How Mortgages Work

For most people, buying a home is the biggest financial transaction of their lives. And for most of those people, that purchase is funded by a mortgage, a loan specifically designed to let you pay for a house over decades instead of all at once. The math of mortgages is straightforward, but the implications often surprise people: in the early years of a 30-year mortgage, you’re mostly paying interest, not building equity.

The short answer

A mortgage is a loan used to buy real estate, where the property serves as collateral. You borrow a large sum (often hundreds of thousands of dollars), agree to pay it back over 15 to 30 years, and pay interest on the outstanding balance. Each monthly payment is split between interest and principal, with the interest portion front-loaded, meaning early payments are mostly interest, not paying down the loan.

The full picture

The basic structure of a mortgage

When you get a mortgage, a lender gives you a lump sum to buy a home. In exchange, you agree to pay back that amount plus interest over a set period, typically 15, 20, or 30 years.

The interest rate is expressed as an annual percentage rate (APR). If you borrow $400,000 at 6% APR, roughly the US average 30-year fixed rate as of early March 2026, per Freddie Mac’s Primary Mortgage Market Survey, you’ll pay interest each year equal to 6% of the remaining balance. Over 30 years, you’ll pay far more in interest than the original loan amount. That’s how lenders make money.

The property itself is the collateral. If you stop making payments, the lender can foreclose, take ownership of the house and sell it to recoup their money. This is why lenders are willing to loan you such large amounts: they have something tangible to claim if things go wrong.

Most conventional mortgages in the US are eventually purchased by Fannie Mae (Federal National Mortgage Association) or Freddie Mac (Federal Home Loan Mortgage Corporation), government-sponsored enterprises that buy loans from lenders, package them into mortgage-backed securities, and sell them to investors. This system frees up capital so lenders can issue more loans. When a lender originates your mortgage, they’re often selling it within weeks to one of these agencies.

Fixed-rate vs. adjustable-rate mortgages

The most important decision when choosing a mortgage is between fixed-rate and adjustable-rate:

Fixed-rate mortgages have an interest rate that never changes for the life of the loan. Your monthly payment stays exactly the same for 15 or 30 years. This predictability is valuable: you always know what you’ll pay, even if rates rise dramatically. Most homeowners choose fixed-rate mortgages.

Adjustable-rate mortgages (ARMs) have an interest rate that’s initially lower than fixed rates, but can change over time. For example, a 5/1 ARM might have a fixed rate for five years, then adjust annually after that. If market rates rise, your payment goes up. If they fall, your payment goes down.

ARMs make sense in specific scenarios: if you plan to sell the house before the fixed period ends, or if you expect to refinance soon. For most people planning to stay in their home long-term, the certainty of a fixed rate is worth paying a slightly higher initial rate.

How mortgage payments work

Every month, you make the same payment. That payment covers two things:

1. Interest: The lender’s charge for borrowing the money. Calculated as your annual rate divided by 12, multiplied by the remaining balance.

2. Principal: The portion that actually pays down the loan amount.

Here’s where it gets interesting. In the first year of a 30-year mortgage, your balance is near its maximum. So the interest charge is near its maximum too. Most of your payment goes to interest, leaving only a small amount for principal.

As years pass, your balance shrinks. Each year, the interest charge gets smaller. More of your payment goes to principal. By the final years of the mortgage, you’re almost entirely paying down principal.

Example: A $400,000 mortgage at 6.5% for 30 years:

• Monthly payment: $2,528
• In month 1: $2,167 goes to interest, $361 goes to principal
• In year 1 total: about $25,600 to interest, $4,700 to principal
• In year 30 total: about $3,400 to interest, $27,000 to principal

The same payment, doing completely different work depending on when you make it.

Down payments and PMI

The down payment is the amount you pay upfront. It’s expressed as a percentage of the home price. A 20% down payment on a $500,000 home is $100,000.

