What this calculator does
Compare two ways to pay off two mortgages:
1. Keep them separate. Make normal and extra payments to each mortgage individually.
2. Combine overpayments. Put all extra payments towards the highest-rate mortgage first.
When one mortgage is cleared, you can roll that monthly payment onto the other debt.
This typically reduces total interest and helps you become mortgage-free sooner.
Use the graph and yearly table to see the difference over time.
A simple real-life example
🏢 Olivia and Lucas each own a flat
đź’Ť When they move in together, they end up with two mortgages but only one home to live in.
They could sell one flat. If they do, they do not need a multiple mortgage calculator.
But if they keep the second flat and rent it out, they will likely have more money available each month:
• two incomes
• additional rental income
• only one home to live in (lower outgoings)
That extra cash could be used to overpay their mortgages.
At that point, they can either:
• keep the mortgages separate
• treat them as one plan and send extra money where it saves the most interest
This calculator helps compare those options.
A rollover example
Sometimes there are no overpayments because budgets are tight.
Even then, combining the mortgage payments can help.
When one mortgage finishes, its monthly payment can roll over to the other.
In this example, that alone saves about 9 years and ÂŁ31,600 compared with treating the mortgages separately.
Open the rollover example to see this in the graph and table. The rollover shows up most clearly in the table.
What counts as “extra payments”
In this calculator, extra payments include:
Monthly overpayments you enter
Payments redirected after one mortgage finishes
The normal monthly payment from the finished mortgage (if rollover is enabled)
What the effective rate means
The effective rate shows the return your extra payments achieved.
For example, if the effective rate is 5.9%, your overpayments saved interest roughly equivalent to earning 5.9% on that money.
If you can reliably earn more than that elsewhere, for example through investments, you might prefer to do that instead.
What this tool does not cover
This calculator does not include:
Lender overpayment limits or fees
Taxes
Property price changes
Changing interest rates
Rental risks or vacancy
It also does not settle the classic “overpay the mortgage vs invest instead” debate. That depends on risk tolerance, taxes, and personal goals.
This tool has a narrower purpose: to model multiple mortgages properly, which is difficult with calculators designed for a single loan.
Planning model only. For illustration, not financial advice.