How to Calculate Your Mortgage Payment (Step by Step)

A mortgage payment is not one number. It is usually four, bundled together: principal, interest, taxes and insurance - the PITI your lender keeps mentioning. Lenders and many online quotes show you only the first two, which is why so many first-time buyers get surprised once the first real bill arrives.

This article shows how the principal-and-interest piece is calculated, works a complete $300,000 example, and then adds the rest of the letters so you can estimate the true monthly cost of a house before you offer on it.

The Formula Behind Every Fixed-Rate Mortgage

Banks use one formula for every fixed-rate loan:

M = P × r × (1 + r)^n / ((1 + r)^n - 1)

Where:

• M is the monthly principal-and-interest payment.
• P is the loan amount - the price minus your down payment.
• r is the monthly interest rate, which is the annual rate divided by 12.
• n is the total number of payments - years multiplied by 12.

Nothing exotic. The formula simply splits each payment so that, if you pay every installment exactly, the balance lands on zero with the final check. The one exception is an adjustable-rate mortgage, where the rate - and therefore r - resets at set intervals and the payment is recalculated from the same formula with a new r and a new remaining balance.

Worked Example: $300,000 at 6.5% for 30 Years

Step 1. Find the monthly rate: 6.5% divided by 12 gives r = 0.0054167.

Step 2. Find the number of payments: 30 years × 12 = n = 360.

Step 3. Compute the compounding factor: (1 + r)^n = 1.0054167^360, which is about 6.992.

Step 4. Put it all together:

M = 300,000 × 0.0054167 × 6.992 / (6.992 - 1) = 1,625 × 6.992 / 5.992 ≈ $1,896

So the principal-and-interest payment is about $1,896 a month. If that felt slow, it is - doing it by hand once teaches you what the number is made of, and then you earn the right to use a calculator.

What the Loan Really Costs

Multiply the payment by every payment you will make: $1,896 × 360 payments is roughly $682,600. You borrowed $300,000, which means about $382,600 of that total is interest. Over a 30-year term, interest often costs more than the amount borrowed - seeing that number in print is the first step toward minimizing it.

Why Half a Point of Interest Matters So Much

Run the same $300,000 loan at 6.0% instead of 6.5% and the payment drops to about $1,798.65. That is $97.55 a month, which sounds minor. Over 360 payments it comes to about $35,100 - for half a percentage point.

This is exactly why it pays to collect loan estimates from more than one lender before signing anything. The loan terms are identical on paper; the totals are not.

Your First Payment Is Mostly Interest

Here is the part that surprises people. On the very first month of the 6.5% loan:

• Interest owed: $300,000 × 0.0054167 = $1,625.00
• Principal paid: $1,896 - $1,625 = about $271

Only $271 of your $1,896 actually reduces the balance in month one. Each month the interest slice shrinks slightly and the principal slice grows, so the balance falls faster and faster in later years. This is also why extra principal payments made early in the loan are so powerful - they skip decades of interest that would have been charged on those dollars. An amortization table shows the whole shift payment by payment; the amortization calculator generates the full schedule for any loan in one step.

The Other Letters: Taxes, Insurance and PMI

Principal and interest go to your lender. The rest goes to other parties, and the amounts depend heavily on where you live:

• Property taxes (T). At a 1.1% tax rate on a $300,000 home, that is $3,300 a year, or $275 a month. Rates vary enormously between states and counties, so look up your local rate rather than borrowing a friend’s number.
• Homeowners insurance (I). A common planning figure is $1,200 to $2,000 a year. At $1,500, that is $125 a month.
• Private mortgage insurance (PMI). If your down payment is under 20%, most lenders add PMI, typically 0.5% to 1% of the loan per year. On a $300,000 loan, budget roughly $125 to $250 a month. PMI can usually be removed once you reach 20% equity.
• HOA fees, where they apply, come on top of all of this and almost never appear in advertised payment estimates.

Adding it up for our example: $1,896 (principal and interest) + $275 (taxes) + $125 (insurance) + $188 (PMI at 0.75%) = about $2,484 a month. That is the number your budget actually has to carry - nearly $590 more than the figure the lender leads with.

Shorter Terms Change Everything

The same $300,000 at 6.5% over 15 years instead of 30 costs about $2,613 a month. That is $717 more per month, but the total interest falls to roughly $170,000 - a savings of well over $200,000. There is no universally right answer between a 15- and 30-year term; there is only what fits your income, your other goals and your tolerance for a larger required payment.

Do the Math on Your Own Numbers

Formulas are for understanding; calculators are for deciding. Enter your price, down payment, rate and term in the mortgage calculator to see the estimated monthly payment with taxes and PMI included, then open the amortization schedule to watch each payment split between principal and interest over the years.

This article is educational and uses simplified, rounded examples - your lender’s figures will reflect your exact rate, local taxes and insurance quotes. The method, however, is always the same one above.