How is my monthly mortgage payment calculated?

The calculator uses the standard fixed-rate amortization formula. The monthly rate is r = (annual rate / 100) / 12 and the number of payments is n = years × 12. The fixed monthly P&I payment is M = P × r × (1 + r)^n / ((1 + r)^n − 1), where P is the loan principal. At a 0% rate it simplifies to M = P ÷ n.

What is the difference between P&I and PITI?

P&I stands for Principal and Interest — the amount that repays the loan itself and the figure the calculator shows by default. PITI adds property tax and homeowners insurance to that base, and with HOA dues included, it reflects your true total monthly housing cost, which is what lenders use to qualify you.

Is this mortgage estimate a loan offer?

No. The result is an estimate for general information and planning only. It excludes PMI, discount points, closing costs, adjustable-rate scenarios, extra payments, and escrow changes. Always verify exact figures with a licensed lender or mortgage professional before making a decision.

Why does my lender qualify me on PITI instead of P&I?

Lenders qualify on PITI because that is the total amount that leaves your checking account each month. A borrower can afford the P&I portion of a payment and still be unable to fund the taxes and insurance portion, so qualifying on PITI gives a more realistic read on whether the loan fits your budget.

Mortgage Calculator Tips and Common Mistakes to Avoid

The standard fixed-rate mortgage payment formula M = P·r(1+r)^n / ((1+r)^n − 1) is the engine behind every reliable mortgage calculator, where P is the loan principal, r is the monthly interest rate, and n is the total number of monthly payments. Run four numbers through that formula — the home price, the down payment, the annual interest rate, and the loan term in years — and it returns the monthly principal-and-interest figure that decides whether a given house actually fits your budget. The Mortgage Calculator does exactly that in your browser, never uploading your inputs, and adds an amortization schedule plus optional property tax, insurance, and HOA so you can see both the loan-repayment number (P&I) and your real monthly housing cost (PITI). The tips below are the ones that change the result by the largest dollar amounts, and the mistakes are the ones that quietly inflate the headline payment you plan around.

What a Mortgage Calculator Actually Computes

Every mortgage calculator does one job: turn your loan inputs into the numbers that govern a housing decision. The Mortgage Calculator uses the four numbers that drive the formula, then offers optional extras for the cash you actually send each month.

Handled by the calculator	Not handled by the calculator
Home price, down payment, annual rate, and term (P&I)	Private mortgage insurance (PMI)
Monthly principal-and-interest payment	Discount points and origination fees
Total interest and total amount paid over the life of the loan	Closing costs and prepaid escrow items
Year-by-year amortization schedule	Adjustable-rate or balloon-loan scenarios
Optional property tax, insurance, and HOA → full PITI total	Extra payments, prepayments, and recasting
Browser-only processing; financial inputs are not uploaded	Escrow changes and lender-specific fee credits

The mechanics are worth knowing because they tell you exactly which inputs move the result and which assumptions are baked in. The calculator first subtracts your down payment from the home price to get the loan principal P. It then converts your annual interest rate into a monthly rate r = (annual rate % / 100) / 12, and your term into a number of monthly payments n = years × 12. The fixed monthly P&I payment is M = P · r · (1 + r)^n / ((1 + r)^n − 1). When the rate is exactly 0%, that collapses to a simple M = P / n, which is just the principal split evenly across every month.

Two assumptions sit inside every number the tool returns. The rate is treated as fixed for the whole term, and interest compounds monthly. Those are the standard conventions for U.S. fixed-rate mortgages, which is why a calculator can give you an answer that lines up with a lender's quote — as long as the inputs are honest.

Tips for a More Reliable Estimate

The accuracy of any mortgage calculator is bounded by the accuracy of what you type into it. These are the habits that consistently produce numbers you can plan around.

Use the down payment you can document, not the one you wish you had. Lenders count only the documented portion, and the calculator does the same. Inflating the down payment by even five percentage points drops the principal and can move the monthly figure by hundreds of dollars.
Pull the rate from a current quote, not a memory. Mortgage rates change weekly. A rounded guess from a year ago can throw the monthly payment and the total interest off by more than a typical property tax estimate.
Match the term to the loan you'd actually take. A 30-year and a 15-year loan on the same principal can differ in monthly payment by close to a factor of two. Decide which term you're considering first, then run the number.
Run more than one rate at a time. If your credit score has wiggle room, run a 0.25% lower rate alongside your base case so you can see the dollar impact of qualifying for a better tier.
Add taxes, insurance, and HOA before you compare totals. The headline number is P&I; lenders qualify you on PITI. Without the extras, two houses can look like the same payment when they aren't.
Change one variable at a time when comparing scenarios. If you want to compare a 30-year loan at 6.5% with a 15-year loan at 6.0%, hold everything else fixed so the difference you see is caused by rate and term, not by accident.
Trust browser-only tools for privacy, not for substituting a lender. The calculator runs locally so your inputs are never uploaded, but a planning figure is still not a loan offer.

