Portable Mortgage Calculator (What U.S. Buyers Might Expect)

Published on January 7th, 2026

This portable mortgage calculator is designed to help U.S. homeowners visualize how mortgage portability might work if it were ever introduced in the United States. While portable mortgages aren’t currently available here, the tool offers a way to explore how keeping a low existing rate could affect monthly payments when moving to a new home.

This calculator may be especially interesting for homeowners who purchased a house or refinanced at a historically low interest rate during the COVID-19 pandemic.

How This Portable Mortgage Calculator Works

The calculator models a scenario where part of your current mortgage, including its lower interest rate, is carried over to a new home purchase.

To generate an estimate, you’ll enter:

Your current mortgage balance and interest rate
The estimated value of your existing home
The price of the home you want to buy
Any equity funds you plan to apply toward the new purchase

Based on those inputs, the calculator assumes a home sale and estimates how much of your loan could be “ported” at your current rate and how much would need to be financed at today’s market rates. It also takes into account whether you are downsizing.

The results are presented for illustrative and informational purposes only.

What the Portable Mortgage Calculator Shows

This tool breaks the scenario into two parts so you can see how the math might look:

Ported loan portion: The part of your existing mortgage that keeps its lower rate
Additional loan portion: The new mortgage amount needed to cover the price difference
Total monthly payment: What you’d pay when both loans are combined
Estimated savings: How that total compares to financing the entire purchase at current rates

If an additional loan is required for move-up buyers, the calculator uses a default current interest rate of 6.2%. You can change this rate and the loan term using the “Advanced Options” feature.

The goal is to help you visualize how portability could change the math — not to predict exact loan terms.

Why This Matters for U.S. Homeowners

More than half of U.S. homeowners have mortgage rates below 4%, many locked in during the pandemic through purchases or refinances. As rates climbed, that gap created what’s often called the rate lock-in or golden handcuffs effect: homeowners want to move, but don’t want to give up their low rate.

Mortgage portability has entered the national conversation as a potential way to ease that tension. While it wouldn’t solve housing affordability on its own, the concept could help explain how homeowners might regain flexibility without fully resetting their mortgage at today’s rates.

Important Limitations

This portable mortgage calculator is intentionally simplified and comes with a few key limitations:

Portable mortgages are not currently offered in the U.S.
Property taxes, homeowners insurance, and private mortgage insurance are not included in estimated monthly payments
Real-world programs would likely include eligibility rules, caps, and underwriting requirements

Due to these limitations, the results should be viewed as educational rather than predictive. It illustrates what might be possible if U.S. lenders offered a portable mortgage solution.

How to Think About Your Next Move

Even without mortgage portability, understanding your rate advantage can help you plan more strategically. Comparing scenarios, exploring equity options, and talking with experienced real estate and mortgage professionals can clarify what’s realistic today — and what changes in the future might mean for you.

This calculator is meant to help you picture one possible path forward as policymakers debate new ideas — and to give you clearer insight into how much your low rate is really worth when you consider making a move.

If you’ve built up significant equity and are looking to make a move, check out HomeLight’s modern Buy Before You Sell program. This innovative buying solution unlocks the equity in your current home, streamlining and simplifying the entire process. You can make a strong, non-contingent offer on your new home and only move once. Watch the short video below to learn more.

At HomeLight, our vision is a world where every real estate transaction is simple, certain, and satisfying. Therefore, we promote strict editorial integrity in each of our posts.