The basic formula for a house payment
Your monthly house payment is built from four parts: principal, interest, property taxes, and homeowners insurance. The acronym is PITI. Principal and interest are calculated together using a fixed formula; taxes and insurance are added on top based on your location and policy.
The principal-and-interest portion uses three numbers: the loan amount (what you borrowed), the interest rate (what the lender charges), and the loan term (how many months you have to pay it back). Plug those into the standard amortization formula, and you get a number that stays the same every month for a fixed-rate mortgage. Adjustable-rate mortgages recalculate when the rate changes.
Property taxes and insurance are not part of the loan itself. Your lender estimates them, adds them to your principal-and-interest payment, and holds the total in escrow—a separate account the lender controls. When your tax bill or insurance premium comes due, the lender pays it from that account on your behalf.
Key Takeaways
- Principal and interest are calculated using the loan amount, interest rate, and loan term; this portion stays the same every month on a fixed-rate mortgage.
- Property taxes and homeowners insurance are estimated by your lender, added to your payment, and held in an escrow account until they are due.
- You can calculate principal and interest yourself using an online amortization calculator or the standard mortgage formula.
- Your actual monthly payment will be higher than principal and interest alone because taxes and insurance are included.
- PMI (private mortgage insurance) is added to your payment if you put down less than 20 percent; it drops off once you reach 20 percent equity.
How to calculate 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 principal (loan amount), r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years times 12).
Example: You borrow $300,000 at 6.5 percent annual interest over 30 years. Your monthly rate is 0.065 ÷ 12 = 0.00542. Your total payments are 30 × 12 = 360. Plugging into the formula gives you roughly $1,896 per month for principal and interest alone.
You do not have to do this by hand. Every mortgage lender provides an amortization schedule—a table showing how much of each payment goes to principal versus interest over the life of the loan. Early payments are mostly interest; later payments shift toward principal. You can also use a free online mortgage calculator; enter the loan amount, rate, and term, and it calculates the payment instantly.
Adding property taxes to your payment
Property taxes vary by county and municipality. Your lender estimates your annual tax bill, divides it by 12, and adds that amount to your monthly payment. If your home is assessed at $400,000 and your local tax rate is 1 percent of assessed value, your annual tax is $4,000, or about $333 per month.
The estimate is not always exact. If your home is reassessed or the tax rate changes, your lender recalculates and adjusts your payment. Some states reassess every year; others do it less often. You can find your current property tax rate and assessment through your county assessor's office website.
If you pay off your mortgage early, you still owe property taxes—they do not disappear. Taxes are a lien on the property, meaning the government can foreclose if you do not pay them, regardless of whether you have a mortgage.
Adding homeowners insurance to your payment
Homeowners insurance protects the structure of your home and your belongings. Your lender requires it as a condition of the loan and estimates the annual premium, divides it by 12, and adds it to your payment. The estimate depends on your home's age, location, construction type, and the coverage limits you choose.
Insurance premiums change when you renew your policy, usually every year. If your premium goes up, your lender recalculates your escrow payment and raises your monthly bill. If it goes down, your payment drops. You can shop for insurance independently; your lender does not choose your insurer, and you can switch at renewal without penalty.
If you have a mortgage, you cannot skip insurance. If your policy lapses, the lender will buy a force-placed policy on your behalf and charge you for it—usually at a much higher rate than you would pay on your own.
Understanding PMI and how it affects your payment
Private mortgage insurance (PMI) is an extra fee added to your payment if you put down less than 20 percent. It protects the lender, not you, in case you default. The cost varies by loan amount, down payment percentage, and credit score, but typically ranges from 0.5 to 1.5 percent of the loan amount per year.
Example: You borrow $300,000 with 10 percent down. PMI might cost $150 to $450 per month. Once you reach 20 percent equity in the home—either by paying down the principal or by the home appreciating in value—you can request that PMI be removed. Some loans remove it automatically once you hit that threshold.
PMI is not the same as homeowners insurance. It is a separate line item on your payment, and it does not cover your home or belongings. It is purely a lender protection. If you are paying PMI, ask your lender when you can stop—the rules vary by loan type.
