Interest vs. principal over loan lifetime

Segment

Value

Percentage

Principal

$200,000

43.9%

Interest

$255,089

56.1%

Month

Payment

Principal

Interest

Balance

$1,264.14

$180.80

$1,083.33

$199,819.20

$1,264.14

$181.78

$1,082.35

$199,637.42

$1,264.14

$182.77

$1,081.37

$199,454.65

$1,264.14

$183.76

$1,080.38

$199,270.89

$1,264.14

$184.75

$1,079.38

$199,086.14

$1,264.14

$185.75

$1,078.38

$198,900.39

$1,264.14

$186.76

$1,077.38

$198,713.63

$1,264.14

$187.77

$1,076.37

$198,525.86

$1,264.14

$188.79

$1,075.35

$198,337.07

$1,264.14

$189.81

$1,074.33

$198,147.26

$1,264.14

$190.84

$1,073.30

$197,956.42

$1,264.14

$191.87

$1,072.26

$197,764.55

$1,264.14

$192.91

$1,071.22

$197,571.64

$1,264.14

$193.96

$1,070.18

$197,377.68

$1,264.14

$195.01

$1,069.13

$197,182.67

$1,264.14

$196.06

$1,068.07

$196,986.61

$1,264.14

$197.13

$1,067.01

$196,789.49

$1,264.14

$198.19

$1,065.94

$196,591.29

$1,264.14

$199.27

$1,064.87

$196,392.03

$1,264.14

$200.35

$1,063.79

$196,191.68

$1,264.14

$201.43

$1,062.70

$195,990.25

$1,264.14

$202.52

$1,061.61

$195,787.73

$1,264.14

$203.62

$1,060.52

$195,584.11

$1,264.14

$204.72

$1,059.41

$195,379.39

What is amortization?

Amortization is the process of spreading a loan into equal periodic payments over its term. Each payment covers the interest owed on the remaining balance plus a portion of the principal. As the balance decreases, more of each payment goes toward principal and less toward interest - a shift that accelerates in the second half of the loan.

The payment formula

The fixed monthly payment M for a loan of principal P, monthly interest rate r (annual rate ÷ 12), and n total payments is:

M = P × [r(1 + r)^n] / [(1 + r)^n − 1]

For a $300,000 mortgage at 6% for 30 years: r = 0.005, n = 360, M ≈ $1,799/month.

Front-loading of interest

In the early years of a loan, the vast majority of each payment is interest. For the $300k / 6% / 30-year example:

Payment #	Principal	Interest	Remaining balance
1	$299	$1,500	$299,701
12	$317	$1,482	$296,202
60 (yr 5)	$382	$1,417	$283,861
180 (yr 15)	$607	$1,192	$238,193
300 (yr 25)	$966	$833	$165,791
360 (yr 30)	$1,790	$9	$0

Over the full 30-year term, total payments equal $647,514 - meaning $347,514 in interest on a $300,000 loan.

Extra payments

Adding even a small amount to each payment reduces the principal faster, shortening the loan term and cutting total interest paid significantly. An extra $200/month on the example above saves approximately 6 years and $76,000 in interest.

When more than half the payment goes to principal

For a 30-year mortgage, the tipping point where each monthly payment contributes more to principal than to interest arrives around year 20 (approximately payment #241). Until then, the majority of every payment services interest on the outstanding balance. This is why refinancing early in a loan can make mathematical sense - you restart the amortization clock and may pay front-loaded interest for years again.

Negative amortization

Negative amortization occurs when a minimum payment does not cover the interest due for that period. The unpaid interest is added to the principal balance, causing the loan to grow larger over time rather than smaller. This scenario was common with certain adjustable-rate mortgage (ARM) products that offered artificially low introductory payment caps. Always verify that your scheduled payment covers at least the full interest accrued each period.

How to read the schedule

Column	What it shows
Payment #	Sequential number of the payment (1 = first payment)
Date	Scheduled payment date based on start date
Payment	Total amount due (fixed for a standard loan)
Principal	Portion of this payment that reduces the outstanding balance
Interest	Portion that pays the interest accrued since the last payment
Balance	Remaining principal after this payment is applied