The basic formula: how your payment splits between principal and interest
Your monthly mortgage payment is divided into two parts: principal (the amount that reduces what you owe) and interest (the lender's fee for lending you the money). The split changes every month, even though your total payment stays the same.
The lender calculates interest first, based on your remaining balance. If you owe $300,000 and your interest rate is 6 percent annually, the lender divides 6 percent by 12 to get the monthly rate (0.5 percent). They multiply your balance by 0.5 percent to find that month's interest charge. Whatever is left over from your payment goes toward principal. Next month, your balance is lower, so the interest charge is smaller, and more of your payment goes to principal.
This is why early payments are mostly interest and later payments are mostly principal — the math works the same way for every mortgage, regardless of the lender or loan type.
Key Takeaways
- Interest is calculated monthly by multiplying your remaining balance by your annual interest rate divided by 12.
- Principal is whatever remains of your payment after the interest charge is subtracted.
- In the first years of a 30-year mortgage, 80 to 90 percent of your payment typically goes to interest; by year 25, that reverses.
- You can see the exact split for each month in an amortization schedule, which your lender provides or which you can generate using an online calculator.
- Making extra principal payments reduces the total interest you pay and shortens the loan term.
Why the split changes every month
The reason the principal-to-interest ratio shifts is that interest is always calculated on the balance you still owe, not the original loan amount. On a $300,000 loan at 6 percent, your first month's interest is $1,500. But after you make your first payment, your balance drops — say to $299,200. Next month, interest is calculated on $299,200, not $300,000, so the interest charge is slightly less.
Over 30 years, this compounds. In month 1, you might pay $1,500 in interest and $300 in principal. By month 360 (the final payment), you pay almost nothing in interest and nearly the full payment in principal. The total payment amount never changes, but the composition does.
This is why paying off a mortgage early saves so much money — you stop paying interest on a balance that would have taken decades to eliminate otherwise.
Reading an amortization schedule
An amortization schedule is a month-by-month table showing your payment, the interest portion, the principal portion, and your remaining balance. Your lender must provide one at closing, and most will email it to you or post it online. You can also generate one using a free calculator on sites like Bankrate or your lender's website.
To read it: find the row for the month you want. The "Payment" column shows your total payment. The "Interest" column shows how much of that payment goes to interest. The "Principal" column shows how much reduces your balance. The "Balance" column shows what you owe after that payment is applied.
If you are considering making extra principal payments, an amortization schedule shows you exactly how much interest you would save. For example, if you pay an extra $200 toward principal in month 1, you can follow the schedule to see how many months shorter your loan becomes and how much total interest you avoid.
The role of interest rate and loan term
Two factors control how much interest you pay overall: your interest rate and your loan term (usually 15, 20, or 30 years). A higher rate means larger monthly interest charges. A longer term means you pay interest for more months, even if each month's charge is smaller.
A 30-year mortgage at 6 percent costs significantly more in total interest than a 15-year mortgage at the same rate, because you are paying interest for twice as long. However, the 15-year payment is much higher each month. A 20-year mortgage splits the difference.
Your lender will show you the total interest you will pay over the life of the loan when you receive your loan estimate. This number assumes you make only the required payment each month and do not pay off the loan early.
How to calculate interest for a single month
If you want to verify the interest charge on your own statement, the math is straightforward. Take your remaining balance, multiply it by your annual interest rate, and divide by 12.
Example: You owe $250,000 with a 5.5 percent interest rate. Multiply $250,000 by 0.055 to get $13,750. Divide by 12 to get $1,145.83. That is your interest charge for that month. If your payment is $1,500, then $1,145.83 goes to interest and $354.17 goes to principal.
This calculation works the same way whether you have a fixed-rate mortgage (rate stays the same) or an adjustable-rate mortgage (rate changes after an initial period). The only difference is that on an ARM, your interest rate itself changes on the adjustment date, which changes your monthly interest charge going forward.
What happens when you make extra principal payments
Any payment above your required monthly amount goes directly to principal (assuming you specify this to your lender — always confirm in writing). Extra principal payments reduce your balance faster, which means less interest accrues in future months.
The earlier you make extra payments, the more interest you save, because you are reducing the balance on which future interest is calculated. An extra $100 in month 1 saves more interest than an extra $100 in month 300, even though the payment amount is identical.
Some borrowers make one extra payment per year (often by paying half the monthly payment every two weeks instead of the full payment once a month). Others round up their payment by $50 or $100 each month. Even small extra amounts add up to years of interest saved over the life of the loan.
The difference between fixed-rate and adjustable-rate mortgages
On a fixed-rate mortgage, your interest rate never changes, so the principal-to-interest split follows the same predictable pattern for the entire loan. Your amortization schedule is accurate from day one through payoff.
On an adjustable-rate mortgage (ARM), your rate is fixed for an initial period (often 3, 5, 7, or 10 years), then adjusts annually or semi-annually based on a market index. When the rate adjusts, your monthly payment changes, and so does the split between principal and interest. Your lender will provide a new amortization schedule after each adjustment.
ARMs typically start with a lower rate than fixed mortgages, which is why the early payments are lower. However, when the rate adjusts upward, your payment can increase significantly. Understanding how interest is calculated becomes more important with an ARM, because your payment and interest charge are not locked in for the full term.
Frequently Asked Questions
Why do I pay so much interest in the first years?
Interest is calculated on your remaining balance each month. Early in the loan, your balance is highest, so the interest charge is largest. As you pay down the balance, the interest charge shrinks and more of your payment goes to principal. This is true for all mortgages and is not a sign of a bad loan.
Can I see how much interest I will pay over the life of my loan?
Yes. Your loan estimate (provided before closing) shows the total interest you will pay if you make only the required payment each month. Your amortization schedule also shows this by adding up all the interest charges. Keep in mind this assumes you do not pay off the loan early or refinance.
What is the difference between principal and balance?
Principal is the amount you pay toward reducing what you owe. Balance is the total amount you still owe after a payment is applied. If you owe $300,000 and pay $500 in principal, your new balance is $299,500.
Does paying extra principal reduce my monthly payment?
No. Your monthly payment amount stays the same (on a fixed-rate mortgage). Extra principal payments shorten the loan term and reduce total interest, but they do not lower the required monthly payment. You would need to refinance to change your monthly payment.
How do I know if my lender calculated interest correctly?
Use the formula: remaining balance × annual interest rate ÷ 12. Compare the result to the interest charge on your statement. If they match, the calculation is correct. Small differences (a few cents) are normal due to rounding.