Interest-Only ARM Calculator
An interest-only ARM keeps payments very low at first by deferring all principal. See that payment and the jump when amortization begins.
Free to use No sign-up needed Updates as you type
How the interest-only ARM calculator works
An interest-only ARM lets you pay only the interest for an opening period, keeping early payments low while the balance stays flat. This tool computes that interest-only payment as the loan balance times the current rate, then models what happens when the IO window closes.
At that point two things change together: principal repayment begins, and the rate may reset on its adjustment schedule. The result is amortized over the shorter remaining term, so the calculator shows both the comfortable early payment and the larger payment that follows.
Worked example: with loan amount of $400,000, interest-only rate of 5.75% and interest-only period (years) of 10, the interest-only ARM calculator shows interest-only payment of $1,916.67.
- Interest-only payment
- $1,916.67
- Payment after IO period
- $3,101.20
- Balance still owed
- $400,000
- Rate after reset
- 7.00%
| Loan amount | Interest-only payment |
|---|---|
| $200,000 | $958.33 |
| $400,000 | $1,916.67 |
| $600,000 | $2,875.00 |
| $800,000 | $3,833.33 |
The formula
During the IO period: payment = current balance multiplied by the current annual rate, divided into the period's installments, with no principal reduction. Afterward the payment re-amortizes the full balance at index + margin, capped, over the remaining months only, which raises it sharply.
How every calculator here computes, what it assumes and how the defaults stay current: our methodology.
- The balance does not fall during the interest-only window because no principal is being repaid in that phase.
- The length of the IO period and the reset schedule are defined by the specific loan terms.
- Rate resets after the IO period use index + margin within caps and depend on unpredictable future benchmark levels.
- Once amortization starts, the full balance is repaid over fewer months, which concentrates the payment increase.
- Escrowed taxes and insurance, plus any HOA dues, are not reflected in the payment figures shown.
Results are estimates for educational purposes and are not financial advice. Confirm exact figures with your lender or a licensed advisor.
Questions about the interest-only ARM calculator
Does paying interest only mean the loan never gets paid down?
During the interest-only window, correct, your balance stays where it started because nothing is going to principal. The loan is not interest-only forever, though.
When the IO period ends, full amortizing payments begin and must clear the entire balance over the remaining term. You can also pay extra toward principal during the IO phase voluntarily if your loan allows it.
How big is the payment shock when the IO period ends?
It is usually substantial, because the increase comes from two sources at once. Principal repayment switches on, compressed into fewer remaining years than the original term, and the rate may have reset higher.
The exact jump depends on your balance, the new rate, and how many months remain, but doubling or more is common when both effects land together.
Is an interest-only ARM ever a sensible choice?
It can be, for the right borrower. It suits people with irregular or bonus-heavy income who want low required payments but intend to pay principal when cash allows.
It also fits those confident they will sell or refinance before the IO period ends. It is risky for anyone relying on the low payment indefinitely, since the later increase is steep.
Is my information saved or shared?
No. Every calculation runs entirely in your browser. We never store, transmit or sell the numbers you enter.
How does an interest-only ARM work?
It combines two risks: for an intro period you pay only interest and build no principal, and the rate is adjustable. When the interest-only period ends and the rate resets, the payment can jump sharply — the payment shock this calculator projects.
Sources
- CFPB — Fixed-rate vs adjustable-rate mortgages
- Freddie Mac — Primary Mortgage Market Survey (weekly average rates)
- CFPB — Interest-only loans
How the figures on this site are calculated and tested: methodology · editorial policy.
Adjustable-Rate Mortgage (ARM) Calculator
Project payments through the fixed period and the rate adjustments that follow on an ARM.
ARM vs. Fixed-Rate Mortgage
Compare an adjustable-rate mortgage against a fixed-rate loan across likely rate scenarios.
ARM & Interest-Only ARM vs. Fixed
Compare a standard ARM, an interest-only ARM and a fixed-rate mortgage side by side.
Option ARM Calculator
See how minimum payments on an option ARM can cause negative amortization and a growing balance.
Fixed-Rate vs. Interest-Only Mortgage
Compare a conventional fixed-rate loan against an interest-only mortgage over time.
Fixed-Rate vs. SOFR ARM
Compare a fixed-rate mortgage with an index-based ARM under rising and falling rate paths.