How to Work Out Your Monthly Mortgage Payment, Step by Step

Your monthly mortgage payment comes from one formula: M = P × r(1 + r)^n ÷ ((1 + r)^n − 1). On a $320,000 loan at 6.5% for 30 years, that gives you $2,022.62 in principal and interest. Add property tax and insurance, and the real bill climbs to about $2,564.

How to calculate your monthly mortgage payment refers to the process of turning a loan amount, interest rate, and loan term into a fixed monthly bill. The standard formula covers principal and interest only. Property taxes, homeowners insurance, and PMI get added on top.

This works best for a 30-year fixed-rate loan on a home you plan to live in. It won’t help if you’re pricing an adjustable-rate or interest-only loan, because those payments change over time.

Let’s walk through every number.

What Goes Into Your Monthly Mortgage Payment?

A monthly mortgage payment includes principal, interest, property taxes, and homeowners insurance, often called PITI. According to the Consumer Financial Protection Bureau, lenders frequently collect taxes and insurance in an escrow account alongside your loan payment, and private mortgage insurance (PMI) usually joins the bill when your down payment is under 20%.

Principal is the part that shrinks your loan balance. Interest is the fee for borrowing. Taxes and insurance flow into an escrow account, which your servicer uses to pay those bills on your behalf when they come due each year.

Most buyers picture a mortgage payment as a single charge. Or maybe I should say it this way: it’s one charge wrapped around four separate costs that all move at different speeds, which is why a quick online number rarely matches the lender’s.

Take the shortcut many first-time buyers try, which is loan amount times the rate, divided by 12, then stop there and call it your payment. On our $320,000 loan at 6.5%, it returns $1,733.33. It’s only the first month’s interest, so it isn’t a payment, and you’ll feel the gap when the lender’s quote arrives.

Buyers who’ve tried that shortcut often end up with a quote hundreds of dollars higher than they expected.

The Mortgage Payment Formula, Step by Step

The monthly mortgage payment formula is M = P × r(1 + r)^n ÷ ((1 + r)^n − 1), where P is the loan amount, r is the monthly interest rate, and n is the number of payments. For a $320,000 loan at 6.5% over 30 years, M equals $2,022.62 a month.

To calculate your monthly mortgage payment, follow these steps:

1. Subtract your down payment from the home price to get the loan amount.
2. Divide your annual interest rate by 12 to get the monthly rate.
3. Multiply the loan term in years by 12 for total payments.
4. Plug all three numbers into the formula.
5. Add monthly taxes, insurance, and PMI for the full payment.

By Hand: A $400,000 Home With 20% Down

Here’s the thing: the formula looks scarier than it is. You need five numbers and a calculator with an exponent key, and the whole process takes about two minutes once you’ve done it once.

• Home price: $400,000
• Down payment (20%): $80,000
• Loan amount (P): $320,000
• Monthly rate (r): 6.5% ÷ 12 = 0.0054167
• Number of payments (n): 30 × 12 = 360

Now raise (1 + r) to the 360th power. That gives about 6.9918. The top of the fraction becomes 0.0054167 × 6.9918 = 0.037872, and the bottom becomes 6.9918 − 1 = 5.9918. Divide the top by the bottom to get 0.0063207, then multiply by your $320,000 loan.

Your principal and interest: $2,022.62 a month.

Rounding the monthly rate too early can shift the answer by a few cents, so keep at least six decimals.

The Same Math in Google Sheets or Excel

Calculating a mortgage payment in Excel or Google Sheets takes one function, PMT. According to Microsoft’s Excel documentation, PMT returns the payment for a loan with constant payments and a constant interest rate. Enter =PMT(rate/12, years*12, -loan) and the sheet returns your monthly principal and interest payment.

=PMT(6.5%/12, 30*12, -320000)

The minus sign in front of the loan matters. Without it, the function returns a negative payment, which trips up a lot of first-time users. To add the rest of the bill, put the loan in B1, the annual rate in B2, the years in B3, yearly property tax in B4, and yearly insurance in B5, then use this:

=PMT(B2/12, B3*12, -B1) + B4/12 + B5/12

With $320,000, 6.5%, 30 years, $4,400, and $2,100, the cell shows $2,564.29.

Quick Comparison

By hand vs spreadsheet: doing it by hand is better suited for learning how a payment is built because every step is visible. A spreadsheet works better when you want to test several home prices or rates quickly. The key difference is transparency versus speed.

