Amortization Calculator

What is amortization?

Amortization is the process of spreading a loan into a series of fixed payments over time. Each payment covers both interest and principal, with the proportion shifting over the life of the loan. Early payments are mostly interest; later payments are mostly principal.

Understanding amortization helps you see exactly where your money goes and how strategies like extra payments can save you thousands in interest.

The amortization formula

The monthly payment for an amortizing loan:

$$M = P \times \frac{r(1+r)^n}{(1+r)^n - 1}$$

Where:

• $M$ = Monthly payment
• $P$ = Principal (loan amount)
• $r$ = Monthly interest rate (annual rate ÷ 12)
• $n$ = Total number of payments

Example calculation

For a $300,000 loan at 7% for 30 years:

$$\begin{aligned} r &= 0.07 / 12 = 0.00583 \\[0.5em] n &= 30 \times 12 = 360 \\[0.5em] M &= \$300,000 \times \frac{0.00583(1.00583)^{360}}{(1.00583)^{360} - 1} \\[0.5em] &= \$1,996 \end{aligned}$$

How amortization works

Each month, your payment is split:

Interest portion:

$$Interest = Balance \times Monthly\ Rate$$

Principal portion:

$$Principal = Monthly\ Payment - Interest$$

First vs last payment comparison

For a $300,000 loan at 7% over 30 years:

	First payment	Last payment
Interest	$1,750 (87.7%)	$12 (0.6%)
Principal	$246 (12.3%)	$1,984 (99.4%)
Total	$1,996	$1,996

Early in the loan, most of your payment goes to interest. By the end, almost all goes to principal.

The true cost of a mortgage

Over 30 years, a $300,000 mortgage at 7% costs:

Component	Amount
Principal	$300,000
Total interest	$418,527
Total cost	$718,527

You pay nearly 1.4x the home's price in interest alone!

Impact of interest rate

For a $300,000 30-year loan:

Rate	Monthly payment	Total interest	Total cost
5%	$1,610	$279,767	$579,767
6%	$1,799	$347,515	$647,515
7%	$1,996	$418,527	$718,527
8%	$2,201	$492,467	$792,467

A 1% rate difference = ~$70,000 in interest over the life of the loan.

15-year vs 30-year mortgage

Term	Payment	Total interest	Interest saved
30 years	$1,996	$418,527	—
15 years	$2,696	$185,367	$233,160

The 15-year mortgage costs $700 more monthly but saves over $230,000 in interest.

Power of extra payments

Adding extra principal payments accelerates payoff and reduces interest:

For a $300,000 loan at 7% over 30 years:

Extra payment	Payoff time	Interest saved	Time saved
$0	30 years	—	—
$100/month	25.3 years	$63,000	4.7 years
$200/month	22.4 years	$105,000	7.6 years
$500/month	17.3 years	$179,000	12.7 years

Even $100 extra per month saves thousands.

Strategies for paying off faster

Biweekly payments

Pay half your monthly payment every two weeks. This results in 26 half-payments (13 full payments) per year instead of 12.

Effect: Pays off a 30-year mortgage in about 25 years.

Round up payments

Round your $1,996 payment to $2,000. Small increases add up over time.

Annual lump sum

Apply tax refunds, bonuses, or windfalls directly to principal.

Refinance to shorter term

When rates drop, refinancing to a 15-year mortgage forces faster payoff.

Reading an amortization schedule

Each row shows:

• Payment number: Which payment in the sequence
• Payment amount: Total monthly payment
• Principal: Amount reducing your balance
• Interest: Amount going to the lender
• Balance: Remaining loan amount

Amortization vs other loan types

Loan type	How it works
Amortizing	Fixed payments, interest front-loaded
Interest-only	Pay only interest initially
Balloon	Small payments, large final payment
Adjustable	Rate and payment can change

Negative amortization warning

Some loans allow payments less than the interest owed. The unpaid interest adds to your balance — your debt grows instead of shrinking. Avoid these loan structures.

When extra payments make sense

Do make extra payments when:

• You've maximized tax-advantaged retirement accounts
• You have emergency savings
• The guaranteed return (interest rate) exceeds investment alternatives
• You value the psychological benefit of debt-free living

Consider alternatives when:

• Interest rate is very low (under 4%)
• You have higher-interest debt
• Investment returns likely exceed mortgage rate
• You need liquidity

Amortization for other loans

The same principles apply to:

• Auto loans: Typically 3-7 years
• Personal loans: 1-5 years
• Student loans: 10-25 years
• Business loans: Various terms

Tax considerations

Mortgage interest may be tax-deductible, which effectively reduces your rate. However, the 2017 tax law changes mean fewer homeowners itemize. Consult a tax professional for your situation.

Tips for managing amortization

1. Understand your schedule — Know how much interest you're paying
2. Consider refinancing — When rates drop significantly
3. Prioritize high-interest debt first — Credit cards before mortgage
4. Automate extra payments — Consistency matters
5. Track progress — Watch your balance decrease for motivation
6. Read the fine print — Some loans have prepayment penalties