Ask three lenders what your monthly mortgage payment will be and you’ll get three different numbers. Not because anyone is lying, but because each one is quoting a slightly different bundle: principal and interest alone, principal and interest plus escrow, or a figure that quietly assumes a rate you’d only get with a 780 credit score.
The core math is one formula you can run on a phone calculator in about two minutes. Once you can produce the number yourself, you’re in a far better position to judge whether a lender’s quote is realistic, and to work out how much house you can really afford before you start touring properties.
The Formula Lenders Use (It’s Shorter Than You Think)
Every fixed-rate mortgage payment comes out of the same equation:
M = P × [ r(1 + r)n ] ÷ [ (1 + r)n − 1 ]
- M — the monthly principal and interest payment
- P — the loan amount, not the purchase price
- r — your annual interest rate divided by 12
- n — the total number of payments (360 for a 30-year loan, 180 for a 15-year)
The two mistakes that wreck the result
A 6.5% rate is not 0.065 here. Divide by 12 first, giving you 0.0054167 per month. Use the annual figure by accident and your payment comes out roughly twelve times too large.
The second slip is fat-fingering the loan amount. On a $400,000 house with 10% down, P is $360,000. Forget the down payment and you’ll overstate the payment by around $250 a month, which is enough to talk yourself out of a house you can comfortably afford.
A Worked Example With Real Numbers
Say you’re borrowing $350,000 at 6.5% over 30 years. Here’s the whole calculation, step by step.
- r = 0.065 ÷ 12 = 0.0054167
- n = 30 × 12 = 360
- (1 + r)n = 6.9918
- Numerator: 0.0054167 × 6.9918 = 0.0379
- Denominator: 6.9918 − 1 = 5.9918
- 0.0379 ÷ 5.9918 = 0.00632
- $350,000 × 0.00632 = $2,212 per month
That’s principal and interest only. Across all 360 payments you’d hand over about $796,000 for a $350,000 loan, which means roughly $446,000 of it is interest. Most people find that number uncomfortable, and it’s worth sitting with before you decide on a term.
The Costs That Never Appear in the Formula
Your real monthly bill is bigger than $2,212, and this is exactly where rough estimates go wrong.
Taxes, insurance and HOA dues
Lenders typically collect property taxes and homeowners insurance in escrow and pay those bills on your behalf. On a $400,000 home you might see $400 a month in taxes and $150 for insurance. Toss in $50 of HOA dues and you’ve added $600 before touching principal.
Private mortgage insurance
Put down less than 20% and you’ll usually pay PMI, generally 0.5% to 1% of the loan amount per year. On a $360,000 loan at 0.6%, that’s $2,160 annually, or $180 a month. This is where a loan-to-value calculator earns its keep: PMI is driven entirely by your LTV ratio, and crossing below 80% is what makes it disappear.
Stack it all up on that $400,000 house and the true payment looks like this: $2,275 principal and interest, $400 taxes, $150 insurance, $180 PMI, $50 HOA. That’s $3,055 a month, nearly 35% more than the number the formula gave you.
What the Loan Term Does to That Payment
Run the same $350,000 through a 15-year term at 6% and the payment jumps to about $2,954. That’s $742 more every month, and the higher payment also squeezes how much house a lender will approve you for.
The payoff is on the interest side. Total interest on the 15-year loan lands around $182,000 versus roughly $446,000 on the 30-year. You’d save somewhere near $264,000 and own the place outright at 45 instead of 60, assuming you start at 30. Neither option is wrong. The 30-year buys flexibility and a lower required payment; the 15-year buys a smaller lifetime bill. What matters is picking deliberately rather than defaulting.
Why Two Buyers Get Different Payments on the Same House
Rate is the biggest lever, and rate is mostly about credit. A borrower at 760 gets a noticeably better deal than one at 680, often half a point to a full point lower. On a $350,000 loan, moving from 6.5% to 7.5% pushes that payment from $2,212 to about $2,447. That’s $235 a month, or $84,600 over the life of the loan, for the same house.
Discount points matter too. One point costs 1% of the loan up front and generally shaves around 0.25% off your rate, which can be a smart trade if you plan to stay put for years and a bad one if you might sell in three.
Adjustable-rate mortgages change the picture again. The introductory payment can look great for five or seven years, then reset based on market conditions. If your budget only works at the teaser rate, you don’t have a budget, you have a gamble.
Doing It Yourself in Under Two Minutes
In Google Sheets or Excel, type =PMT(0.065/12, 360, -350000) into any cell and you’ll get 2212.02. Change the rate, the term, or the loan amount and the answer updates instantly. It’s the same equation the lender uses, just without the paperwork.
Then handle escrow separately: look up the actual property tax rate for the county you’re shopping in, get an insurance quote rather than a guess, and check whether PMI applies. There’s a solid walkthrough of the free mortgage tools you need before buying a home if you’d rather not build the spreadsheet from scratch.
Pressure-Test the Payment Against Your Income
A payment you can technically get approved for is not the same as one you can live with. The old rule of thumb says keep housing costs under 28% of gross monthly income, with total debt payments under 36%. Lenders have loosened that considerably, with many now approving total debt-to-income ratios up to 43% or even higher.
On $8,000 of gross monthly income, 28% is $2,240. The $3,055 payment from our example would land at 38% of income on its own, which is why escrow matters so much. The lender’s number counts taxes and insurance against you too.
If you want to see how the pieces fit together in sequence, there’s a useful breakdown of the best mortgage tools every home buyer should use and when to reach for each one.
What to Ask a Lender Once You’ve Run the Numbers
You’ll get a Loan Estimate within three business days of applying, and it’s the document that tells you whether a quote is real. Request one from at least three lenders within a 14-day window, since mortgage inquiries in that period typically count as a single credit pull.
Compare the APR, not just the interest rate, because APR folds in lender fees and points. Ask whether escrow is required or optional, and how large a cushion they’ll hold in your escrow account. On a $400,000 home, a two-month cushion is $1,100 of your money sitting in someone else’s account.
Finally, ask what the process looks like for dropping PMI, and at what point in your loan they’ll automatically review it. Some servicers require you to request it in writing; others handle it themselves. Either way, a monthly payment that’s $180 lighter is worth the phone call.
The number you calculate yourself won’t match a lender’s quote to the dollar, and it isn’t supposed to. What it gives you is a baseline solid enough to spot a padded quote, a rate that doesn’t fit your credit, or a house that only works if you ignore half the costs. A payment you can comfortably cover for thirty years, at a rate you actually shopped for, beats one that looked perfect in month one.