The basic formula: principal, interest, taxes, and insurance

Your monthly mortgage payment is built from four separate pieces. The lender calculates the payment using your loan amount, interest rate, and loan term. Then your servicer adds property taxes and homeowners insurance, which are held in an escrow account and paid on your behalf. The acronym PITI — principal, interest, taxes, and insurance — describes these four parts.

The principal and interest portion stays the same every month (on a fixed-rate mortgage). The taxes and insurance portions can change year to year, which is why your total payment may shift even though your loan itself does not.

Key Takeaways

  • Principal and interest are calculated using a formula based on your loan amount, interest rate, and the number of months you have to repay the loan.
  • Property taxes and homeowners insurance are added on top and held in escrow, meaning they can change annually even if your loan payment stays the same.
  • The interest portion of your payment is highest at the start of the loan and decreases over time as you pay down the principal.
  • You can calculate your own principal and interest payment using an amortization formula or an online calculator, but your servicer will provide the exact figure.

How principal and interest are calculated

Lenders use an amortization formula to divide your monthly payment between principal (the amount borrowed) and interest (the cost of borrowing). The formula accounts for three inputs: the loan amount, the annual interest rate, and the loan term in months.

On a $300,000 loan at 6.5% interest over 30 years, for example, the monthly principal and interest payment would be roughly $1,896. In your first payment, about $1,625 goes to interest and $271 goes to principal. By payment 360 (the final payment), almost all of it goes to principal because you have paid down the loan so much that interest accrues on a smaller balance.

The exact calculation uses this formula: M = P × [r(1+r)^n] / [(1+r)^n − 1], where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments. You do not need to do this by hand — mortgage calculators on lender websites and third-party sites like Bankrate or NerdWallet will compute it for you.

Why your payment changes even when your loan does not

Property taxes and homeowners insurance are not part of your loan. Instead, your servicer collects a portion of each in escrow — a separate account held in your name — and pays the bills when they come due. Because tax assessments and insurance premiums change, your escrow payment can increase or decrease.

If your county reassesses your home value and raises your property tax, or if your insurer raises rates, your servicer will recalculate your escrow payment and adjust your monthly bill. You will receive a notice showing the new amount. This is separate from any change in your principal and interest payment, which remains fixed on a standard 30-year or 15-year fixed-rate mortgage.

The difference between fixed-rate and adjustable-rate mortgages

On a fixed-rate mortgage, your principal and interest payment never changes. You pay the same amount every month for the entire loan term — whether that is 15, 20, or 30 years. This makes budgeting predictable, though your total payment can still shift if taxes or insurance change.

On an adjustable-rate mortgage (ARM), the interest rate is fixed for an initial period (often 3, 5, 7, or 10 years) and then adjusts periodically based on market conditions. When the rate adjusts, your monthly payment recalculates. An ARM may start with a lower payment than a fixed-rate loan, but the payment can rise significantly after the initial period ends. ARMs are riskier because you cannot predict your payment years ahead.

What happens to your payment over time

In the early years of a 30-year mortgage, most of your payment goes to interest. On that $300,000 loan at 6.5%, your first payment is about 86% interest and 14% principal. By year 15, the split is roughly 50-50. By year 25, you are paying mostly principal.

This front-loaded interest structure is built into the amortization formula. It means you build equity slowly at first, even though you are making full payments. If you sell or refinance in the first five years, you have paid a lot of interest but reduced the principal only modestly.

How down payment and loan term affect your payment

A larger down payment lowers the loan amount, which lowers your monthly payment. A 20% down payment on a $400,000 home means borrowing $320,000 instead of $400,000, reducing your principal and interest payment by roughly $477 per month (at 6.5% over 30 years).

Loan term also changes the payment significantly. A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same loan amount, because you are repaying the principal faster. On that $300,000 loan at 6.5%, a 15-year term costs about $2,899 per month, while a 30-year term costs about $1,896. The 15-year loan costs less in total interest because you pay it off sooner, but the monthly burden is higher.

Where to find your actual payment breakdown

Your lender will provide a Loan Estimate within three business days of your application. This document shows your estimated principal and interest payment, property taxes, homeowners insurance, and any other costs. The numbers are estimates because final tax and insurance amounts may not be known yet.

After closing, your servicer sends a monthly statement showing exactly how much of that month's payment went to principal, interest, taxes, and insurance. Your annual statement (often called a 1098-T for tax purposes) shows the total interest paid that year, which you may be able to deduct on your tax return if you itemize deductions.

Frequently Asked Questions

Can I pay more toward principal to shorten my loan?

Yes. Extra payments go directly to principal and reduce the total interest you pay and the loan term. Some mortgages have prepayment penalties, though these are rare in the United States. Check your loan documents or ask your servicer whether extra payments are allowed without penalty.

Why is my payment higher than the calculator showed?

The calculator likely showed only principal and interest. Your actual payment includes property taxes, homeowners insurance, and possibly mortgage insurance (PMI) if you put down less than 20%. These can add $300 to $800 or more per month depending on your home value and location.

What is mortgage insurance and when do I pay it?

Mortgage insurance protects the lender if you default. It is required when you put down less than 20%. The cost is added to your monthly payment and typically ranges from 0.5% to 1.5% of the loan amount per year, divided into 12 monthly payments. You can request to remove it once your principal balance drops to 80% of the original home value.

Does refinancing change how my payment is calculated?

Refinancing replaces your old loan with a new one, so the calculation starts over. Your new payment depends on the new loan amount, the new interest rate, and the new term. You might refinance to a lower rate (reducing your payment), a shorter term (paying off faster), or to cash out equity (increasing the loan amount and payment).

How do I know if my escrow payment is correct?

Your servicer conducts an escrow analysis once a year and sends you a statement showing estimated taxes and insurance for the coming year. If the analysis shows a shortage or surplus, your servicer adjusts your payment. You can request an analysis anytime if you believe the estimate is wrong.