Mortgage Calculator

Why a mortgage payment is more than principal and interest

A generic loan calculator gives you one number: principal and interest. A real monthly house payment is usually four numbers bundled together, often abbreviated PITI — Principal, Interest, property Taxes, and homeowner's Insurance — plus mortgage insurance when your down payment is small. This calculator models all of it, then projects the total cost over the life of the loan and checks whether the payment fits your income. Understanding each piece is what turns a quoted rate into a budget you can actually live with.

How the principal and interest figure is built

The core payment uses the standard fixed-rate amortization (annuity) formula:

M = P × [ r(1+r)^n ] / [ (1+r)^n − 1 ]

Here P is the loan amount (home price minus down payment), r is the monthly interest rate (the annual rate divided by 12), and n is the total number of monthly payments (years × 12). The same fixed payment M covers every month, but its split shifts over time — which is exactly what amortization describes.

A worked example

Take a $400,000 home with 20% down ($80,000), leaving a $320,000 loan at 6.5% over 30 years. The monthly rate is 0.065 ÷ 12 ≈ 0.005417, and n = 360. Plugging in, principal and interest come to roughly $2,022 per month. Add $4,800 a year in property tax ($400/mo) and $1,200 in insurance ($100/mo), and the total monthly payment is about $2,522. Over the full 30 years you pay around $728,000 in principal and interest combined — meaning roughly $408,000 of that is interest alone, more than the original loan. Seeing that total is the strongest argument for a larger down payment or a shorter term.

Reading the amortization table

The first-year table reveals the counter-intuitive heart of a mortgage: early payments are mostly interest. In month one of the example above, interest is the balance times the monthly rate — $320,000 × 0.005417 ≈ $1,733 — so only about $289 of your $2,022 payment chips at the principal. As the balance falls, the interest slice shrinks and the principal slice grows, accelerating near the end. This is why paying a little extra toward principal early saves so much: every dollar that skips ahead avoids years of compounding interest. It is also why selling in the first few years builds almost no equity from payments alone.

PMI: the cost of a small down payment

Put down less than 20% and most lenders require Private Mortgage Insurance (PMI), which protects the lender (not you) against default. The calculator adds it automatically only when your down payment falls below 20%, estimating it as an annual percentage of the loan (commonly around 0.5%) charged monthly. On a $360,000 loan at 0.5%, that is $1,800 a year or $150 a month of pure cost that builds no equity. The good news: PMI is not permanent. Once you reach roughly 20% equity (an 80% loan-to-value ratio) through payments or appreciation, you can typically request its removal — which is why crossing the 20% down-payment threshold is such a meaningful target.

The affordability check

Enter your gross monthly income and the tool divides your total housing payment by it to give a front-end ratio. The long-standing guideline is the "28/36 rule": keep housing costs at or below 28% of gross income, and total debt (housing plus car loans, student loans, credit cards) at or below 36%. The calculator color-codes the result — green up to 28%, amber from 28% to 36%, red above — so you can see at a glance whether a price stretches you. On a $7,000 monthly income, the $2,522 example payment is about 36%, right at the upper edge, a signal to consider a lower price or a bigger down payment.

Tips and common mistakes

• Don't shop by the principal-and-interest number alone. Taxes, insurance, and PMI can add hundreds a month. Budget the full PITI figure.
• Compare 15-year and 30-year terms. A 15-year loan has a higher monthly payment but dramatically less total interest. Toggle the term and watch the lifetime interest figure move.
• Reaching 20% equity ends PMI. Track that milestone; canceling PMI is one of the easiest ways to cut a payment without refinancing.
• The down-payment percent and dollar fields stay in sync. Edit either one and the other updates from the home price, so you can think in whichever unit is natural.
• Estimates, not quotes. Property tax rates, insurance premiums, and PMI vary by location and lender. Use this to plan and compare; rely on a lender's official Loan Estimate for exact figures.