How to Calculate Your Monthly Mortgage Payment (Step-by-Step)

You're about to borrow a few hundred thousand dollars, and your lender hands you a monthly payment number. But where does that figure actually come from? Most people accept it without question — and end up confused when their payment doesn't drop the way they expected, or surprised by how slowly their loan balance shrinks.

Understanding how your monthly mortgage payment is calculated takes about five minutes. Once you do, you'll be able to spot errors, run your own scenarios before talking to a lender, and make smarter decisions about loan term, down payment, and refinancing. Here's everything you need to know.

What Makes Up a Monthly Mortgage Payment?

A mortgage payment is not just "the money you owe on your house." It's made up of four distinct components, often abbreviated as PITI:

• Principal — the portion that reduces your actual loan balance. In the early years of a 30-year mortgage, this is a surprisingly small slice of your payment.

• Interest — the lender's charge for lending you money, calculated as a percentage of your remaining balance. This dominates your early payments.

• Taxes — property taxes, collected monthly by your lender and held in escrow, then paid to your local government on your behalf.

• Insurance — homeowner's insurance, also typically escrowed. If your down payment is under 20%, private mortgage insurance (PMI) is added here too.

When mortgage calculators ask for a "monthly payment," they're usually calculating only the principal + interest (P&I) portion — the part tied to your loan terms. Taxes and insurance vary by location and insurer, so they're added separately.

How to Calculate Your Monthly Mortgage Payment — Step by Step

The formula lenders use is called the fixed-rate mortgage payment formula. You don't need to memorise it, but understanding it helps you see exactly what drives your payment up or down.

The formula:

M = P × [r(1+r)^n] ÷ [(1+r)^n − 1]

Where:

• M = your monthly payment

• P = principal loan amount (purchase price minus down payment)

• r = monthly interest rate (annual rate ÷ 12)

• n = total number of payments (loan term in years × 12)

Step 1: Find your loan amount (P)

Subtract your down payment from the purchase price. On a $400,000 home with 10% down ($40,000), your loan amount is $360,000.

Step 2: Convert the annual interest rate to monthly (r)

Divide your annual rate by 12. At 6.8% annual interest, your monthly rate is 6.8% ÷ 12 = 0.5667%, or 0.005667 as a decimal.

Step 3: Calculate total number of payments (n)

For a 30-year loan: 30 × 12 = 360 payments. For a 15-year loan: 15 × 12 = 180 payments.

Step 4: Plug into the formula

M = 360,000 × [0.005667 × (1.005667)^360] ÷ [(1.005667)^360 − 1]

M = 360,000 × [0.005667 × 7.074] ÷ [7.074 − 1]

M = 360,000 × 0.04007 ÷ 6.074

M = $2,375/month (principal + interest only)

Step 5: Add taxes and insurance

For a $400,000 home in a typical US market, property taxes average around $4,000–$6,000/year ($333–$500/month) and homeowners insurance runs $1,200–$2,000/year ($100–$167/month). Add these to get your total monthly housing cost.

Worked Example: A Home Purchase in Austin, Texas

Meet Sarah — 34 years old, buying her first home in Austin. Here are her numbers:

• Purchase price: $420,000

• Down payment: 10% ($42,000)

• Loan amount: $378,000

• Interest rate: 6.9% (fixed, 30-year)

• Property tax rate: 1.8% of home value/year

• Homeowners insurance: $1,800/year

P&I calculation:

Monthly rate = 6.9% ÷ 12 = 0.575% = 0.00575

n = 360

M = 378,000 × [0.00575 × (1.00575)^360] ÷ [(1.00575)^360 − 1]

M = $2,498/month (principal + interest)

Adding PITI:

• Property tax: $420,000 × 1.8% ÷ 12 = $630/month

• Insurance: $1,800 ÷ 12 = $150/month

• PMI (down payment < 20%): ~$190/month

Total monthly payment: ~$3,468

Sarah's lender quoted her $3,450 — close enough, with minor rounding. Now she can verify any lender's quote herself.

Monthly Payment by the Numbers

How much does your interest rate or loan term actually move the needle? More than most buyers expect.

*P&I only. Based on fixed-rate mortgage. Taxes, insurance, and PMI not included.*

A 0.9% difference in interest rate on a $350,000 loan costs you an extra $212/month — or $76,320 over 30 years. This is why shopping lenders and improving your credit score before applying can save you tens of thousands of dollars.

Common Mistakes to Avoid

1. Forgetting about PITI. Calculating only principal and interest and then being blindsided by taxes and insurance at closing is one of the most common first-time buyer mistakes. Always budget for the full payment, not just the P&I figure.

2. Using the purchase price instead of the loan amount. Your payment is based on what you're borrowing — not what you're paying. Always subtract your down payment first.

3. Ignoring PMI. If your down payment is under 20%, PMI typically adds 0.5%–1.5% of the loan amount per year to your payment. On a $350,000 loan, that's $145–$437/month until you reach 20% equity. According to the Urban Institute (2024), over 30% of first-time buyers underestimate this cost.

4. Assuming your payment stays fixed forever. For a fixed-rate mortgage, P&I stays the same. But property taxes and insurance are reassessed annually and can push your total payment up over time — even if your rate doesn't change.

5. Not accounting for HOA fees. If the property is in a planned community or condo building, HOA fees can add $200–$800/month on top of your PITI. These are separate from your mortgage but absolutely part of your housing cost.

The Bottom Line

Your monthly mortgage payment is a formula — not a mystery. Loan amount, interest rate, and loan term are the three variables that drive your P&I payment, and you can run any scenario yourself in seconds. The full housing cost (PITI + PMI) adds taxes, insurance, and sometimes PMI on top of that.

The best move before talking to any lender is to know your number first. Our free Mortgage Calculator gives you the answer in under 30 seconds.