Add Taxes, Insurance and PMI to See Your Real Payment

Principal and interest are only part of the bill, so run the same loan through a fuller budget. The example assumes property tax of 1.1% of the home’s value, homeowners insurance of $2,100 a year, and no HOA. These are illustrations, not financial advice or a lender quote. Yours will differ.

Taxes, Insurance and HOA Dues

Divide each yearly cost by 12. Tax runs $4,400 ÷ 12 = $366.67 a month, and insurance runs $2,100 ÷ 12 = $175. HOA dues, if your home has them, are usually paid straight to the association rather than through escrow, but they still count toward what you can afford.

What most guides skip is that escrow isn’t fixed. Property taxes get reassessed and insurance premiums reset, so a fixed-rate loan can still produce a payment that rises.

PMI and the Down-Payment Trade-Off

Look, if you’re deciding between 10% and 20% down, here’s what actually works: run both versions side by side before you talk to any lender.

At 10% down, the same house costs $492.82 more each month, even though you put $40,000 less down.

Same house. Very different monthly bill.

PMI is the line that surprises people. I’ve seen conflicting data on what it costs, some sources say 0.3% to 1.5% of the loan per year, others quote closer to 2% for lower credit scores. My read is that 0.5% to 1% is a sensible planning range for buyers with decent credit. The example uses 0.8%.

Some experts argue you should always put 20% down to skip PMI. That’s valid for buyers with cash to spare. But if reaching 20% would drain your emergency fund, paying PMI for a while can be the safer trade.

Federal law helps here. The Homeowners Protection Act of 1998 lets you request PMI cancellation at 80% loan-to-value and requires automatic termination at 78% on most conventional loans.

Why Are Your First Payments Mostly Interest?

Most people assume their first payment mostly shrinks the loan. The data says otherwise: about 86% of month one goes to interest. Your first payment splits into $1,733.33 of interest and just $289.29 of principal, because interest is charged on the full balance every single month.

That’s amortization. In a spreadsheet, =IPMT(6.5%/12, 1, 360, -320000) returns the month-one interest, and =PPMT(6.5%/12, 1, 360, -320000) returns the principal.

After five years, you’ve paid about $121,000 and cut the balance by only around $20,400. Over the full 30 years, interest alone totals about $408,143, which is more than the $320,000 you borrowed. That’s why extra principal payments early in the loan carry so much weight.

Quick Answers About Monthly Mortgage Payments

Short answers for the questions buyers ask out loud.

What’s the best way to calculate a monthly mortgage payment?

Use the PMT function in Google Sheets or Excel for speed, and the formula by hand to check it. Then add monthly property tax, homeowners insurance, and PMI to get your full payment.

How do I calculate my mortgage payment with taxes and insurance?

Calculate principal and interest first, then divide your annual property tax and annual insurance premium by 12. Add both to that payment, plus PMI and HOA dues if they apply.

Should I put 20% down to avoid PMI?

Not always. Twenty percent removes PMI, but it can drain your cash reserves. Compare the PMI cost against your emergency fund, and remember conventional PMI can be cancelled once you reach about 20% equity.

Why does my lender’s quote differ from an online calculator?

Calculators often leave out or guess at taxes, insurance, PMI, and HOA dues. Your lender uses your actual rate, credit profile, and local costs, which is why the quote can differ.

When should I use a mortgage calculator instead of doing the math by hand?

Use a calculator when you’re comparing many prices, rates, or down payments quickly. Do it by hand once to understand the formula, then check any calculator’s result against it so you know the inputs are right.

Compare Lenders and Test the Payment Against Your Budget

Rate quotes matter as much as the formula. Freddie Mac research (2018) found that borrowers who requested one extra quote saved about $1,500 over the life of the loan on average, and those who collected five saved about $3,000. Knowing how a payment is built lets you compare two offers line by line instead of trusting a headline number.

Ask every lender for a Loan Estimate, the standardized three-page form created under CFPB rules, and compare the projected monthly payment on each.

Then test the payment against your pay. The common 28% rule of thumb caps housing at 28% of gross income, and I’d push back on it. After taxes and payroll deductions, 28% of gross often lands above a third of take-home pay, which leaves less breathing room than the rule suggests.

This guide covers fixed-rate purchase loans. It does NOT address adjustable-rate mortgages, FHA mortgage insurance premiums, or refinancing. Everything here is general education, not financial advice, so confirm your final numbers with a licensed lender.

Run your own numbers. Copy the Sheets formula above into a blank sheet, swap in your price, rate, and down payment, and see where you land.