Mortgage Overpayment vs Invest Calculator

Path A ; Overpay then invest. Every month, your spare cash goes onto the mortgage as overpayment. The mortgage clears earlier than the original term. From the month it clears onwards, the full freed-up contractual payment plus the original spare cash get redirected into investments for the rest of the original term.

Path B ; Invest from day one. Mortgage stays on its contractual schedule. Spare cash goes into investments from the first month. When the mortgage finishes naturally at the end of the term, the freed-up contractual payment gets invested too ; though there's no remaining time for it to compound much.

Overpaying tends to win mathematically when your mortgage rate is high relative to realistic investment returns ; say a 6% mortgage against an assumed 5% return, especially outside a tax wrapper. The sensitivity grid above will show this clearly: a block of green cells in the high-rate, low-return corner.

Overpaying tends to win practically when you're close to retirement (less time for investments to recover from a downturn), when your income is unstable (debt reduction is the bigger risk-management win), when you'd otherwise hold the money in low-yielding savings rather than actually investing it, or when the psychological certainty of clearing debt matters more than a higher expected outcome.

Investing tends to win mathematically when realistic expected returns comfortably exceed your mortgage rate (say 7% returns versus a 4% mortgage), and especially when the investing happens inside an ISA or SIPP. Time matters: the longer you have, the more compound growth amplifies the gap.

Investing can look particularly strong for higher-rate taxpayers using a pension/SIPP, because pension tax relief changes the net cost of contributions. With long timeframes, that can make the investing route more resilient across a range of assumptions.

Should I overpay or invest?
Mathematically, investing tends to win when after-tax expected returns exceed your mortgage rate, particularly inside an ISA or SIPP. Overpaying tends to win in high-rate, low-return scenarios ; and when the certainty of clearing debt is worth more to you than a higher expected outcome.

Why does SIPP do so well?
Because you get tax relief on contributions (effectively up to 25%/67%/82% boost depending on rate band) and only the 75% taxable portion of withdrawals is taxed, often at a lower retirement marginal rate.

What about early repayment charges?
Most UK mortgages allow 10% overpayment per year penalty-free during a fixed term. The calculator doesn't model charges ; check your specific mortgage terms.

Doesn't being debt-free have value too?
Yes ; and it isn't in the maths. If a smaller, debt-free outcome lets you sleep better, that's a legitimate reason to overpay even when investing would win on paper.

Should I always max my SIPP?
Not automatically. Pensions can be tax-efficient, but money is locked up until at least age 55 for most people, rising to 57 from April 2028. If you might need access sooner, an ISA may be more flexible.

What's the sensitivity grid for?
To check whether your answer is robust. If the grid shows a wide block of cells favouring the same path around your inputs, you can be more confident. If your cell is one step from a flipped result, your conclusion is fragile.