You borrow $400,000 to buy a house. Your first monthly payment is $2,528.27 — and $2,166.67 of it is interest. Only $361.61 goes toward the house. That is not a mistake, and it is not a bad loan. It is amortization: the way every fixed-rate mortgage splits each payment between interest and principal. Once you understand the split, you understand the mortgage — and you can see exactly why extra payments and refinancing behave the way they do.
What Amortization Actually Means
Amortization is the schedule that pays a loan off in equal monthly instalments. The payment never changes, but what it does changes every month. Each month, the lender first charges interest on the balance you still owe; whatever is left over reduces the principal. Early on, the balance is large, so the interest charge is large and the principal portion is small. As the balance shrinks, the interest charge shrinks with it, and more of each payment goes to principal.
On a 30-year mortgage the split starts out extremely lopsided. In month 1 of our example, nearly 86% of the payment is interest. The principal portion does not overtake the interest portion until month 233 — more than 19 years in. After one full year, you have paid $30,339.24 and your balance has fallen by only $4,470.90. After five years, you still owe $374,443.91 of the original $400,000. That slow start is why extra payments made early are so powerful.
The Formula
The monthly payment comes from one formula:
M = P × r(1 + r)^n / ((1 + r)^n − 1)
- M = monthly payment; P = amount borrowed; r = monthly rate (annual rate ÷ 12); n = number of payments
For our example: P = $400,000, annual rate 6.5%, so r = 0.065 / 12 = 0.0054166667, and n = 360 months.
Worked Example: $400,000 at 6.5% for 30 Years
Step by step:
- Monthly rate: 0.065 / 12 = 0.0054166667
- Growth factor: (1.0054166667)^360 = 6.9917979739
- Numerator: 400,000 × 0.0054166667 × 6.9917979739 = 15,149.28
- Denominator: 6.9917979739 − 1 = 5.9917979739
- Monthly payment: 15,149.28 / 5.9917979739 = $2,528.27
So the payment is $2,528.27 a month: 360 payments add up to $910,177.95 in total, which means $510,177.95 of interest — more than the loan itself. Skip the arithmetic anytime with the free mortgage amortization calculator — it runs the same formula and shows the full breakdown.
The Amortization Schedule: Where Each Payment Goes
Here is what happens to that $2,528.27 over the life of the mortgage:
| Month | Interest | Principal | Balance left |
|---|---|---|---|
| 1 | $2,166.67 | $361.61 | $399,638.39 |
| 12 | $2,144.53 | $383.74 | $395,529.10 |
| 60 | $2,030.93 | $497.34 | $374,443.91 |
| 120 | $1,840.54 | $687.73 | $339,104.51 |
| 180 | $1,577.27 | $951.01 | $290,236.56 |
| 240 | $1,213.20 | $1,315.07 | $222,661.13 |
| 360 | $13.62 | $2,514.65 | $0.00 |
In month 1, $2,166.67 of your $2,528.27 is interest; in month 360, only $13.62 is. In year 1, $25,868.36 of the $30,339.24 you pay is interest. A free mortgage amortization schedule shows the same split for any loan, rate, or term.
Three Ways to Pay Less Interest
Shorten the term. The same $400,000 at 6.5% over 15 years instead of 30 costs $3,484.43 a month — $956.16 more — but total interest falls from $510,177.95 to $227,197.30, saving $282,980.65.
Pay a little extra. Add $200 a month ($2,728.27) and the mortgage ends in 293 months instead of 360 — nearly six years early — with total interest of $398,286.42, saving $111,891.54. Or pay half the payment every two weeks: 26 half-payments a year equals 13 full payments, which pays the loan off in about 290 months and saves $117,495.70.
Refinance when rates fall — but check the break-even. Refinancing the same $400,000 from 6.5% to 5.5% cuts the payment to $2,271.16, saving $257.11 a month, and breaks even after 24 months with $6,000 in closing costs. Beware the reset trap: refinancing five years in restarts the 30-year clock, and the new loan costs $12,897.48 more overall despite the lower payment.
5 Mistakes That Cost Real Money
- Only looking at the monthly payment. A longer term always lowers the payment and raises the total cost.
- Ignoring the first-year split. In year one, $25,868.36 of $30,339.24 paid was interest — the balance barely moved.
- Refinancing for the payment, not the total cost. A lower payment after a reset can still mean more total interest.
- Buying the maximum the lender approves. Lenders approve more than you can repay comfortably — your affordable payment decides the house, not their letter.
- Never recalculating. One extra payment a year, or a refinance when rates fall, can cut years off a mortgage.
The Bottom Line
A $400,000 mortgage at 6.5% for 30 years is $2,528.27 a month, $910,177.95 in total, $510,177.95 of it interest — and every one of those numbers comes from one formula and one schedule you can now read yourself. The levers are in your hands: a shorter term, a lower rate, or a little extra each month. For the instant version, use the free mortgage amortization calculator — plug in any amount, rate, and term, and see the payment, total interest, and full amortization table before you sign.
