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

Your monthly mortgage payment is built from four parts, often called PITI: principal, interest, taxes, and insurance. The lender calculates principal and interest using a fixed formula based on your loan amount, interest rate, and how many months you have to repay it. Taxes and insurance are added on top, and those amounts change based on your home's value and location.

The principal and interest portion stays the same every month for a 30-year or 15-year fixed-rate mortgage. The taxes and insurance portions can shift annually or when your home is reassessed, which is why your total payment may creep up even though your loan balance is shrinking.

Key Takeaways

  • Principal and interest are calculated using loan amount, interest rate, and loan term; this portion never changes on a fixed-rate mortgage.
  • Property taxes and homeowners insurance are added to your principal and interest payment, and these can increase each year.
  • You can calculate your own principal and interest using the standard amortization formula or use an online calculator with your loan details.
  • Early in the loan, most of your payment goes to interest; later, most goes to principal — this split is determined by the amortization schedule.
  • If you put down less than 20 percent, mortgage insurance (PMI) is added to your payment until you reach 20 percent equity.

How principal and interest are calculated

Lenders use the amortization formula to divide your monthly payment between principal and interest. The formula is: M = P × [r(1 + r)^n] / [(1 + r)^n − 1], where M is your monthly payment, P is the loan amount, r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments.

You do not need to do this math yourself. Your lender provides an amortization schedule that shows exactly how much of each payment goes to interest and how much reduces your loan balance. In the first months, nearly all your payment is interest. By the end of the loan, nearly all is principal. This is why paying extra toward principal early in the loan saves you the most money.

For example, on a $300,000 loan at 6.5 percent over 30 years, your principal and interest payment is roughly $1,896 per month. In month one, about $1,625 goes to interest and $271 to principal. In month 360 (the last payment), almost all $1,896 goes to principal.

Where property taxes fit in

Property taxes are set by your county or municipality and are based on your home's assessed value, not its purchase price. The tax rate varies widely — from less than 0.5 percent of home value in some states to over 2 percent in others. Your lender collects one-twelfth of your annual tax bill each month and holds it in an escrow account, then pays the full bill when it is due.

When your home is reassessed (usually every one to three years), your tax bill may rise, and your monthly payment rises with it. Some states cap how much taxes can increase in a single year, but others do not. If you dispute your assessment, you can file an appeal with your county assessor, though the process and timeline vary by location.

How homeowners insurance is added

Your lender requires homeowners insurance to protect the house as collateral. You buy the policy from an insurance company, and your lender collects the premium each month through escrow, just like taxes. The premium depends on your home's replacement cost, your location (flood risk, crime rate, weather exposure), and the coverage limits you choose.

Insurance premiums can increase when you file a claim, when your home ages, or when your insurer raises rates across a region. You can shop for a new policy every year or two to find better rates. If your current insurer drops you or raises rates sharply, you may be able to move to a different company without penalty.

Mortgage insurance (PMI) and when it applies

If you put down less than 20 percent, your lender adds private mortgage insurance (PMI) to your monthly payment. PMI protects the lender if you default; it does not protect you. The cost is usually 0.5 to 1.5 percent of your loan amount per year, divided into 12 monthly payments.

PMI stays on your loan until you reach 20 percent equity (the point where your loan balance is 80 percent of the home's value). You can request removal once you hit that mark, though some lenders remove it automatically. If you refinance later and your home has appreciated, you may be able to drop PMI sooner.

How adjustable-rate mortgages change the formula

With an adjustable-rate mortgage (ARM), your interest 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 recalculates using the new rate, and your payment can rise significantly.

The adjustment is capped by rate caps built into your loan agreement — a per-adjustment cap (how much the rate can move at one adjustment) and a lifetime cap (the highest rate you can ever pay). Even with caps, a payment can jump hundreds of dollars per month. If you have an ARM, know your adjustment date and rate caps so you are not surprised.

Using a calculator versus doing the math yourself

Online mortgage calculators let you enter your loan amount, interest rate, and term to see your principal and interest payment instantly. Many also let you add estimated taxes and insurance to see your full PITI payment. These calculators use the same amortization formula lenders use, so the result is accurate for comparison purposes.

If you want to understand the math, you can work through the amortization formula by hand or use a spreadsheet. Most people find a calculator faster and clearer. What matters is understanding that your payment is not arbitrary — it is built from specific numbers that you can see and verify.

Frequently Asked Questions

Why does my payment go up if my interest rate is fixed?

Your principal and interest payment stays the same, but property taxes or insurance can increase. Taxes rise when your home is reassessed; insurance rises when your insurer raises rates or you file a claim. These are added to your fixed principal and interest payment, so your total PITI can climb even though your loan terms have not changed.

Can I pay off my mortgage faster by paying extra toward principal?

Yes. Any payment above your required monthly amount goes directly to principal (after you tell your lender to apply it that way). Paying extra shortens your loan term and saves you thousands in interest. Paying an extra $100 or $200 per month can cut years off a 30-year loan, though your lender may charge a fee if you pay off the entire loan early.

What is the difference between my interest rate and my APR?

Your interest rate is the cost of borrowing the principal. Your APR (annual percentage rate) includes the interest rate plus other costs like origination fees, closing costs, and PMI, expressed as a yearly rate. The APR is higher than the interest rate and gives you a fuller picture of what the loan actually costs.

Does my payment include property taxes and insurance?

Only if your lender collects them through escrow. Most lenders require this, so taxes and insurance are rolled into your monthly payment. Some lenders offer loans without escrow, meaning you pay taxes and insurance separately. Ask your lender whether escrow is required or optional on your loan.

What happens to my payment if I refinance?

Refinancing replaces your old loan with a new one, so your payment recalculates based on the new loan amount, interest rate, and term. You might refinance to a lower rate (lowering your payment), a shorter term (raising your payment but saving interest), or to cash out equity (raising your payment). Your new lender will provide a new amortization schedule showing your new payment breakdown.