Mortgage interest is calculated on the unpaid balance of your loan, using a daily rate that compounds monthly

Your lender divides your annual interest rate by 365 to get a daily rate, then multiplies that by your current loan balance to find how much interest accrues each day. When your monthly payment arrives, the lender applies it first to interest owed, then to principal. Because your balance shrinks with each payment, the interest portion of your payment shrinks too — and the principal portion grows. This is why a 30-year mortgage costs roughly twice the original loan amount, while a 15-year mortgage costs significantly less.

The calculation happens the same way whether your rate is fixed or adjustable. The difference is that a fixed rate stays the same for the life of the loan, while an adjustable rate (ARM) changes on a schedule set in your loan documents — typically every year, every three years, every five years, or every seven years. When the rate adjusts, the lender recalculates your payment using the new rate and your remaining balance.

Key Takeaways

  • Your lender calculates daily interest by dividing your annual rate by 365 and multiplying by your current balance, then compounds it monthly into your payment.
  • Each monthly payment covers interest first, then reduces principal, so the interest portion of your payment decreases over time while the principal portion increases.
  • A fixed-rate mortgage uses the same interest rate for the entire loan term, while an adjustable-rate mortgage (ARM) changes on a schedule you can find in your loan documents.
  • The total interest you pay depends on your rate, loan term, and how much principal you still owe — paying extra toward principal reduces future interest.

How the daily interest rate works

Lenders convert your annual percentage rate (APR) into a daily rate by dividing by 365. If your rate is 6.5 percent, the daily rate is 0.065 ÷ 365, or about 0.0001781 percent per day. That daily rate is then multiplied by your current loan balance to find how much interest accrues that day.

This happens every single day your loan is outstanding. On day one of a $300,000 loan at 6.5 percent, you accrue roughly $54.43 in interest. On day two, if you have not made a payment, you accrue interest on $300,054.43. The interest compounds — meaning you pay interest on the interest — but only when your monthly payment is due. At that point, the lender totals all the daily interest accrued since your last payment and bills you for it.

The timing of your payment matters slightly. If you pay on the first of the month instead of the fifteenth, you accrue 14 fewer days of interest. Over a 30-year loan, that small difference adds up to thousands of dollars in total interest saved.

Why your payment splits between interest and principal

Your monthly payment is fixed (in a fixed-rate loan), but the way it splits between interest and principal changes every month. Early in the loan, most of your payment goes to interest because your balance is highest. Late in the loan, most goes to principal because your balance is lowest.

On a $300,000 loan at 6.5 percent over 30 years, your monthly payment is roughly $1,896. In month one, about $1,625 goes to interest and $271 to principal. In month 360 (the final payment), nearly all $1,896 goes to principal because so little interest accrues on the tiny remaining balance. This is why paying extra toward principal early in the loan saves the most interest — you are reducing the balance that future interest accrues on.

You can see this split on your loan statement or amortization schedule, which your lender provides at closing. The schedule shows every payment for the life of the loan, broken down into interest and principal.

Fixed-rate mortgages versus adjustable-rate mortgages

A fixed-rate mortgage locks in the same interest rate for the entire loan term — 15 years, 30 years, or whatever you agreed to. Your monthly payment never changes (unless you refinance). The interest calculation stays the same every month: daily rate times current balance, compounded monthly.

An adjustable-rate mortgage (ARM) starts with a lower initial rate, usually called the "teaser rate," for a set period — often three, five, seven, or ten years. After that period ends, the rate adjusts based on a formula in your loan documents. The formula typically ties your rate to a market index (like the Secured Overnight Financing Rate, or SOFR) plus a margin set by your lender. When the rate adjusts, your monthly payment recalculates using the new rate and your remaining balance.

ARMs usually include rate caps that limit how much the rate can jump at each adjustment and over the life of the loan. A typical ARM might cap increases at 2 percent per adjustment and 6 percent over the loan's life. Even with caps, your payment can rise significantly when the rate adjusts, so ARMs carry more payment risk than fixed-rate loans.