Putting down 20% or more avoids PMI (Private Mortgage Insurance), which is an extra monthly cost that protects the lender, not you. PMI typically costs 0.5% to 1% of the loan amount per year. On a $400,000 loan, that’s $200 to $400 per month.

You can also put down less than 20%. FHA loans (backed by the Federal Housing Administration) allow down payments as low as 3.5% and are popular with first-time buyers, but you’ll pay mortgage insurance for the life of the loan if you put down less than 10%. VA loans, guaranteed by the Department of Veterans Affairs, allow eligible veterans and active-duty service members to buy with zero down payment and no PMI. USDA loans serve rural buyers with similar zero-down terms in qualifying areas.

The amortization schedule

An amortization schedule is a table showing exactly how each payment is split between interest and principal over the life of the loan. It reveals the hidden truth of mortgages: the interest you pay in the early years is massive.

On a 30-year mortgage at 6.5%, you’ll pay roughly $510,000 in total payments for a $400,000 loan. More than $110,000 of that (about 22%) goes entirely to interest in the first five years.

This is why extra payments matter so much. If you pay an extra $200 per month on that same mortgage, you cut about eight years off the loan and save roughly $60,000 in total interest. The math works because every extra dollar goes directly to principal, skipping the interest-first allocation of regular payments.

Closing costs

The mortgage payment isn’t the only cost. Closing costs, fees paid at the time of purchase, typically range from 2% to 5% of the loan amount. On a $400,000 loan, that’s $8,000 to $20,000.

Closing costs include:

• Loan origination fees
• Appraisal fees
• Title insurance
• Recording fees
• Survey costs
• Attorney fees

These are one-time costs, but they add significantly to the cost of buying a home. It’s worth shopping around: different lenders charge different fees, and some will cover a portion of your closing costs in exchange for a slightly higher interest rate.

Refinancing: when it makes sense

Refinancing means replacing your current mortgage with a new one, typically to get a lower interest rate. If rates have dropped since you got your mortgage, refinancing can reduce your monthly payment and save thousands in interest.

But refinancing isn’t free. You pay closing costs again. The math only works if you plan to stay in the home long enough to recoup those costs through lower monthly payments. A common rule of thumb: if you can reduce your rate by at least 1%, refinancing likely makes sense.

Cash-out refinancing, borrowing more than you owe and taking the difference in cash, is popular but risky. Also note that inflation can work in a borrower’s favor: debt repaid in future dollars that are worth less in real terms is effectively cheaper than it looks nominally. You’re increasing your debt and potentially extending your loan term back to 30 years, which can cost more in the long run.

Why it matters

A mortgage is a 15- to 30-year commitment. The interest rate you get, the down payment you make, and the extra payments you choose to pay (or not) all have massive implications for your total cost.

The difference between a 6% mortgage and a 7% mortgage on a $400,000 loan is roughly $90,000 in total interest over 30 years, and your credit score is often the deciding factor in which rate you qualify for. That’s the cost of a new car, every single year for three decades.

Understanding how mortgages work helps you negotiate better terms, decide whether to pay points, evaluate whether to refinance, and determine how much house you can truly afford. It’s not just about the monthly payment. It’s about the total cost of ownership over decades.

Common misconceptions

“The monthly payment is what matters most.” The monthly payment is important for budgeting, but the interest rate and total cost matter more. A lower monthly payment with a longer term (like 30 years vs. 15) often means paying much more total interest.

“I should wait to buy until I can put down 20%.” Waiting can backfire if home prices rise faster than you save. In many markets, a smaller down payment with PMI costs less than waiting for prices to climb. PMI is an expense, but it’s often temporary. Once you reach 20% equity, it goes away.

“All mortgages from different lenders are basically the same.” They vary enormously in interest rate, fees, and flexibility. Shopping around can save tens of thousands of dollars over the life of the loan. Getting quotes from at least three lenders is standard practice for a reason.