Mortgage Overpayment Break-Even Calculator

On a 200,000 mortgage at 4% with 20 years remaining, adding 200/month overpayment may not reach cash break-even within the remaining term — cumulative overpayment cash can exceed cumulative interest saved for the full period. This is normal: the big wins come from term reduction and interest saved at the end of the mortgage, even if pure-cash break-even is 'beyond term'.

How to use it

Enter current balance, monthly overpayment amount, interest rate, and years remaining. The tool computes when cumulative interest saved exceeds cumulative overpayment contributed.

Why break-even takes years

Interest saved compounds slowly at first because balance reductions are small. Over time each reduced balance saves more interest, and the compounding accelerates. The cash break-even point can fall anywhere from partway through the remaining term to beyond it, depending on the overpayment size, rate, and years left — before that point the cumulative cash paid in exceeds the interest saved so far.

Run it with sensible defaults

Using outstanding balance of 200,000, monthly overpayment of 200, interest rate of 4%, years remaining of 20 years, the calculation works out to Beyond Term. The defaults are meant as a starting point, not a recommendation.

The levers in this calculation

The inputs — Outstanding Balance, Monthly Overpayment, Interest Rate, and Years Remaining — do not pull with equal force. Not every input has equal weight. Adjusting one input at a time toward extreme values shows which ones move the result most.

How the math works

Simulates the mortgage with and without monthly overpayment month by month. Tracks cumulative interest saved (compared to no overpayment) versus cumulative overpayment cash. Returns the first month where saved exceeds overpaid.

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.