Your monthly principal-and-interest payment comes from one formula: M = P · [ r(1+r)n ] / [ (1+r)n − 1 ]. Plug in the loan amount, the monthly interest rate and the number of monthly payments, and you have your number — but that figure isn't the whole story, and we'll get to the parts most people forget.
What the letters mean
- M — the monthly payment you're solving for (principal + interest only).
- P — the principal: the amount you actually borrow (home price minus your down payment).
- r — the monthly interest rate. Take the annual rate, divide by 100, then divide by 12. A 6.5% rate is 0.065 ÷ 12 = 0.005417.
- n — the total number of monthly payments. A 30-year loan is 30 × 12 = 360.
A worked example
Say you borrow $300,000 at 6.5% for 30 years.
- P = 300,000
- r = 0.065 ÷ 12 = 0.0054167
- n = 360
Run those through the formula and the monthly principal-and-interest payment lands at about $1,896. Over 360 payments that's roughly $682,600 paid in total — meaning about $382,600 of pure interest on a $300,000 loan. Seeing that number is usually the moment people start caring about the interest rate and the loan term.
You don't have to do this by hand. The Mortgage Calculator runs the same formula instantly and shows the full amortization schedule, so you can watch how each payment splits between interest and principal.
The part the formula leaves out: PITI
The formula only covers principal and interest. Your real monthly bill is usually PITI — Principal, Interest, Taxes and Insurance — and sometimes more:
- Property taxes — collected monthly and held in escrow, then paid to your local government.
- Homeowners insurance — also escrowed and paid yearly on your behalf.
- PMI (private mortgage insurance) — see below.
- HOA dues — if your home is in an association.
It's normal for taxes and insurance to add a few hundred dollars on top of the formula's answer, so always budget with PITI, not just principal and interest.
PMI and the 20% rule
If your down payment is under 20% of the home price, most lenders require PMI until you've built about 20% equity. On a typical loan that's often $100–$250 a month for nothing you keep — it only protects the lender. This is exactly why a bigger down payment can lower your cost twice: smaller loan and no PMI.
Term length changes everything
Keep the loan amount and rate the same but switch from 30 years to 15 and the monthly payment jumps — yet the total interest drops dramatically. Which one is right depends on your cash flow and goals; we break the trade-off down in 15-year vs 30-year mortgage. You can also test paying a little extra each month with the Biweekly Mortgage Calculator.
The fast way
Understanding the formula helps you see why the payment moves the way it does. But for real decisions, change one input at a time in the Mortgage Calculator and watch the payment and total interest react — that's how you find a number you can actually live with. If you're not sure how big a loan you can take on in the first place, start with the Loan Affordability Calculator.
For an independent explainer on how mortgage costs work, the U.S. Consumer Financial Protection Bureau is a solid, ad-free reference.
Try it yourself
Run your own numbers in the free Mortgage Calculator — instant, private, no sign-up.
Open the Mortgage Calculator →Frequently asked questions
- What is the formula for a monthly mortgage payment?
- M = P · [ r(1+r)^n ] / [ (1+r)^n − 1 ], where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (years × 12). It returns the principal-and-interest portion only.
- Does the mortgage formula include taxes and insurance?
- No. The standard formula only calculates principal and interest. Property taxes, homeowners insurance, PMI and any HOA dues are added on top — together these are called PITI and make up your real monthly housing cost.
- Why is so much of my early payment going to interest?
- Interest is charged on the remaining balance, which is highest at the start. Early payments are mostly interest and slowly shift toward principal — an amortization schedule shows the exact split for every month.
- How can I lower my monthly mortgage payment?
- A lower interest rate, a longer term, a larger down payment (which also avoids PMI), or buying a less expensive home all reduce the monthly payment. Try each one in the Mortgage Calculator to see the effect before you commit.
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