Fixed vs. Adjustable Rate Mortgage Calculator  
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The Fixed vs. Adjustable calculator will help you determine whether you should choose an fixed rate loan or an adjustable rate loan. The interest rate for a fixed rate loan stays the same for the life of the loan. But, with an adjustable rate mortgage (ARM), the interest rate fluctuates periodically and is based on an index like the U.S. Treasury Security Yields (1 Year T Bill), the Cost of Funds Index (COFI) and the London Inter-Bank Offer Rate (LIBOR).

ARMs are attractive to home buyers because they generally offer a lower initial interest rate than a fixed rate mortgage, saving you more money up front. They also can save you more in the long term, provided interest rates remain stable or decrease. And often, home buyers choose an ARM because they can qualify for a larger loan amount. But for all these benefits there is a tradeoff – the risk of rates going up. When comparing fixed versus adjustable, it is important to look at the index against which your interest rate will be measured, as well as the historical and predicted growth or decrease of that index. Margins are also an important factor, especially when comparing loans between lenders

  • Enter ballpark values by dragging the sliders left or right. Enter exact numbers by clicking in the box and type your number (no commas).
  • Click on tabs for details and for more options.

General Loan Information

Loan Amount

Enter the amount of money you need to borrow.

Loan Term

The loan period is the length of time in years you will need to borrow the money before your loan is fully paid back. Typical fixed loan terms are 15 or 30 years.

Fixed Rate Loan Information

Fixed Rate

Enter the interest rate for the fixed loan.

Adjustable Rate Loan Information

Initial Rate

Enter the interest rate for the adjustable loan's initial period. This rate is sometimes called the teaser rate.

Initial Period

This is the period in which you receive the teaser rate, or the first contracted interest rate, which is usually lower than the current fixed rate. During this period, typically 1,3, 5, or 7 years, your interest rate will not fluctuate.

Adjustable Period

This is the time between changes in the interest rate, once the initial period has passed.. On some adjustable rate loans, the adjustable period can change monthly.

Period Cap

This is a limit set on how much the adjustable rate can increase or decrease in the adjustable period. If your adjustable period is 1 year and your adjustable cap is 2%, that means your interest rate cannot increase or decrease more than 2% each year.

Lifetime Cap

Like the adjustable cap, this is a limit set on interest rates, but this cap is the maximum increase you will receive over the lifetime of the loan. If you have your loan for 15 years and your lifetime cap is 6%, your interest rates cannot increase more than a total of 6% over the loan's starting rate during this 15 year period.

Current Index Rate

The index is a published interest rate used as a base interest rate on an adjustable rate loan. Typical indexes are the U.S. Treasury bill index, LIBOR (London Inter-Bank Offered Rate), or the COFI (Cost of Funds Index). Enter the value of your current index rate.


Margins are the percentage points added to the index by the lender to determine the final adjustable rate. The margin doesn't usually change over the life of the loan, but will often change from lender to lender.

Yearly Index Change

The number of percentage points you estimate the selected index will rise or fall. Select a value that makes sense to you or ask your lender to suggest a value.

Payment Graph Tab

The Payment Graph Tab contains an interactive line chart that compares your fixed vs. your adjustable monthly payment over 10 years. Grab the slider at the bottom of the graph and move it to compare monthly payments during a particular year.

Comparison Tab

The Comparison Tab presents an interactive table comparing the fixed and adjustable rate options. Rows show values for monthly payment, maximum monthly payment, interest paid, equity built, loan balance and total paid for the number of years in thes home selected in the Years in Home slider.

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