The basic formula: your loan balance times the interest rate, divided by the number of days in a year
Mortgage interest is calculated daily on the outstanding balance you owe. The lender takes your remaining loan balance, multiplies it by your annual interest rate, and divides by 365 (or sometimes 360, depending on the lender's method). That gives you the interest charge for one day. When you make a monthly payment, that daily amount is multiplied by the number of days in that month, and that is the interest portion of your payment.
This means your interest cost changes every month because your loan balance shrinks with each payment. Early in the loan, most of your payment goes toward interest. Later, most goes toward principal. A $300,000 mortgage at 6.5% might cost you $1,625 in interest alone in month one, but only $1,200 by month 60, even though your monthly payment stays the same.
Key Takeaways
- Interest is calculated daily on your remaining balance, so the amount you owe in interest changes every month as your principal decreases.
- Your monthly payment is fixed, but the split between interest and principal shifts over time — early payments are mostly interest, later payments are mostly principal.
- The lender uses either a 365-day year or a 360-day year to calculate daily interest, and this choice affects your total cost slightly.
- Paying extra principal reduces the balance immediately and cuts the total interest you will pay over the life of the loan.
- Your interest rate is locked in at closing for a fixed-rate mortgage, but adjustable-rate mortgages (ARMs) recalculate interest at set intervals.
Why your payment stays the same but the interest portion shrinks
When you sign a mortgage note, the lender calculates an amortization schedule — a table showing exactly how much principal and interest you will pay each month for the entire loan term. Your total payment amount never changes (on a fixed-rate mortgage), but the recipe inside that payment changes constantly.
In month one of a 30-year, $300,000 loan at 6.5%, you might pay $1,625 in interest and $296 in principal. By month 120, you might pay $900 in interest and $1,021 in principal. By month 360 (the final payment), you pay almost nothing in interest and nearly the full payment in principal. The amortization schedule is designed so that by the final payment, the loan is paid off completely.
This front-loaded interest structure is why paying extra principal early in the loan saves you the most money. A single extra $100 payment in year one reduces your balance by $100 and saves you roughly $200 in interest over the remaining 29 years (depending on your rate). The same $100 payment in year 25 saves you much less.
The difference between 360-day and 365-day interest calculations
Some lenders calculate daily interest using a 360-day year (called the "ordinary interest" or "banker's year" method), while others use 365 days. This is a small but real difference in how much you pay.
Using 360 days makes each day's interest slightly higher, because you are dividing the annual rate by a smaller number. On a $300,000 loan at 6.5%, the difference between the two methods amounts to roughly $50 to $100 per year, or about $1,500 to $3,000 over a 30-year loan. Your loan documents should state which method your lender uses. Most conventional mortgages use 365 days, but some portfolio loans and certain lenders use 360.
How adjustable-rate mortgages recalculate interest differently
With a fixed-rate mortgage, your interest rate stays the same for the entire loan term, so your interest calculation method never changes. With an adjustable-rate mortgage (ARM), the interest rate resets at specific intervals — often every 3, 5, 7, or 10 years — based on a market index plus a margin the lender adds.
When your ARM rate adjusts, the lender recalculates your monthly payment using the new rate and your remaining balance. Your payment usually increases (though it can decrease if rates fall). The interest calculation itself works the same way — daily balance times rate divided by days in the year — but the rate itself changes, which changes how much interest you owe each month.
ARM documents specify caps on how much your rate can rise at each adjustment and over the life of the loan. A 5/1 ARM might cap increases at 2% per adjustment and 6% over the life of the loan. Understanding these caps matters because they determine your worst-case payment scenario.
What happens to interest when you pay extra principal
Every dollar of extra principal you pay reduces your loan balance immediately. The next month's interest is calculated on the lower balance, so you pay less interest that month. Over the life of the loan, this compounds.
If you pay an extra $200 per month on a $300,000, 30-year mortgage at 6.5%, you will pay off the loan in roughly 23 years instead of 30, and you will save approximately $80,000 in total interest. The lender must apply your extra payment to principal, not to future interest — this is required by law. Some lenders allow you to specify "principal only" payments, which makes the application explicit.
Interest calculations during the first month and at closing
Your first mortgage payment works differently from the rest. At closing, you sign documents and receive the loan funds, but closing usually happens mid-month. The lender calculates interest from the closing date to the end of that month and collects it at closing or rolls it into your first payment.
This is called a "per diem" interest charge — interest for the partial month. If you close on the 15th of a 30-day month, you owe interest for 16 days (including closing day). The lender calculates this using the daily interest formula: loan amount times annual rate, divided by 365, times the number of days. This amount appears on your Closing Disclosure as "Prepaid Interest" or "Interest Paid at Closing."
Your first full monthly payment (due 30 days after your closing date) then covers the next full month of interest, calculated the normal way. This is why your first payment date is usually about 45 days after closing, not 30.
How to read the interest breakdown in your mortgage statement
Your monthly mortgage statement shows your payment amount, the date it was received, and a breakdown of how much went to principal and how much to interest. It also shows your remaining balance after that payment.
The interest shown is always the interest accrued during the previous month, calculated on the balance you owed at the start of that month. If you made an extra principal payment last month, this month's interest will be slightly lower. Over time, as your balance shrinks, the interest portion of your regular payment shrinks too, and the principal portion grows.
Some statements also show year-to-date interest paid, which is useful for tax purposes if you itemize deductions. Mortgage interest is deductible on loans up to $750,000 of principal, though you must itemize rather than take the standard deduction for this to benefit you.
Frequently Asked Questions
Why do I pay so much interest in the early years?
Interest is calculated on your remaining balance each month. Early on, your balance is at its highest, so the daily interest charge is largest. As you pay down principal, the balance shrinks, and interest shrinks with it. This is true for all amortizing loans and is built into the payment schedule from day one.
Can I change how my interest is calculated?
No. The calculation method — daily balance times annual rate divided by days in the year — is set by your loan documents and by law. You cannot negotiate a different method. You can only reduce the total interest you pay by paying extra principal or refinancing to a lower rate.
What is the difference between APR and the interest rate on my mortgage?
Your interest rate is the percentage used to calculate daily interest on your balance. Your APR (Annual Percentage Rate) includes the interest rate plus closing costs, expressed as an annual rate. The APR is higher than the interest rate and is meant to show the true cost of borrowing. Your monthly payment is based on the interest rate, not the APR.
If I pay my mortgage bi-weekly instead of monthly, do I save interest?
Yes, but only slightly. Bi-weekly payments mean you make 26 half-payments per year instead of 12 full payments, which equals 13 full payments per year instead of 12. That extra payment per year goes entirely to principal and reduces your total interest cost and loan term. The savings are real but modest — roughly 3 to 5 years off a 30-year loan.
Does refinancing reset my interest calculation?
Refinancing replaces your old loan with a new one, so yes, the calculation resets. You get a new interest rate, a new amortization schedule, and a new per diem interest charge at closing. If you refinance a 25-year-old mortgage back to 30 years, you restart the amortization process and pay more total interest, even if your new rate is lower.