The basic formula: principal, interest, taxes, and insurance
Your house payment has four parts, and lenders add them together to give you one monthly number. The first two parts — principal and interest — go to the bank that lent you the money. Principal is the amount you borrowed; interest is what the bank charges you for lending it. The other two parts — property taxes and homeowners insurance — go to your local government and your insurance company, not the bank.
Most people call the whole monthly payment a "mortgage payment," but technically the mortgage is just the loan itself. The payment includes the loan plus taxes and insurance bundled together. If you know the loan amount, the interest rate, and the loan length (usually 15 or 30 years), you can calculate the principal and interest portion. Then you add your local property tax and insurance costs to get the full picture.
The reason lenders bundle everything together is practical: they want to make sure the taxes and insurance get paid, because unpaid property taxes or a lapsed insurance policy can put the house at risk. So they collect a little extra each month, hold it in an account called an escrow account, and pay those bills on your behalf when they come due.
Key Takeaways
- A house payment combines four separate costs: principal and interest to the lender, plus property taxes and homeowners insurance paid through an escrow account.
- You can calculate the principal and interest portion using the loan amount, interest rate, and loan term (15 or 30 years), or use an online calculator to do it when ready.
- Property taxes vary by location and are based on your home's assessed value, which you can find through your county assessor's office.
- Homeowners insurance costs depend on the home's location, age, and condition, and you should get quotes from at least two insurers before committing.
- Your actual payment may be higher if you put down less than 20 percent, because lenders require mortgage insurance (PMI) until you build enough equity.
Calculating principal and interest with the standard formula
If you want to do this by hand, the formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ]. In plain terms: M is your monthly payment, P is the principal (the amount you borrowed), r is your monthly interest rate (the annual rate divided by 12), and n is the total number of payments (years times 12).
Here is a concrete example. Say you borrow $300,000 at 6.5 percent annual interest over 30 years. Your monthly interest rate is 0.065 divided by 12, which equals 0.00542. Your total number of payments is 30 times 12, which equals 360. Plugging those numbers in gives you a principal and interest payment of roughly $1,896 per month. That is just the loan part — you still need to add taxes and insurance.
Most people do not do this math by hand. Online calculators (sometimes called mortgage calculators) do it when ready and let you change the numbers to see how different loan amounts, rates, or terms affect your payment. The math is always the same; the calculator just saves you the arithmetic.
Finding your property tax amount
Property taxes are set by your county or municipality and are based on the assessed value of your home, not the price you paid for it. The assessed value is often lower than the market value, and it is recalculated periodically — sometimes every year, sometimes every few years, depending on where you live.
To find your property tax, contact your county assessor's office (search online for "[your county] assessor" or "[your county] property appraiser"). They can tell you the assessed value and the tax rate for your area. Tax rates are usually expressed as a percentage or as dollars per $1,000 of assessed value. If your home is assessed at $250,000 and the rate is 1.2 percent, your annual property tax is $3,000, or $250 per month.
Property taxes can change year to year, especially if your home is reassessed or if your local government raises the tax rate. When you are estimating your payment, use the current tax amount, but know that it may go up. Some lenders add a small cushion to your escrow payment to account for this.
Estimating homeowners insurance costs
Homeowners insurance protects your home and belongings if there is a fire, theft, weather damage, or liability (someone gets hurt on your property and sues). Lenders require you to carry it as long as you have a mortgage. The cost depends on your home's location, age, construction type, and condition, plus your chosen deductible (the amount you pay out of pocket before insurance kicks in).
The only way to know what insurance will cost is to get quotes. Contact at least two insurance companies — your current auto insurer often offers homeowners insurance, and there are national carriers like State Farm, Allstate, and GEICO, plus local or regional companies. Give them the same information about the home (address, year built, square footage, number of bedrooms) and ask for quotes with the same deductible. Prices vary significantly, so comparing is worth the time.