How extra payments reduce total interest

Any payment above your required monthly amount goes directly to principal (assuming your lender does not hold it in escrow). Reducing principal immediately reduces the balance that future interest accrues on, which compounds into substantial savings over time.

On that $300,000 loan at 6.5 percent, paying an extra $200 per month reduces the loan term from 30 years to about 24 years and cuts total interest paid by roughly $80,000. Paying an extra $500 per month shortens the term to about 20 years and saves roughly $150,000 in interest. The earlier you make extra payments, the more interest they save, because the balance is higher and the remaining term is longer.

Before making extra payments, check your loan documents for prepayment penalties. Most mortgages have none, but some older loans or loans with special features (like certain ARM products) may charge a fee if you pay off the loan early. Your lender can tell you whether your specific loan has a prepayment penalty.

What happens when rates adjust on an ARM

When an adjustable rate resets, your lender recalculates your monthly payment using three pieces of information: the new interest rate, your remaining loan balance, and the remaining loan term. The new payment is then set for the next adjustment period.

If rates have risen, your payment rises. If rates have fallen, your payment falls. Some ARMs include a floor (a minimum rate you will never go below) and a ceiling (a maximum rate you will never exceed), so even if market rates drop significantly, your rate may not fall as far as you might expect.

You can find the adjustment schedule and rate caps in your Adjustable Rate Note or ARM disclosure, documents you received at closing. These spell out exactly when your rate adjusts, what index it is tied to, what margin your lender adds, and what the caps are. If you cannot find these documents, your lender can provide copies.

How to estimate your total interest cost

To estimate total interest, multiply your monthly payment by the number of months in your loan term, then subtract the original loan amount. On that $300,000 loan at 6.5 percent over 30 years, the monthly payment is roughly $1,896. Over 360 months, you pay $682,560 total. Subtract the original $300,000, and you owe roughly $382,560 in interest.

Online mortgage calculators let you adjust the loan amount, rate, and term to see how each changes total interest. Many lender websites offer calculators, and sites like Bankrate and Mortgage Calculator also provide them. These tools are useful for comparing scenarios — for instance, seeing how much interest you save by choosing a 15-year term instead of 30 years, or by putting down 20 percent instead of 10 percent.

Keep in mind that these estimates assume you make only the required monthly payment and do not refinance. If you make extra payments or refinance to a lower rate partway through, your actual total interest will be lower.

Frequently Asked Questions

Does my interest rate include the APR?

Your interest rate and your APR are related but not identical. The interest rate is the percentage used to calculate daily interest on your balance. The APR includes the interest rate plus other costs like origination fees, closing costs, and mortgage insurance, expressed as an annual rate. Lenders must disclose both on your Loan Estimate and Closing Disclosure.

Can I pay interest-only for part of my loan?

Some loans allow an interest-only period, usually at the start of an ARM, where your payment covers only interest and no principal for a set time — often five or seven years. After that period ends, your payment recalculates to cover both interest and principal over the remaining term, which causes your payment to jump. Interest-only periods are less common now than they were before 2008, but some lenders still offer them.

What is the difference between simple interest and compound interest on a mortgage?

Mortgages use daily simple interest that compounds monthly. Each day, interest accrues on your current balance at the daily rate. When your payment is due, all that daily interest is added together (compounded) into one monthly charge. This is different from true compound interest, where interest accrues on interest continuously throughout the month.

If I make a payment mid-month, does it reduce my next month's interest?

Yes. Any payment you make reduces your balance immediately, so interest accrues on the lower balance from that point forward. A mid-month payment saves you interest for the rest of that month and reduces the balance that interest accrues on in the following month. This is why paying biweekly instead of monthly saves interest over time.

How does refinancing change my interest calculation?

Refinancing replaces your old loan with a new one. The new loan has its own interest rate, term, and balance (usually your current remaining balance plus closing costs). Interest calculation starts over using the new rate and term. If you refinance from a 30-year loan to a 15-year loan at a lower rate, your new payment will likely be higher, but you will pay significantly less total interest because the term is shorter and the rate is lower.