An interest only mortgage calculator compares the payment during an interest-only period with the higher payment required when principal repayment begins. U.S. borrowers can use it to model the payment transition and estimated interest across both phases of a fixed-rate scenario.
Interest-only mortgage calculator
Your entries stay in your browser. This calculator makes no external requests.
How to use this interest only mortgage calculator
- Enter the mortgage principal that remains outstanding.
- Add the annual interest rate used for the entire example.
- Enter the number of years during which scheduled payments cover interest only.
- Provide the amortizing term that begins after the interest-only phase, then compare both payments and total interest.
How interest-only and amortizing payments differ
During the initial phase, the scheduled monthly amount is the outstanding balance multiplied by the monthly rate:
Interest-only payment = P × (annual rate ÷ 12 ÷ 100)
This model assumes no scheduled principal reduction during that period, so the same balance enters the next phase. The mortgage calculator interest only result then re-amortizes that balance using the standard fixed-payment formula over the term entered for principal repayment. Total modeled interest equals all interest-only payments plus all later amortizing payments minus the original principal.
Worked interest-only example
Consider an example $300,000 mortgage with a 5% annual rate and a 10-year interest-only period. The monthly rate is 0.05 ÷ 12. Multiplying that rate by $300,000 produces an initial scheduled payment of $1,250.00 per month. Because that amount covers interest only, the modeled principal remains $300,000 after the first phase.
If the balance is then amortized over 20 years at the same example rate, the estimated post-interest-only payment is about $1,979.87. The interest only mortgage payment calculator combines 120 initial payments with the later 240-payment schedule to estimate total interest. It does not assume that refinancing or a balloon payoff will be available. Compare transitions before considering any loan offer.
Assumptions and limits
- The same fixed interest rate is used during both phases solely for a consistent comparison.
- Actual interest-only products may have adjustable rates, balloon features, qualification rules, or different payment structures.
- No principal is scheduled during the initial period, although optional voluntary payments may be handled differently by a servicer.
- Taxes, insurance, HOA dues, mortgage insurance, fees, and closing costs are excluded.
- Rates and terms are user-entered examples that must be verified directly with a lender.
Interest only mortgage calculator FAQ
Does the loan balance decline during the interest-only period?
Not under the schedule modeled here. The required payment covers accrued interest, leaving principal unchanged unless the borrower separately pays additional principal.
Why can the payment rise after the initial period?
The same principal must begin amortizing over fewer remaining years. The later payment therefore includes both interest and enough principal to repay the balance within the entered amortizing term.
Does a smaller initial payment mean the loan costs less?
No. Delaying principal can increase total interest and creates a later payment transition. Compare lifetime cost, payment risk, fees, and alternatives rather than evaluating only the introductory amount.
Compare the two payment phases
Compare a fully amortizing loan with the mortgage payment calculator, study the balance path in the amortization schedule calculator, or review lifetime cost with the total-interest calculator. The savings guide discusses accessible reserves. The Consumer Financial Protection Bureau explains the payment transition in its guide to interest-only loans.
Last reviewed: October 5, 2026
Author: Harsh Kardam
Our approach and limitations are detailed in the Editorial Policy, Financial Disclaimer, and Privacy Policy.
Disclaimer: This is an educational estimate, not a rate quote, approval, or financial, legal, or tax advice. Verify current rates, fees, and terms directly with your lender or provider.
