Invest vs Pay Off Mortgage Calculator UK

Once you have a mortgage and a bit of spare cash, you face a question with no obvious answer. Should you overpay the mortgage and become debt-free sooner, or should you invest the same money and let it grow in the market? The answer depends on your interest rate, your expected investment return, your tax situation, and a few personal factors that no rule of thumb can capture for you.

Our invest vs pay off mortgage calculator does the maths. Enter your mortgage balance, your rate, your remaining term, and an expected investment return. The calculator runs both paths over your full mortgage term and tells you which one leaves you with more money at the end.

Contents

• The Core Question
• How to Use the Invest vs Pay Off Mortgage Calculator
• The Underlying Maths
• Why Tax Wrappers Tilt the Answer
• When Overpayment Wins
• When Investing Wins
• Frequently Asked Questions

The Core Question

Every pound of spare cash you have can do exactly one of two things: knock down debt or buy assets. If your mortgage is at 4.5% and you can earn 7% a year in a global tracker, the maths leans toward investing. If your mortgage is at 6.5% and your investment return is 5%, overpayment is the obvious winner.

But neither of those simple comparisons is the full picture. Investment returns are taxed unless they sit inside an ISA or pension. Mortgage interest is paid from your post-tax income. And the certainty of guaranteed debt reduction is not the same as the uncertainty of expected market returns. The calculator handles all of this in the background and tells you the net effect on your wealth.

How to Use the Invest vs Pay Off Mortgage Calculator

The calculator needs five pieces of information. Most are sitting on your most recent mortgage statement.

1. Mortgage Balance

The amount you currently owe on your mortgage. If you have multiple mortgaged properties, run the calculator separately for each one because the rates and terms usually differ.

2. Mortgage Rate

Your current rate, after any fixed-rate deal. If you are part-way through a fixed term, use the actual rate you are paying. If you are due to remortgage soon, run the calculator twice: once at your current rate and once at the rate you expect to roll onto.

3. Remaining Term

How many years you have left on the mortgage. The calculator simulates the full term, so a 25-year remaining term gives the comparison enough time for compounding to do its thing.

4. Monthly Spare Cash

The amount of extra money you have available each month after covering your mortgage minimum and other essentials. This is what gets either thrown at the mortgage or invested into the market. £200 a month is a useful starting point if you are not sure.

5. Expected Investment Return

The annual rate of return you expect from the invested alternative. Use a real (post-inflation) rate of around 5% to 7% for a globally diversified equity portfolio. Our piece on reasonable rates of return explains where these numbers come from and why the headlines are usually optimistic.

You can also tell the calculator whether the investment is held in a tax wrapper. An ISA or pension means returns are not taxed. A general investment account means the calculator applies a haircut for the dividend tax and capital gains you would owe.

The Underlying Maths

Mortgage overpayment is a guaranteed return at your mortgage rate. If your rate is 4.5%, every £100 you overpay saves you £100 worth of future interest, which is mathematically identical to earning a 4.5% return on that £100. There is no risk and no tax to pay because mortgage interest is settled with post-tax money.

Investing is an expected return with risk. If you invest £100 and the market returns 7% on average, you expect to have around £197 after a decade. But the actual outcome depends on the path of returns. A bad sequence early on, especially in a long-term comparison, can mean the investment underperforms the certain mortgage saving even if the average return is higher.

The calculator assumes a smooth average return rather than modelling sequence risk. That is a simplification but a useful one. If you want to stress-test the answer against a poor sequence of returns, our drawdown calculator shows how the same dynamic plays out for retirement portfolios.

Why Tax Wrappers Tilt the Answer

The comparison changes a lot once you factor in UK tax wrappers. Inside a Stocks and Shares ISA, all gains and dividends are tax-free. Inside a SIPP, you get income tax relief on contributions and the gains are tax-free until withdrawal.

