Mortgage basics: how a home loan really works

What is a mortgage, really?

A mortgage is just a long-term loan you use to buy a house. The bank gives you most of the money up front (the loan amount), you put in some of your own money (the down payment), and you pay the bank back a little every month for a long time — usually 15 to 30 years. The house itself is the collateral, which is a fancy way of saying that if you stop paying, the bank can take the house back. That's called a foreclosure.

The thing that makes mortgages feel complicated is that you're not paying back the loan in equal pieces. You're paying back the loan plus interest, and how those two add up changes every single month. Once you understand that one piece, the rest gets a lot easier.

The four parts of a monthly payment (PITI)

When people say "my mortgage payment is $2,400," they usually mean the full bundle that comes out of their account each month. Lenders call this PITI:

• P — Principal

  The part of your payment that goes to paying down the actual loan balance. Each month, the principal portion gets a little bigger.

• I — Interest

  What the bank charges you for lending you the money. Interest is calculated on the remaining balance, which is why early payments are mostly interest and later payments are mostly principal.

• T — Taxes (property)

  Property taxes go to your county or city. Most lenders bundle them into your payment and pay the tax bill on your behalf — that’s called escrow.

• I — Insurance (homeowners)

  Homeowners insurance protects the house in case of fire, theft, storm damage, etc. Like taxes, it usually gets escrowed.

On top of PITI, there are two extra line items some borrowers see:

• Mortgage insurance (MI) if you put less than 20% down on a Conventional loan, or for any FHA / USDA loan. See our guide to mortgage insurance for the details.
• HOA dues if your home is in a homeowners association. HOA dues aren't part of your loan and aren't escrowed — you pay them separately to the HOA.

Why early years feel like you’re not making progress

Here's the part that surprises new homeowners. After 5 years on a 30-year loan, you've usually paid down only about 9% of the loan. After 10 years, around 22%. The math is front-loaded with interest because interest is charged on the balance, and the balance is biggest at the start.

This is also why extra principal payments early save the most money. A single extra $1,000 payment in year 1 can save several thousand dollars in lifetime interest. The same payment in year 25 saves almost nothing — by then there's barely any interest left to avoid.

Loan term: 15-year vs 30-year (and everything in between)

The loan term is how long you have to pay the mortgage off. Most loans are 30 years, but 15 and 20 are common, and 10 exists too. Here's the trade-off in plain terms:

• 30-year fixed: Lowest monthly payment, highest total interest paid. Most flexibility because the payment is smaller.
• 15-year fixed: Higher monthly payment, much lower total interest. You also build equity (the part of the home you actually own) much faster. Rates on 15-year loans are usually about 0.5–0.75% lower than 30-year rates.
• 20-year fixed: A middle ground. You finish faster than 30 but the payment isn't as steep as 15.

A common rule of thumb is to take a 30-year loan for the lower payment and then make extra principal payments when you can. That gives you flexibility (you can drop back to the regular payment in a tight month) without being locked into a 15-year payment commitment.

Fixed-rate vs adjustable-rate (ARM)

On a fixed-rate mortgage, your interest rate is set for the entire life of the loan. The principal-and-interest part of your payment never changes. (Taxes and insurance can still change.)

On an adjustable-rate mortgage, or ARM, the rate is fixed for an initial period — like 5, 7, or 10 years — and then it adjusts, usually once a year, based on a market index. ARMs usually start with a lower rate than fixed mortgages, but you take on the risk that rates could be higher when the adjustment kicks in.

How interest is actually calculated

For the math-curious: each month, the lender calculates interest using a simple formula:

Whatever's left of your payment after that interest is taken out goes to principal, which reduces the balance for next month. That's why the math shifts over time. The formula behind your full payment (the one used in our payment calculator) is the amortization formula:

Common questions