Annual homeowners insurance typically ranges widely depending on location and home characteristics, but you should get actual quotes rather than guessing. Once you have a number, divide it by 12 to get your monthly cost, which the lender will add to your escrow payment.
Adding it all together: the complete payment
Once you have the four numbers — principal and interest, property tax, homeowners insurance, and any mortgage insurance (explained below) — add them together to get your total monthly payment. Here is a realistic example:
| Component | Monthly Cost |
| Principal and interest | $1,896 |
| Property tax | $250 |
| Homeowners insurance | $120 |
| Mortgage insurance (PMI) | $150 |
| Total monthly payment | $2,416 |
This is what you would owe the lender each month. The lender collects the full $2,416, keeps the $1,896 for the loan, and sets aside $250 + $120 + $150 = $520 in the escrow account to pay taxes, insurance, and mortgage insurance when those bills come due.
Understanding mortgage insurance (PMI) if you put down less than 20 percent
If you borrow more than 80 percent of the home's value — meaning you put down less than 20 percent — the lender requires you to pay mortgage insurance, usually called PMI (private mortgage insurance). This protects the lender if you stop paying, not you. It is an extra monthly cost that disappears once you have paid down the loan enough to own at least 20 percent of the home's value.
PMI costs vary but typically run between 0.5 and 1.5 percent of the loan amount per year, divided into 12 monthly payments. On a $300,000 loan, that could be $125 to $375 per month. You can ask the lender to remove PMI once you reach 20 percent equity, or in some cases the lender will remove it automatically when you hit that threshold.
If you are putting down less than 20 percent, factor PMI into your payment estimate. Some people save longer to reach 20 percent down specifically to avoid this extra cost; others accept PMI as the trade-off for buying sooner. Either way, knowing the number helps you decide.
Using online calculators to test different scenarios
Once you understand the four components, you can use an online mortgage calculator to see how changes affect your payment. Try different loan amounts, interest rates, or loan terms to see what fits your budget. If a 30-year loan feels tight, see what a 15-year loan costs (the payment is higher, but you pay less interest overall). If the payment is too high, see what a lower purchase price would mean.
Calculators also let you account for a down payment. If you are planning to put down $60,000 on a $300,000 home, you would borrow $240,000, not $300,000. Entering the actual loan amount gives you a realistic number.
Keep in mind that calculators show estimates based on the information you enter. Your actual payment may differ slightly because property taxes and insurance rates change, and the lender may adjust the escrow amount if those costs rise. But a calculator gives you a solid starting point for understanding what you can afford.
Frequently Asked Questions
Does the interest rate change after I lock it in?
No. Once you lock in a rate with the lender, it stays the same for the life of the loan (assuming a fixed-rate mortgage, which is the most common type). Your principal and interest payment never changes. Property taxes and insurance can go up, which raises your total payment, but the loan portion stays fixed.
What is the difference between a 15-year and 30-year mortgage?
A 15-year mortgage has a higher monthly payment but you pay the home off faster and pay much less interest overall. A 30-year mortgage has a lower monthly payment but takes twice as long to pay off and costs significantly more in interest. The choice depends on your budget and how long you plan to stay in the home.
Can I pay extra toward principal to pay off the loan faster?
Yes. Most lenders allow you to pay extra without penalty, and any amount above your required payment goes directly to principal. This shortens the loan term and reduces the total interest you pay. Check your loan documents or ask your lender about their policy on extra payments.
What happens if property taxes or insurance go up?
Your lender adjusts your escrow payment to cover the increase. You may see your total monthly payment go up even though your principal and interest stays the same. The lender should notify you of the change in advance.
Is there a rule of thumb for how much house I can afford?
A common guideline is that your total monthly housing payment (including taxes, insurance, and mortgage insurance) should not exceed 28 percent of your gross monthly income. So if you earn $5,000 per month before taxes, your housing payment should be no more than $1,400. This is a starting point, not a rule — your actual comfort level depends on your other debts and expenses.