The basic formula: your loan balance times the interest rate, divided by the number of days in a year
Mortgage interest is calculated by taking the amount you still owe on your loan, multiplying it by your interest rate, and dividing by 365 (or 360, depending on the lender). That gives you the daily interest charge. The lender adds up those daily charges for each month and includes the total in your monthly payment.
This means your interest cost changes every month as your loan balance shrinks. Early in the loan, when you owe the most, you pay the most interest. Later, when you owe less, the interest portion of your payment drops—even though your total monthly payment stays the same.
The math is straightforward, but the timing of when interest gets calculated and when payments are applied matters more than most borrowers realize.
Key Takeaways
- Interest is calculated daily on your remaining loan balance, so paying extra principal early in the loan saves you thousands in total interest.
- Your lender uses either a 365-day or 360-day year to calculate daily interest, which changes the amount slightly—ask your lender which method they use.
- Most of your early payments go toward interest rather than principal, which is why the loan balance drops slowly at first.
- If you pay your mortgage early in the month versus late in the month, the timing affects how much interest accrues before your payment is applied.
Why your interest amount changes every month
When you make a mortgage payment, part of it goes to interest and part goes to principal (the actual loan amount). The interest portion is calculated fresh each month based on what you still owe.
On a $300,000 loan at 6.5% interest, your first month's interest is roughly $1,625. After you make your first payment, your balance drops—let's say to $299,500. The next month's interest is calculated on that smaller number, so it's slightly less. This repeats every month for 30 years.
This is why a mortgage payment schedule (called an amortization schedule) shows the interest and principal portions of each payment. Early payments are mostly interest. Later payments are mostly principal. The total payment amount stays the same, but the split changes.
How lenders calculate daily interest
Most lenders calculate interest daily rather than monthly. They take your loan balance, multiply it by your annual interest rate, and divide by either 365 or 360 to get the daily charge.
A lender using the 365-day method divides by 365. A lender using the 360-day method (sometimes called the "banker's year") divides by 360. The 360-day method results in slightly higher interest because you're dividing by a smaller number. Ask your lender which method they use—it's in your loan documents, usually in the section on how interest is calculated.
Once you know the daily interest charge, the lender multiplies it by the number of days since your last payment to get the interest portion of your next payment. If you pay on the 1st of every month, that's roughly 30 or 31 days. If you pay late, more days have passed, so more interest accrues.
What happens when you pay early or late in the month
Because interest accrues daily, the date you make your payment matters. If you pay on the 5th instead of the 25th, fewer days of interest have accumulated, so less of your payment goes to interest and more goes to principal.
This is why paying early in the month—or making extra payments early in the month—saves you money over the life of the loan. You reduce the principal balance before the next month's interest is calculated, which lowers the interest charge for that month and every month after.
The difference from a single early payment is small. But if you pay 15 days early every month for 30 years, you will pay noticeably less total interest and may shorten your loan by several months.
The difference between interest rate and APR
Your interest rate is the percentage used to calculate interest on your loan balance. Your APR (annual percentage rate) includes the interest rate plus other costs, like origination fees and closing costs, expressed as a yearly rate.
The interest calculation itself uses only the interest rate, not the APR. But the APR is what you should compare when shopping for mortgages, because it shows the true cost of borrowing. Two lenders might offer the same interest rate but different APRs if one charges higher fees.
Why you pay so much interest early on
On a 30-year mortgage, your first payment might be 80% interest and 20% principal. This surprises many borrowers, but it's how the math works: you owe a large balance, and interest is calculated on that balance.
As you pay down the principal, the interest portion shrinks. By year 15, your payment might be 40% interest and 60% principal. By year 25, it might be 10% interest and 90% principal. The total payment stays the same, but the split shifts dramatically.
This is why paying extra principal early in the loan is so powerful. An extra $100 payment in year 1 saves you far more in total interest than an extra $100 payment in year 25, because it reduces the balance that interest is calculated on for the remaining 29 years.
How adjustable-rate mortgages change the calculation
With a fixed-rate mortgage, your interest rate stays the same for the entire loan, so the interest calculation is predictable. With an adjustable-rate mortgage (ARM), your interest rate changes at set intervals—usually after an initial fixed period of 3, 5, 7, or 10 years.
When your rate adjusts, your lender recalculates your monthly payment based on the new rate and your remaining balance. If rates have risen, your payment goes up. If rates have fallen, your payment goes down. The interest calculation method stays the same; only the rate changes.
Frequently Asked Questions
Does paying my mortgage twice a month instead of once change how interest is calculated?
Yes. If you make two smaller payments instead of one large payment, the second payment reduces your balance before the next month's interest accrues, saving you interest. This is called bi-weekly or semi-monthly paying. The savings are modest but real over 30 years.
What's the difference between simple interest and compound interest on a mortgage?
Mortgages use simple interest, not compound interest. Interest is calculated once per month (or daily, then summed monthly) on your current balance. Compound interest would charge interest on interest, which mortgages do not do. This is one reason mortgages are more manageable than some other debts.
If I pay off my mortgage early, do I save on interest?
Yes, significantly. If you pay off a 30-year mortgage in 15 years, you stop accruing interest after 15 years. You will have paid far less total interest because the balance was lower for half the loan term. Check your loan documents for prepayment penalties—most mortgages have none, but some older loans do.
Can I see how much interest I'll pay over the life of my loan?
Yes. Your lender provides an amortization schedule showing every payment, how much goes to interest, and how much goes to principal. You can also use an online mortgage calculator to see the total interest for different loan amounts and rates. The schedule changes if you make extra payments or refinance.
Why does my lender use a 360-day year instead of 365?
The 360-day method is an industry standard that simplifies calculations and slightly favors the lender. It results in about 0.1% higher interest annually. It's legal and disclosed in your loan documents, but you should know about it when comparing offers from different lenders.