Understanding mortgages: principal, interest, and amortization

A practical guide to what a monthly mortgage payment is actually buying you.

If you look at a 30-year mortgage on a $400,000 loan at 7%, the monthly principal-and-interest payment comes out to about $2,661. Over the life of the loan you'll pay roughly $958,000 — more than double the amount you borrowed. Where does the extra $558,000 go? Almost entirely to interest. And the way that interest is front-loaded is the single most surprising thing about how mortgages work.

Principal vs interest, payment by payment

Every mortgage payment is split between two things: paying down the loan balance (the principal) and paying the bank for the privilege of borrowing (the interest). On a fixed-rate mortgage, the total payment stays the same month after month, but the split between those two pieces shifts dramatically over time.

At the start of a 30-year loan at 7%, only about 17% of your monthly payment chips away at the loan balance. The other 83% is interest. Year 15 — exactly halfway through the loan term — your balance is still about 72% of what you borrowed. You haven't paid off "half your house" at the halfway point. Not even close.

This is why the amortization schedule matters. It's not a financial gotcha; it's just math. Interest each month is charged on the remaining balance. The balance is highest at the start, so the interest portion is highest at the start, so very little of your payment goes to principal early on. As the balance shrinks, less interest accrues each month, so more of the fixed payment goes to principal — and the curve accelerates.

The amortization formula

The fixed monthly payment P on a loan of amount L with monthly interest rate r (annual rate divided by 12) over n months is: 

P = L · r · (1 + r)^n / ((1 + r)^n − 1)

Plug in a $400,000 loan at 7% annual (so r = 0.07 / 12 ≈ 0.00583) over 360 months and you get the $2,661 figure. The formula doesn't care about your credit score, your down payment, or your local property taxes — those affect what number gets plugged in for L and r, not the formula itself.

Once you have P, the month-by-month schedule is easy:

  1. This month's interest = balance × r
  2. This month's principal = P − interest
  3. New balance = old balance − principal
  4. Repeat 360 times.

Our mortgage calculator does all of this for you and shows the full amortization table so you can see the principal/interest split for any given month.

Extra payments: where the leverage is

Because interest each month is charged on the remaining balance, any extra principal you pay early in the loan removes future interest that would have been charged on that balance for years. A single extra $5,000 principal payment in year 1 of the example loan above saves about $32,000 in total interest and shortens the loan by roughly 9 months.

The same $5,000 paid in year 25 saves only about $1,400. Same dollar, radically different impact, just based on when it lands. This is the reason extra-payment calculators exist: timing isn't a minor detail, it's the whole game.

Rate vs APR: the cost of the loan, not just the rate

The interest rate is what the bank charges on the balance. The APR (annual percentage rate) bakes in the upfront costs of getting the loan — origination fees, points, certain closing costs — and expresses the whole thing as one annualized rate.

A 6.75% rate with $8,000 in fees can have a higher APR than a 6.95% rate with zero fees, even though the second rate is bigger. APR is the closer apples-to-apples comparison, but it's not perfect either: APR assumes you'll keep the loan to maturity, and almost no one does. If you'll refinance or sell in 5 years, the upfront fees matter more than APR makes them look.

The "rules of thumb" worth ignoring

You'll see a lot of these. A few that don't hold up well:

  • "Spend no more than 28% of gross income on housing." This is a lender underwriting guideline, not personal-finance advice. It ignores your other obligations, your stage of life, and where you live. In high-cost cities almost no one hits it.
  • "A 15-year mortgage saves you a fortune." True in nominal interest paid, but the higher monthly payment crowds out investing in tax-advantaged accounts. The right answer depends on the spread between your mortgage rate and what you'd reasonably earn on those investments, which has flipped sign multiple times in the last 20 years.
  • "Always put 20% down." 20% down avoids PMI and lowers the loan amount, but in a fast housing market the cost of waiting another year to save the full 20% can be greater than several years of PMI. Run the numbers; don't follow the rule blindly.

What to actually compare when shopping

When you're getting quotes from multiple lenders, line up:

  • Interest rate, on the same loan amount and term.
  • Discount points (each point is 1% of loan, usually buying you ~0.25% off the rate).
  • Lender fees: origination, application, underwriting, processing.
  • Third-party fees the lender controls or has lower-cost vendors for.
  • APR, as the summary number.
  • Total cash to close, as the other summary number.

Two quotes with the same rate can differ by $5,000+ in upfront cost. Use our mortgage compare tool to put quotes side by side, and our refinance calculator if you already have a loan and are weighing a new one.

The short version

  • Interest is charged on the remaining balance. Early payments are mostly interest because the balance is highest then.
  • The amortization formula is fixed; everything else is just inputs.
  • Extra payments early have huge leverage. Late, almost none.
  • APR is a better comparison number than rate alone, but it assumes you hold the loan forever.
  • Trust the math, not the rules of thumb.