For a basic-rate taxpayer with a 4.5% mortgage and 7% expected returns:

• Invest in a GIA (taxable account) - net return after dividend and CGT might be 5.5% to 6%, marginal call against the mortgage
• Invest in an ISA - net return stays at 7%, clear win over the mortgage
• Invest in a SIPP via salary sacrifice - effective return on contribution is around 13% in year one due to tax relief, demolishes any mortgage rate

This is why most UK households should max out ISAs and pensions before they think about overpaying their mortgage. The tax wrapper is doing more work than the rate-versus-rate comparison alone implies.

When Overpayment Wins

Despite the maths usually favouring investing, there are scenarios where mortgage overpayment is the right answer.

• Your mortgage rate is high - At 7% or above, the certain return from overpayment beats the expected return from a balanced portfolio. Recent UK fixed rates have made this scenario much more common.
• You have already used your tax wrappers - Once your ISA and pension allowances are full and you would be investing in a taxable GIA, the post-tax return often falls below your mortgage rate.
• You hate debt - There is real psychological value in being mortgage-free. If owing money keeps you up at night, the optimal strategy is the one you can stick with.
• You are nearing retirement - Reducing fixed costs before income falls is sensible even if it costs you a small amount of expected wealth. A paid-off mortgage means you can drawdown a smaller pension pot.

When Investing Wins

Investing the spare cash usually wins for households with a low or middle mortgage rate, ISA and pension headroom, and a long time to compound.

• Your mortgage rate is below your expected return - The classic case. A 3.5% mortgage and a 6% expected return inside a tax wrapper is a 2.5% spread that compounds for decades.
• You have unused tax wrapper allowances - A £20,000 annual ISA and a £60,000 annual pension allowance is a lot of room. Filling these first is almost always the right move.
• You are a higher-rate taxpayer with pension headroom - The 40% income tax relief plus NI savings on pension contributions creates an effective return that no mortgage rate can match.
• Your mortgage is not worrying you - If the cashflow is comfortable, locking in long-term growth in markets builds more wealth than slightly accelerating debt repayment.

For a wider view of where mortgage overpayment fits in the UK personal finance priority list, see our UK personal finance flowchart guide. It places overpayment after the basics but ahead of taxable investing.

Frequently Asked Questions

Should I pay off my mortgage or invest in 2026?

It depends on your rate. If your mortgage is below 5% and you have unused ISA or pension allowance, investing usually wins. If your rate is 6% or higher, overpayment becomes the better mathematical bet. Run your real numbers through the calculator before deciding.

Does the calculator account for inflation?

Yes, indirectly. Use a real (post-inflation) investment return such as 5% to 6% for a global equity portfolio. The mortgage rate is already a nominal figure, and inflation erodes the real cost of the debt over time. Comparing real return to nominal mortgage rate is a slightly conservative framing in favour of overpayment, which is fine.

What about overpayment fees and lock-in penalties?

Most UK fixed-rate mortgages allow up to 10% overpayment per year without penalty. Above that, early repayment charges of 1% to 5% apply. The calculator does not model these fees, so if you are planning to overpay aggressively, check your mortgage terms first.

Is it worth investing if I have a 0% interest period like an offset mortgage?

If you genuinely have no interest accruing on offset cash, leaving the cash there is basically equivalent to a "guaranteed return" at your mortgage rate. Anything you invest needs to beat that rate net of tax. Run both scenarios through the calculator.

How do I balance overpayment with building an emergency fund?

Always build a 3 to 6 month emergency fund before overpaying anything. The calculator assumes you have spare cash genuinely available; using money that should be in your emergency fund creates real risk if you lose your income. Our piece on insurance for FIRE covers the buffer you should have in place.

Further Reading:

Smarter Investing - Tim Hale - The definitive UK guide to evidence-based investing. Tim Hale explains why a simple, low-cost equity portfolio almost always beats other uses of long-term cash, including mortgage overpayment for most households. (Affiliate link - we may earn a small commission at no extra cost to you.)