How to Calculate Your Monthly Mortgage Payment (With PMI)

I Almost Bought a House With the Wrong Numbers

I was sitting in my car outside a showing last spring, punching numbers into my phone's calculator app, and I kept getting a monthly payment that seemed way too low. Like, suspiciously low. The listing was 285,000, I was putting 10% down, and my rough math said something around 1,350 a month. The lender's estimate came back at 1,587. That's a 237-per-month gap, which over a year is almost 2,850 I hadn't accounted for.

Turns out I'd forgotten about PMI. And I'd also been using the wrong formula for the loan payment itself — I was basically just dividing the loan amount by the number of months, which isn't how amortization works at all. I nodded along when the loan officer explained it. I didn't really get it until I sat down and worked through it myself.

So here's what I wish someone had told me back then.

The Actual Mortgage Payment Formula (It's Uglier Than You'd Expect)

Most people think calculating a mortgage payment is straightforward division. Loan amount divided by months. But that ignores interest compounding, which is honestly the whole reason banks make money on mortgages. The real formula looks like something out of a finance textbook, and it kind of is, but once you see it broken down it's not that scary.

That exponent is what trips people up. The (1 + r)^n part. It's doing the heavy lifting of compounding, and it's why your payment isn't just a simple split.

Let me walk through a real example because I think that's the only way this actually clicks. Say you're looking at a property listed at 285,000. You're putting 10% down, so that's 28,500 out of pocket, leaving you with a loan of 256,500. Your rate is 7.0% (which is roughly where things have been hovering lately), and you're going with a standard 30-year term.

Here's the math step by step:

• P = 256,500
• r = 0.07 ÷ 12 = 0.005833
• n = 30 × 12 = 360
• (1 + r)^n = (1.005833)^360 ≈ 8.1166
• Numerator: 256,500 × (0.005833 × 8.1166) = 256,500 × 0.04735 ≈ 12,145
• Denominator: 8.1166 – 1 = 7.1166
• M = 12,145 ÷ 7.1166 ≈ 1,707 per month

That's just principal and interest. We haven't even touched PMI yet, or taxes, or insurance. And already you can see why my original "about 1,350" estimate was wildly off — I was basically pretending interest didn't compound.

Now Add PMI (Because Your Lender Definitely Will)

PMI — private mortgage insurance — kicks in whenever your down payment is less than 20% of the purchase price. It protects the lender, not you, which is a fun detail nobody loves hearing. The cost varies, but for most conventional loans you're looking at somewhere between 0.5% and 1.5% of the original loan amount per year, divided into monthly chunks.

For our 256,500 loan, let's say PMI runs 0.75% annually. That's 256,500 × 0.0075 = 1,924 per year, or about 160 per month. So your real monthly obligation jumps from 1,707 to roughly 1,867 — and that's before property taxes and homeowner's insurance pile on.

Here's a quick reference for how PMI rates shift depending on your down payment and credit score (these are approximate annual percentages of the loan amount):

Down Payment	Credit 760+	Credit 700-759	Credit 640-699	Credit Below 640
3%	0.55%	0.85%	1.25%	1.75%+
5%	0.41%	0.70%	1.10%	1.50%+
10%	0.30%	0.55%	0.90%	1.30%+
15%	0.20%	0.40%	0.65%	1.05%+

The difference between a 760 credit score and a 680 score on a 256,500 loan with 10% down? That's the difference between about 64/month in PMI and 192/month. Over the years before you hit 20% equity and can drop it, that adds up to thousands.

And here's the thing a lot of first-time buyers miss: PMI isn't permanent. Once you've paid down the loan to 80% of the original purchase price (or the home appreciates enough), you can request cancellation. By law, your servicer has to automatically terminate it when you hit 78%. So it's annoying, but it's temporary. Track your

to know exactly when that threshold hits.

What Your Full Monthly Payment Actually Looks Like

Lenders talk about PITI — principal, interest, taxes, and insurance. PMI is technically a fifth component that gets lumped in. Here's what our 285,000 example looks like fully loaded (I'm estimating taxes and insurance based on a pretty typical suburban property):

Component	Monthly Amount	Annual Amount
Principal & Interest	1,707	20,484
Property Taxes (est. 1.1%)	261	3,135
Homeowner's Insurance	145	1,740
PMI (0.75%)	160	1,924
Total	2,273	27,283

2,273 a month. That's a far cry from the 1,350 I originally calculated in my car. Almost 70% more!

If you're evaluating this as a rental property, these numbers matter even more. Your

needs to account for the full cost of ownership, not just the mortgage principal. I've seen people run their

using only P&I and then wonder why their actual cash flow is negative. PMI alone can eat 160/month of margin — on a property where you're maybe clearing 200-300 in cash flow, that's the difference between a deal and a money pit.

A Few Things Worth Knowing

Putting down exactly 20% isn't always the best move. I know that sounds backwards. But if putting 20% down on a 285,000 property means draining your reserves to 57,000 and leaving yourself with almost nothing for repairs, vacancies, or unexpected costs — you might be better off putting 10% down, eating the PMI for a few years, and keeping that cash cushion. Run the numbers both ways using a

and see which scenario actually leaves you in a stronger position.

Also, if you're comparing two properties and trying to figure out which one makes more sense from an investment standpoint, don't just look at the sticker price. A 240,000 property with a higher tax rate and PMI can easily cost more monthly than a 260,000 property in a lower-tax area where you can put 20% down. Use a

to see where you stand, and check your

before you get too attached to any one listing.

So yeah — the formula itself isn't complicated once you've seen it a couple times. The hard part is remembering to include everything. PMI, taxes, insurance. They're the invisible weight that turns a "totally affordable" payment into something that makes you sweat a little.

Run the full numbers before you make an offer. Not after. I learned that one the expensive way.

When can I stop paying PMI?

For conventional loans, you can request PMI cancellation once your loan balance drops to 80% of the original purchase price. Your lender is required to automatically cancel it at 78%. If your home has appreciated significantly, you might be able to get a new appraisal and use the higher value to reach that 80% threshold sooner — though your lender may charge for the appraisal and has some discretion here. FHA loans are different; FHA mortgage insurance often sticks around for the life of the loan if you put less than 10% down.

Does PMI affect my debt-to-income ratio?

Yes. Lenders include PMI in your total monthly housing payment when calculating your DTI. So if you're borderline on qualification, PMI could push you over the limit. Use a

to check.

Is there a way to avoid PMI with less than 20% down?

A few options exist. Some lenders offer "lender-paid PMI" where they cover the insurance cost but charge you a slightly higher interest rate — you're still paying for it, just indirectly. Piggyback loans (an 80-10-10 structure, for example) are another route. And VA loans don't require PMI at all, regardless of down payment, though they have their own funding fee. Each option has tradeoffs, so compare the total cost over your expected holding period, not just the monthly payment.