You borrow $400,000 to buy a house. Thirty years later you have paid back $910,178. The extra $510,178 is the price of borrowing — more than the loan itself. It comes from one formula, and once you can run it by hand, no mortgage offer can surprise you.
What a Monthly Mortgage Payment Actually Is
Every month, your fixed payment splits in two: interest on the balance you still owe, and principal that shrinks the balance. Early on, interest eats most of the payment because the balance is large; late in the mortgage, almost the whole payment goes to principal. That slow shift is called amortization. Your payment never changes (on a fixed-rate mortgage), but what it does changes every single month.
The Formula
The monthly payment formula is:
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 $910,178 in total, which means $510,178 of interest — about 56.1% of everything you pay is interest, not the house. Skip the arithmetic anytime with the free mortgage calculator — it runs the same formula and shows the full breakdown.
The Amortization Table: 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 |
Read the first row: in month 1, $2,166.67 of your $2,528.27 is interest — nearly 86% of the payment buys you nothing. By the final month, only $13.62 is. After one full year (12 payments, $30,339.27 paid), the balance has fallen by only $4,470.90 — the other $25,868.36 went to interest. After five years (60 payments, $151,696.20 paid), you still owe $374,443.91. That is normal amortization on a 30-year mortgage: the balance starts high, so interest starts high, and it stays high for years.
How Much House Can You Afford?
Most people ask the question backwards: they pick a house price, then discover the payment. Flip it — start from the payment you can afford and work backwards. If you can afford $2,500 a month at 6.5% over 30 years, the most you can borrow is $395,527 — not $400,000, because $400,000 at those terms costs $2,528.27 a month. At $2,000 a month you can borrow $316,422; at $3,000 a month, $474,632. That backwards question — how much house can i afford on the payment I can actually afford — is the one that keeps a mortgage from becoming a burden. Remember the down payment sits on top of this: a $500,000 house with 20% down ($100,000) is exactly our $400,000 loan, while 10% down means a $450,000 loan and a $2,844.31 monthly payment.
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 $510,178 to $227,197, saving $282,981.
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 $398,287, saving $111,891.
Hunt the rate, not the monthly payment. A lender who stretches the term to lower your payment is not giving you a deal — a longer term means more interest, every time. On our $400,000 over 30 years, 5.5% means $2,271.16 a month and $417,616 of interest; 7.5% means $2,796.86 a month and $606,869. Two percentage points cost an extra $189,253 in interest on the same house.
5 Mistakes That Cost Real Money
- Comparing interest rates instead of APRs. Fees hide in the gap between the two numbers — always compare APR to APR.
- 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 $30,339.27 paid was 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, $910,178 in total, $510,178 of it interest — and every one of those numbers comes from one formula you can now run yourself. The payment is fixed, but the split between interest and principal moves every month, and 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 calculator — plug in any amount, rate, and term, and see the payment, total interest, and full amortization table before you sign.
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