How a Mortgage Actually Works (Amortization, Explained)

Most people sign a mortgage without ever seeing the one number that explains the whole thing: how little of their early payments actually pays down the house. You send the bank the same amount every month for thirty years, and it feels like you are steadily buying your home. You are not, at least not at first. For the first several years you are mostly renting money, and the payment schedule is designed to make that invisible.

The mechanism behind all of it is amortization, a schedule that splits every fixed payment into interest and principal. The split is not even, and it is not supposed to be. Understanding how it works changes how you think about a 15-year versus a 30-year loan, whether points are worth it, and why one extra principal payment early on is worth more than five extra payments later.

The one idea: interest is charged on the balance

Here is the whole engine in one sentence. Each month, the lender charges interest on whatever you still owe, takes that interest out of your payment first, and applies only the leftover to your balance.^[1] That is it. Everything else about amortization falls out of that rule.

Say you borrow $400,000 at a 7% fixed rate for 30 years. The monthly rate is 7% divided by 12, about 0.583%. Your fixed payment works out to roughly $2,661 a month for principal and interest. In month one, the interest is 0.583% of $400,000, which is about $2,333. That leaves only around $328 to actually reduce what you owe. Your $2,661 payment moved the balance by $328. The other 88% vanished into interest.

Now roll it forward. Next month the balance is a hair lower, so the interest is a hair lower, so slightly more of the fixed payment goes to principal. Every month the principal chunk grows and the interest chunk shrinks. It is a slow crossover. On that 30-year 7% loan, you do not reach the point where half your payment goes to principal until somewhere around year 18. The back half of the loan is where you finally start buying the house in earnest.

What an amortization schedule actually looks like

Abstract percentages do not land the way a real table does. Here is the shape of that $400,000, 7%, 30-year loan at a few points in its life. The payment is a constant $2,661. Watch the interest and principal columns trade places.

Look at what that table is telling you. After five full years of paying $2,661 every single month, roughly $160,000 in total, you have knocked the balance down by only about $24,000. The rest, more than $135,000, was interest.^[2] That is not a scam, it is just what borrowing $400,000 for a long time costs. But it is worth seeing plainly, because it reframes every decision that follows.

Your payment is PITI, not just P and I

The $2,661 in my example is only the principal and interest, the part that pays off the loan. Your actual monthly bill is bigger, because it is usually PITI: principal, interest, taxes, and insurance.^[3] Taxes means your annual property tax. Insurance means your homeowners policy, plus private mortgage insurance if you put down less than 20%.

Here is the part people get surprised by. The lender typically collects those taxes and insurance premiums as part of your monthly payment and holds them in an escrow account, then pays the tax bill and the insurance premium for you when they come due.^[4] You send one number every month, the servicer splits it up. The upside is you are not hit with a surprise multi-thousand-dollar tax bill once a year. The catch is that when your property taxes or insurance premium rise, your monthly payment rises too, even on a fixed-rate loan. The rate is fixed. The escrow is not.

15-year vs 30-year: the real tradeoff

This is where amortization stops being trivia and starts costing or saving you real money. A 15-year mortgage is not just a 30-year loan paid twice as fast. It usually comes with a lower interest rate, because the lender takes on less risk over a shorter window.^[5] Combine the shorter term with the lower rate and the total interest difference is enormous.

Take that same $400,000. On a 30-year loan at 7%, you pay around $558,000 in interest. On a 15-year loan at, say, 6.25%, the monthly principal-and-interest payment jumps to roughly $3,430, a lot higher, but the total interest over the life of the loan is only about $217,000. You cut the interest bill by more than half, and you own the place free and clear in 15 years instead of 30. The price of that is roughly $770 more every month.

So which is better? My honest take: the 15-year is mathematically cheaper and it is not close, but the 30-year is not the mistake people online make it out to be. The 30-year's real product is flexibility. Your required payment is lower, so in a bad month, a job loss, a medical bill, a broken furnace, you are not on the hook for the higher number. If you have the discipline to make extra principal payments voluntarily on a 30-year, you can capture most of the 15-year's savings while keeping the escape hatch. The catch is the discipline part. Most people do not actually send the extra. If you are the kind of person who will, the 30-year with extra payments is the smart, flexible play. If you know you will spend the difference, the 15-year forces the good outcome on you. How this fits your broader picture depends on the math in renting versus buying a home.

Points: paying up front to lower the rate

A mortgage point, also called a discount point, is prepaid interest. One point costs 1% of the loan amount and typically buys down your rate by about 0.25%, though the exact amount varies by lender.^[6] On the $400,000 loan, one point costs $4,000 up front and might drop your rate from 7% to 6.75%.

Whether that is worth it is pure break-even math. Figure out how much the lower rate saves you each month, then divide the cost of the points by that monthly saving. That is how many months it takes to get your money back. If a $4,000 point saves you around $67 a month, you break even at roughly 60 months, five years. Stay in the loan past that and you are ahead. Sell or refinance before it and you lit the money on fire.

Extra principal is the best return you can get

Now the move I actually feel strongly about. If you have spare cash and you want a guaranteed return, sending extra money to principal early in a mortgage is one of the best deals in personal finance. Here is why it works so well. Every dollar you apply to principal permanently removes that dollar from the balance, which means it erases all the future interest that dollar would have generated for the rest of the loan.^[7]

The timing is everything, and it flows straight from the amortization table above. A dollar of principal paid in year one avoids nearly 30 years of compounding interest. The same dollar paid in year 25 avoids only a few years of it. Early principal is worth dramatically more than late principal. On that 30-year $400,000 loan, adding just one extra monthly payment per year can shave roughly five to six years off the term and save tens of thousands in interest.

There is one operational trap. When you send extra money, you have to tell the servicer to apply it to principal. If you do not, many of them will just treat it as an early payment toward next month, which does nothing for you. Mark it "apply to principal" every time. And before you accelerate, check that your loan has no prepayment penalty, most modern conforming loans do not, but confirm it. The one honest counterargument is opportunity cost: if your mortgage rate is low and you could reliably earn more elsewhere, the math flips. If your rate is high, prepaying is a risk-free return equal to your rate, which is hard to beat. This is closely related to the choice in a home equity loan versus a refinance when you want to change the loan itself instead of just paying it down faster.

What I'd do

The way I think about a mortgage is that the term and the rate, not the price of the house, decide what you actually pay. A quarter-point of rate or ten years of term moves the total by six figures. That is where the attention belongs.

Concretely: get the amortization schedule for any loan before you sign it and look at the year-five balance, not just the monthly payment. Budget for PITI and expect the escrow portion to creep up over time. If the 15-year payment is genuinely comfortable, take it, the interest savings are real and large. If it is a stretch, take the 30-year and actually send extra principal early, marked to principal, treating it like the forced-savings account it is. Skip points unless you are certain you will hold the loan past the break-even. None of this is complicated. It just requires looking at the one table the lender never puts in front of you. A lot of what shapes the rate itself is out of your hands, which is the whole story in why mortgage rates don't follow the Fed.