Months before a monthly overpayment produces more interest saved than the overpayment cash itself.

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. Break-even is typically 40-60% of the way through the remaining term — before that, you're paying in; after, you're reaping net savings.

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. Nudge the inputs toward your own situation and the output recalculates instantly. 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. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.

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. The working is transparent — you can verify every step yourself in the formula section below. No black box, no opaque "proprietary model".

Why this matters before you sign

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. Running the numbers here before committing is the cheapest form of due diligence available.

What this doesn't capture

The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.