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Pick when you could have started. This replays real mortgage rates, stock market returns and savings rates from then to the latest month.
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Both paths spend the same each month. The difference is where the extra goes.
| Overpaid | Invested |
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This overpay mortgage or invest calculator replays the pay off mortgage early or invest question with what actually happened, not an assumed return. Pick the UK or the US and a year from 2000 onwards, and it runs month by month from that January to the latest data.
Both paths spend the same each month. One pays the mortgage as scheduled and invests the extra. The other overpays: it puts the extra into the mortgage as extra payments and, once the mortgage is cleared, invests the whole payment until the original term would have ended. Each ends as investments minus mortgage owed, so overpaying vs investing is compared like for like.
Unless you type your own, the mortgage owed is the average UK home or typical US home in the start month, less a 25% deposit or 20% down payment, at the average new two-year fix (UK) or 30-year fixed rate (US). The £200 or $300 a month is an example, not an average. You can type over the extra, a lump sum, the mortgage owed and the rate, and set the years left, how often the rate is refixed, and stocks or savings. Returns are tax-free with no fund fees, as in a stocks and shares ISA or Roth IRA. Early repayment charges (ERCs), pension or 401(k) tax relief and the mortgage interest deduction are left out; the page warns when UK overpayments pass the 10% a year most fixed deals allow.
These use the calculator's starting figures for one UK and one US case, and are refreshed when the data updates.
Had you put £200 a month into the stock market from January 2015, instead of overpaying a £131,727 mortgage, by August 2026 you'd be £38,000 better off having invested.
Had you put $300 a month into the stock market from January 2015, instead of overpaying a $151,210 mortgage, by August 2026 you'd be $57,000 better off having invested.
This page can't say what will happen next, only what would have happened. Pick UK or US and a start year above, then type in your own mortgage, rate and monthly amount; "How it played out" also shows how far behind the winner fell at its worst. Returns are counted tax-free, so money invested outside an ISA or Roth IRA would do somewhat worse than shown.
The invest path works like a stocks and shares ISA: returns aren't taxed and there are no fund fees. For the UK, the stock market figures are the S&P 500 with dividends reinvested, converted to pounds each month, as a stand-in for a global tracker. Real platform and fund fees would pull the invest path down a little.
Pensions aren't modelled. A pension adds tax relief on the way in, tax on most of what comes out, often an employer contribution, and money you can't reach until later life. None of that is here, so read the invest path as an ISA, not a pension.
A 401(k) isn't modelled. Traditional 401(k) contributions come out of pay before income tax and are taxed when withdrawn, and many employers add a match; neither the tax saving nor the match shows up here. The invest path is closer to a Roth IRA.
The invest path behaves like a Roth IRA: returns grow untaxed, with no fund fees. With the US selected, stocks are the S&P 500 with dividends reinvested and savings earn the 3-month Treasury bill yield. The model doesn't apply Roth IRA contribution limits, so a large monthly amount or lump sum may be more than one account would take in a year.
The mortgage interest deduction is left out, because most households take the standard deduction and get nothing from it. If you itemise, your mortgage interest cost less after tax than shown here, which would tilt the result towards investing.