A mortgage looks like a single monthly number, but it is actually five different payments bundled together. Understanding what makes up that number, why the early payments are mostly interest, and how a small change in the interest rate can cost tens of thousands of dollars is the difference between buying a home blind and buying one with your eyes open.

This guide walks through every part of a mortgage payment, the math behind amortization, when PMI goes away, and the strategies that actually move the needle on total interest paid. Use the mortgage calculator to model your own numbers as you read.

The Five Parts of a Mortgage Payment

Lenders use the acronym PITI: Principal, Interest, Taxes, and Insurance. Most homeowners also pay PMI (if the down payment was small) and HOA dues (if the community has them). That is everything in a typical monthly housing cost.

Where Does Each Dollar Go?

On a $340,000 loan at 6.79% for 30 years, your scheduled principal and interest payment is about $2,215 per month. In month one, almost $1,925 of that is interest and only about $290 reduces the balance. By year 20, the split flips: roughly $1,400 is principal and $815 is interest. By the final payment, almost the entire amount is principal. This is how amortization works.

How Amortization Actually Works

Each month, the lender computes interest on the current outstanding balance using your annual rate divided by 12. Whatever is left over from your scheduled payment goes to principal. Because the balance starts high and shrinks slowly, the early payments are dominated by interest.

That formula produces a level monthly payment. The split between principal and interest is what shifts month to month. Knowing this is what makes extra payments so powerful: every extra dollar of principal skips you ahead on the amortization schedule and erases the corresponding interest at the back end of the loan.

The 78% Rule: When PMI Goes Away

Private mortgage insurance is required by most conventional lenders when the down payment is less than 20%. PMI typically costs 0.3% to 1.5% of the original loan amount per year, paid monthly.

Under the federal Homeowners Protection Act, PMI is automatically terminated when your loan balance reaches 78% of the original home value. You can also request removal at 80% LTV, but you have to ask in writing. Either way, PMI is temporary and does not last for the life of the loan.

The mortgage calculator on this site uses the 78% rule automatically. Set the down payment below 20% and switch to the amortization view to see exactly which month PMI drops off.

What a 0.25% Rate Change Actually Costs

Most people underestimate how much a small rate change matters over 30 years. Here is the same $340,000 loan at three different rates:

Each quarter point of rate adds about $56 to the monthly payment and roughly $20,000 to lifetime interest. This is why locking in a good rate, or paying for discount points up front, often pays for itself many times over.

Strategies for Paying Off Early

Because of amortization, an extra dollar of principal in year one saves you many dollars of interest at the end of the loan. Three practical patterns:

1. Round up every monthly payment

Pay $2,300 instead of $2,215. The extra $85 a month, applied to principal, will knock about three years off a 30-year loan and save roughly $40,000 in interest at a 7% rate. Set it as an auto-pay and forget about it.

2. One extra payment a year

Take your tax refund, an annual bonus, or one month of windfall income and apply it to principal. On a 30-year loan, a single extra monthly-equivalent payment per year cuts about four years off the loan term. The calculator's "extra yearly payment" field models this exactly.

3. Biweekly equivalent

Paying half your payment every two weeks results in 26 half-payments per year, which equals 13 monthly payments instead of 12. It is the same idea as one extra payment per year, packaged as a payment cadence. Confirm your lender applies the half-payments to principal immediately rather than holding them.

15 vs 20 vs 30 Year: The Real Tradeoff

Shorter terms come with lower interest rates (often 0.5% to 0.75% less than 30 year) and far less lifetime interest. The catch is cash flow: the 15-year payment is about 30% higher than the 30-year, even with the better rate. Many buyers prefer the 30 year for flexibility, then make extra principal payments when their budget allows.

When to Refinance: The Rule of Thumb

The common heuristic is that refinancing makes sense when you can cut your rate by at least 0.75% to 1.0% and plan to stay in the home long enough to recoup the closing costs. To estimate your break-even:

Also factor in the reset on amortization. A new 30-year loan in year 7 of your current loan resets the interest-heavy front-end. Sometimes a shorter-term refi ends up cheaper even at a slightly higher rate.

Put It Together

The numbers that matter most for a mortgage are interest rate, down payment, and loan term. Everything else (PMI, taxes, insurance, HOA, extras) shifts the monthly number, but those three set the long-term cost. Use the side-by-side comparison feature in the calculator to test combinations and find your own break-even.