How to Run Your Numbers in the Calculator

The interface is short on purpose. Three steps get you a complete P&I estimate with an amortization schedule, and a fourth step turns it into a PITI budget number.

Enter the home price and your down payment. The calculator subtracts the down payment from the home price to get the loan principal, which is the number the formula actually runs on.
Type your annual interest rate and pick a loan term. Choose 15 or 30 years, or enter a custom number of years. The tool converts the annual rate to a monthly rate and the term to a count of monthly payments.
Read your monthly payment, total interest, and total paid. These three numbers update instantly. Then expand the amortization schedule to see how each payment splits between principal and interest over the life of the loan.
Add property tax, insurance, and HOA to see the full monthly total. With those optional fields filled in, the tool also displays an estimated PITI (Principal, Interest, Taxes, Insurance) plus HOA figure — the number lenders actually use to qualify you.

Mistakes That Quietly Skew the Result

The same inputs produce wildly different plans depending on how they're entered. These are the errors that show up most often — and the ones with the largest dollar consequences.

Planning around P&I when the lender will qualify you on PITI. The headline figure is what repays the loan; PITI is what leaves your checking account. Forgetting taxes and insurance makes the payment look smaller than it really is.
Treating the calculator as if the rate changes monthly. The tool assumes a fixed rate for the entire term. If you have an adjustable-rate mortgage or expect to refinance, the number you see is a snapshot, not a forecast.
Using a guessed property tax rate. A guess of 1% versus the real 1.4% in your county changes the monthly figure more than a 0.125% change in the mortgage rate on a typical loan. Pull the actual rate from the county assessor.
Forgetting that the calculator doesn't include PMI. If your down payment is under 20%, most loans add private mortgage insurance, which is a real monthly cost the tool does not model. Add it yourself when budgeting.
Comparing two homes by changing every input at once. If you change price, down payment, rate, term, taxes, and HOA simultaneously, you can't tell which lever moved the result. Hold the inputs constant and vary one at a time.
Ignoring the amortization schedule. The schedule shows that early payments are mostly interest and later payments are mostly principal. That timing affects how much interest you can deduct in the early years and how quickly you build equity.

To see exactly what the formula produces, take one concrete example. Suppose you borrow P = $300,000 at an annual rate of 6% for n = 30 years. The monthly rate is r = 6% / 12 = 0.005, and the number of payments is 30 × 12 = 360. Plugging in:

M = 300,000 × 0.005 × (1.005)^360 / ((1.005)^360 − 1)

(1.005)^360 ≈ 6.0226, so the calculation reduces to M = 300,000 × 0.005 × 6.0226 / (6.0226 − 1) = 9,033.90 / 5.0226 ≈ $1,798.65 per month. Multiply that monthly figure by the number of payments to get the total paid: $1,798.65 × 360 = $647,514. Subtract the original $300,000 principal to get the total interest: $647,514 − $300,000 = $347,514. Those are the planning numbers for one specific loan. For any other principal, rate, or term, run the same inputs through the Mortgage Calculator rather than recomputing by hand.

P&I Versus PITI — Budget the Right Number

Two related acronyms carry two very different meanings, and confusing them is the single most expensive mistake in this article.

P&I — Principal and Interest — is the figure that repays the loan itself. Every dollar of it either reduces your balance or compensates the lender for the time value of that balance. The calculator shows P&I by default because that is what the formula solves for.

PITI — Principal, Interest, Taxes, and Insurance — is the figure that leaves your bank account. Property tax is estimated as home value × tax rate / 12, insurance as the annual premium / 12, and HOA dues are added as-is. None of those add-ons changes how fast the loan is paid off; they only raise the cash you send each month. That is exactly why lenders qualify borrowers on PITI rather than P&I alone: a borrower who can afford the P&I payment may still be unable to fund the taxes and insurance portion.

When you compare two homes, compare the PITI totals, not the headlines. A house with a lower price but a higher tax rate can carry a larger real monthly bill than a pricier home in a lower-tax county.

When the Estimate Is Enough — and When to Verify With a Lender

A calculator is a planning tool, not an approval tool. It is the right instrument for choosing a price range, comparing rate quotes side by side, deciding between a 15-year and a 30-year term, and estimating how long it will take to reach a target down payment. It is not a substitute for a loan officer, a closing disclosure, or a written pre-approval.

The estimate deliberately leaves out things only a lender can quote: PMI premiums specific to your credit profile, discount points and origination fees that change your effective rate, closing costs and prepaid escrow items, the impact of any adjustable-rate structure on future payments, and how extra payments or recasting would alter the schedule. If any of those factors will shape your decision, the right next step is to take the same four inputs into a licensed lender or mortgage broker and compare their quote — including all add-ons — to the calculator's PITI figure.

For everything the calculator does cover, the numbers are trustworthy to the same precision as a lender's quote for a fixed-rate, fully amortizing loan with the same inputs. Treat the result as the planning baseline, then layer the lender-specific costs on top before you sign.