How interest rates change your payment
A higher interest rate means a higher monthly payment. On a $300,000 loan over 30 years, the difference between 5 percent and 7 percent is roughly $400 per month—$1,610 versus $2,010. That $400 difference compounds over 360 payments, adding up to $144,000 more in total interest paid.
Fixed-rate mortgages lock in your rate for the entire loan term. Your principal-and-interest payment never changes, even if market rates rise. Adjustable-rate mortgages (ARMs) start with a lower rate for a set period—often 3, 5, 7, or 10 years—then adjust annually or semi-annually based on a market index. When the rate adjusts, your payment recalculates and usually goes up.
Your interest rate depends on your credit score, down payment, loan term, and current market conditions. A higher credit score typically gets you a lower rate. A larger down payment also lowers your rate slightly. Shorter terms (15 years instead of 30) have lower rates but higher monthly payments.
What happens when taxes or insurance estimates are wrong
Your lender estimates taxes and insurance at closing, but those estimates can be off. If your actual taxes or insurance costs more than estimated, your lender will increase your escrow payment to make up the difference. If they cost less, your payment may drop, or you may receive a refund.
Lenders are required to review your escrow account annually. If there is a shortage—meaning the account does not have enough to cover upcoming bills—they raise your payment. If there is a surplus, they either lower your payment or send you a check, depending on the size of the surplus and your lender's policy.
You can request an escrow analysis at any time if you believe the estimate is significantly wrong. Provide your lender with updated tax assessments or insurance quotes, and they will recalculate. This is especially important if you have made home improvements that increased your assessed value or if you switched to a cheaper insurance policy.
Using online calculators versus doing it yourself
An online mortgage calculator is the fastest way to see what your payment will be. Enter the loan amount, interest rate, loan term, and estimated property taxes and insurance, and the calculator shows your total monthly payment instantly. Most calculators also show you an amortization schedule, breaking down how much of each payment goes to principal versus interest.
Doing the math yourself using the amortization formula is more work but gives you the same answer for principal and interest. The advantage of calculating it yourself is understanding exactly how the formula works and seeing how small changes in rate or term affect your payment. For taxes and insurance, you need actual numbers from your county assessor and insurance quotes—no formula can replace those.
Your lender will provide a Loan Estimate within three business days of your application. This document shows your estimated principal, interest, taxes, insurance, PMI (if applicable), and total monthly payment. It is the most accurate estimate you will get before closing, because it is based on the actual property and your actual loan terms.
Frequently Asked Questions
Does my down payment affect my monthly payment?
Yes, in two ways. A larger down payment means you borrow less, so your principal-and-interest payment is lower. It also gets you to 20 percent equity faster, which means PMI drops off sooner. A down payment of 20 percent or more eliminates PMI entirely from day one.
What is the difference between a 15-year and 30-year mortgage payment?
A 15-year mortgage has a higher monthly payment but much lower total interest. On a $300,000 loan at 6 percent, a 15-year payment is roughly $2,000 per month; a 30-year payment is roughly $1,800. Over the life of the loan, you pay about $120,000 less in interest with the 15-year term, but your monthly cash flow is tighter.
Can my house payment go down after I close?
Yes, if property taxes or insurance premiums decrease, or if you reach 20 percent equity and PMI is removed. Your principal-and-interest payment on a fixed-rate mortgage never changes, but the tax and insurance portions can fluctuate. If your home value drops significantly, you may be able to request a tax reassessment, which could lower your property tax bill.
What if I want to pay extra toward principal?
Most mortgages allow you to make extra principal payments without penalty. Any amount you pay above your regular monthly payment goes directly to principal, reducing the loan balance and the total interest you pay over the life of the loan. Check your loan documents or ask your lender whether there are any restrictions on extra payments.
How do I know if my escrow payment is correct?
Request an escrow analysis from your lender. They will compare what they estimated for taxes and insurance against what you actually owe, and adjust your payment if needed. You can also get your own property tax assessment from your county assessor and shop for insurance quotes to see if the lender's estimates are in the ballpark.