Mortgage overpayment vs ISA calculator

Whether cashing in is the right move depends on your circumstances — some people prefer the certainty of being mortgage-free, others prefer keeping the flexibility of an investment pot — but knowing when you'd reach that point is useful for the decision either way. The calculator simulates both balances month by month: the mortgage balance falling as you pay it down (plus any overpayment), and the ISA pot growing with your contributions and the assumed growth rate.

One thing surprises most people: how much you put in each month matters far more than the growth rate you achieve. A higher return helps, but you can't control the market. Increasing your monthly contribution by even £50 typically moves the crossover point more than pushing the assumed growth rate from 5% to 8%. The contribution is money you're actually committing — the rate is an assumption, shown in the sensitivity comparison below the chart.

ISAs are a tax wrapper, not an investment in themselves, and the amount you can pay in each tax year is capped — £20,000 across all your ISAs combined for 2025–26. If your monthly ISA contribution here would take you over that when annualised, you'd need to hold the excess outside an ISA (or spread it across a partner's allowance too), which changes the tax treatment this calculator doesn't model.

Mortgage overpayment savings are calculated exactly from the terms you enter. ISA growth is illustrative only — it assumes a single consistent annual rate throughout, when real investment returns vary year to year and can fall as well as rise. Most mortgage lenders also cap penalty-free overpayments, typically at 10% of the outstanding balance per year, flagged automatically above if your numbers exceed it.