Invest vs Pay Off Mortgage Calculator

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

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 (illustrative figures only - your real position will depend on dividend yields, realised gains, allowances and your full tax position):

• Invest in a GIA (taxable account) - net return after dividend and CGT might land around 5.5% to 6%, which is a marginal call against the mortgage
• Invest in an ISA - net return stays at 7%, which tends to beat the mortgage rate over a long horizon (returns are not guaranteed)
• Invest in a SIPP via salary sacrifice - the effective first-year boost on a contribution can be around 13% once income tax relief is included, which often dwarfs the mortgage rate. Pension withdrawals above the 25% tax-free lump sum are taxed as income, so the long-run after-tax position depends on your retirement tax band.

This is why many UK households look at filling ISAs and pensions before considering mortgage overpayment. The tax wrapper is doing more work than the rate-versus-rate comparison alone implies. This is general information, not personalised advice - speak to a regulated adviser if you are unsure which path suits you.

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 (expected returns are not guaranteed).
• You have unused tax wrapper allowances - A £20,000 annual ISA and a £60,000 annual pension allowance is a lot of room, and filling these first is a common starting point in UK personal finance.
• You are a higher-rate taxpayer with pension headroom - The 40% income tax relief plus NI savings on pension contributions can create an effective uplift that mortgage rates rarely match. Pension funds are locked away until age 57 (rising to 58 in 2028 onwards) and withdrawals above the tax-free lump sum are taxed as income.
• Your mortgage is not worrying you - If the cashflow is comfortable, the historical pattern is that long-term investment in diversified equities has built more wealth than accelerating debt repayment, but this comes with capital risk and is not guaranteed.

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.

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.)