an escrow deposit for property taxes and homeowners insurance, that portion isn’t part of the interest math either. Interest is charged only on the loan’s principal balance.
Why the Interest Portion Shrinks Every Month
On a fixed-rate loan, your total payment stays the same for the entire term, but the split between interest and principal changes with every payment. Early on, the balance is large, so most of the payment goes to interest. As the balance shrinks, the interest charge falls and more money flows to principal, which shrinks the balance faster in a self-reinforcing cycle. This is called amortization, and your lender produced a full amortization schedule at closing that maps every payment over the life of the loan.
The effect is stark. On a $300,000 loan at 6% over 30 years, only about $299 of the first payment reduces the debt. By year 15, roughly half of each payment goes to principal. In the final year, almost the entire payment chips away at the balance. Total interest over the full 30 years runs to about $347,000, more than the amount borrowed. Front-loading interest this way is how lenders earn their return even when borrowers sell or refinance after just a few years.
One warning about the shrinking-balance rule: it can run in reverse. If a scheduled payment is too small to cover the interest owed, the unpaid interest is added to the principal balance and the loan actually grows. This is negative amortization, and it can happen with certain adjustable products that permit minimum payments below the full interest charge.2Consumer Financial Protection Bureau. What Is Negative Amortization?
Daily Interest for Closings and Payoffs
The monthly formula covers regular payments, but closings, payoffs, and refinances land on specific calendar dates, so lenders switch to a daily figure. The per diem rate is the annual rate divided by the number of days in the year. Some lenders divide by 365, others by 360, which treats every month as exactly 30 days. On a $300,000 loan at 6%, that works out to about $49.32 per day using 365 or $50.00 per day using 360.3Bank of America Corporation. Explanation of Simple Interest Calculation The 360-day method produces a slightly higher daily rate and pulls slightly more interest.
You’ll see per diem interest most often on the Closing Disclosure when you buy a home. You owe prepaid interest from the closing date through the end of that month. Close on March 10 and you pay 21 days of per diem interest to cover March 10 through March 31; your first full monthly payment then starts May 1.4Consumer Financial Protection Bureau. What Are Prepaid Interest Charges? Per diem figures also appear on payoff statements when you sell or refinance, because the lender has to account for interest through the exact date the loan is retired.
How the Calculation Changes on an ARM
Everything above assumes a fixed rate. On an adjustable-rate mortgage the rate changes on a set schedule, so the monthly interest charge changes with it. A 5/1 ARM holds the initial rate for five years, then adjusts every year after that; a 7/1 works the same way with a seven-year initial period.5Consumer Financial Protection Bureau. Consumer Handbook on Adjustable-Rate Mortgages
At each adjustment, the new rate is the current value of a benchmark index plus a fixed margin your lender set when you applied. The margin never changes; the index moves with the market. The lender adds the two together, then recalculates the monthly payment using the updated rate and the remaining balance.6Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage (ARM), What Are the Index and Margin, and How Do They Work?
ARMs come with caps on how far the rate can move. An initial adjustment cap limits the first change after the fixed period ends, commonly two or five percentage points. A subsequent adjustment cap limits each later change, commonly one or two points. A lifetime cap limits total movement over the life of the loan, most often five points above the starting rate.7Consumer Financial Protection Bureau. What Are Rate Caps With an Adjustable-Rate Mortgage (ARM), and How Do They Work? A 5/1 ARM starting at 5% with a five-point lifetime cap can eventually reach 10%, nearly doubling the monthly interest charge.
How to Cut the Interest You Pay
Because interest is recalculated each month on whatever balance remains, any extra dollar you put toward principal immediately shrinks the base that future interest is charged on. Modest additional payments compound over time.
Extra Monthly Payments
Adding a fixed amount to your regular payment and designating it for principal is the simplest approach. On a $200,000 loan at 4% over 30 years, paying an extra $100 per month toward principal saves more than $26,500 in total interest. Doubling that to $200 extra per month saves over $44,000. Tell your servicer in writing to apply the extra funds to principal, not to the next scheduled payment.
Biweekly Payments
Instead of 12 monthly payments, you make 26 half-payments every two weeks. Twenty-six halves equal 13 full payments, so you make one extra payment per year without noticing. On a $250,000 loan at 5%, that can cut roughly five years off the loan term. Not every servicer offers a formal biweekly program, and some charge a setup fee, so ask before enrolling.
Mortgage Recasting
If a large sum lands in your hands, a recast lets you make a lump-sum principal payment and have your lender recalculate your monthly payment based on the new, lower balance. Your rate and term stay the same, but the required monthly payment drops permanently. A recast avoids the closing costs and credit check of a refinance, though most lenders charge a small administrative fee and require a minimum lump sum.
A Note on Partial Payments
Sending less than a full monthly payment usually doesn’t reduce your balance immediately. Most servicers park partial payments in a suspense or unapplied-funds account until enough accumulates to cover a full installment, and interest keeps accruing on the full balance in the meantime.8Consumer Financial Protection Bureau. 12 CFR 1026.41 Periodic Statements for Residential Mortgage Loans Paying half on the first and half on the fifteenth won’t help unless your servicer credits the partial payments right away. Confirm the handling in writing before relying on a split-payment strategy.
Checking Your Statement and Disputing Errors
Your monthly mortgage statement breaks the payment into principal, interest, and escrow. The interest figure should match the formula (current balance × annual rate ÷ 12) within a few cents. Rounding produces small differences; a gap of more than a dollar or two usually points to a real problem.
If something looks wrong, federal law gives you a formal way to challenge it. Under the Real Estate Settlement Procedures Act, you can send your servicer a written notice of error. The servicer must acknowledge receipt within five business days and either correct the error or explain why it believes none occurred within 30 business days.9Consumer Financial Protection Bureau. 12 CFR 1024.35 Error Resolution Procedures Borrowers who show a servicer violated these requirements can recover actual damages and, in some cases, additional statutory damages and attorney’s fees.10Office of the Law Revision Counsel. 12 U.S. Code 2605 – Servicing of Mortgage Loans and Administration of Escrow Accounts
Each January your servicer also sends Form 1098 showing the total interest paid during the prior year. Compare it against your own running total from monthly statements before tax season; catching a discrepancy early is much easier than fixing it later.