Option ARM Calculator
An option (pick-a-payment) ARM lets you choose among several payment levels — but the minimum payment can cause your balance to grow. See all the options.
How the option arm calculator works
A payment-option ARM gives you several payment choices each month, including a minimum that can fall below the interest actually owed. This tool shows what happens when you repeatedly pick that minimum: the shortfall between what you pay and the interest due is added to your balance, so the loan grows instead of shrinking.
That is negative amortization. The calculator tracks the rising balance, applies any recast trigger where the loan is forced back onto a full repayment schedule, and reveals how a low monthly payment today can leave you owing more than you borrowed.
Worked example: with loan amount of $400,000, note interest rate of 6.75% and minimum-payment rate of 2.50%, the option arm calculator shows minimum payment of $1,580.48.
- Minimum payment
- $1,580.48
- Interest-only
- $2,250.00
- 30-year payment
- $2,594.39
- 15-year payment
- $3,539.64
The formula
Each month, interest due = balance times the current rate. If the minimum payment is less than that, the unpaid interest is added to the balance (negative amortization). The balance keeps compounding until a recast or balance cap triggers a forced re-amortization at a fully covering payment.
- Choosing the minimum payment is assumed each period; selecting a higher option would slow or stop balance growth.
- Negative amortization continues only until a balance cap or scheduled recast forces a full repayment schedule.
- The rate adjusts using index + margin within caps and rests on future benchmark levels that cannot be predicted.
- A typical balance cap limits how much the loan can grow, often a set percentage above the original amount.
- Taxes, insurance, and association dues are separate from the principal and interest figures shown here.
Results are estimates for educational purposes and are not financial advice. Confirm exact figures with your lender or a licensed advisor.
Questions about the option arm calculator
How can I owe more than I originally borrowed on this loan?
When your minimum payment is smaller than the month's interest, the gap does not disappear; it is added to your principal. Repeat that month after month and the balance climbs above the original loan amount.
This is negative amortization, the defining hazard of option ARMs, and it quietly erodes your equity even while property values hold steady.
What is a recast, and why does it matter so much here?
A recast is the point where the loan is forced back onto a fully amortizing payment, either after a set number of years or once the balance hits a cap, often around 110 to 125 percent of the original amount.
The new payment must repay the grown balance over the remaining term, so it can leap dramatically. Recast is when deferred cost finally arrives.
Are option ARMs still common in 2026?
They are far rarer than before the 2008 housing crisis, when minimum-payment option ARMs were widely blamed for unsustainable borrowing. Post-crisis ability-to-repay rules made them difficult to originate, and most lenders stopped offering them to typical borrowers.
You may still encounter the structure in portfolio or niche lending, which is why understanding negative amortization remains worthwhile.
Is the Option ARM Calculator free to use?
Yes. Every calculator on MortgageLoansCalculator is completely free, with no sign-up, login or paywall. Run as many scenarios as you like.
What is an option ARM, and why is it risky?
An option ARM lets you pick a minimum payment that may not even cover the interest, so the shortfall is added to your balance — negative amortization. Your debt can grow while you pay, then recast to a much higher payment. It is one of the riskiest loan types.
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