First vs Second Mortgage Calculator
Compare refinancing first mortgage vs adding a second.
Compare refinancing your first mortgage against taking out a second mortgage for the same cash need. Enter first mortgage balance to size affordability.
What this tool does
This calculator compares two financing paths when you need cash: refinancing your entire first mortgage at a new rate, or keeping your current first mortgage and adding a second mortgage on top. It models the monthly payment outcome for each scenario, showing which route produces a lower combined payment based on your cash need, existing balance, current rate, available refinance rate, and second mortgage rate over your chosen term. The result illustrates how interest rate spreads and loan structure interact to affect your payment obligations. Note that this calculation assumes both mortgages run for the same duration and doesn't account for closing costs, fees, tax implications, or changes in rates over time. The output is for educational comparison only and models a single point-in-time snapshot of your financing options.
Enter Values
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Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
Say you need 50,000 in cash and have an existing 200,000 mortgage at 4%. Refinancing the whole balance to 250,000 at 4.5% gives a payment of about 1,389 a month. Keeping the first at 4% and adding a 50,000 second at 7% comes to about 1,056 plus 354, or roughly 1,409 a month. Here the refinance route is lower by about 19 a month, a small margin that rate and fee differences can easily reverse.
A worked example
With the defaults: current first mortgage balance of 200,000, current first rate of 4%, cash needed of 50,000, new refinance rate of 4.5%. The tool returns 19.48. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.
What moves the number most
The result responds to Current First Mortgage Balance, Current First Rate, Cash Needed, New Refinance Rate, and Second Mortgage Rate. Two inputs usually tip the answer one way or the other. Flipping each value past a round threshold shows which input moves the result most.
The formula behind this
Compute amortised payment for each route. Difference is the winner's saving. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.
Why this matters
A mortgage is usually the biggest single financial commitment a person makes. The difference between a well-chosen product and a hasty one can run into tens of thousands over the life of the loan. Modelling the numbers ahead of a decision shows how sensitive the outcome is to the rate and structure chosen.
What this doesn't capture
The figure shown reflects the core calculation; additional costs such as arrangement fees, valuation, legal fees, insurance, and any early-repayment charges (where applicable) sit on top and can add materially to the total cost of borrowing. Rates and product terms can also change over the life of the loan, which can shift the picture relative to this fixed-snapshot estimate.
Comparing a 4.5% refinance against a 7% second mortgage on £200,000, the two routes differ by about $19.48 a month.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
The calculator computes the monthly payment required under two scenarios: refinancing the entire first mortgage balance plus cash needed at the new refinance rate, versus keeping the current first mortgage and adding a second mortgage for the cash needed at the second mortgage rate. Both payments are calculated using standard amortisation over the selected term in years, treating the loan balance, interest rate, and term length as fixed inputs. The difference between the refinance payment and the combined first-plus-second payment indicates the monthly cost variance between the two approaches. The model assumes constant interest rates throughout the term, regular monthly payments, and no fees, closing costs, or early repayment penalties. It does not account for potential changes in rates, differences in loan origination costs, or tax implications. Because both routes use the same selected term, the kept first mortgage is modelled as re-amortised over that term rather than its remaining original schedule; matching the term to the time left on the existing mortgage gives a like-for-like comparison.
Frequently Asked Questions
When does second-mortgage win?
What about second-mortgage risk?
Fees to consider?
HELOC alternative?
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