Mortgage vs Investments Calculator

50,000 lump sum vs 200,000 mortgage at 4% for 25 years: paying down saves 47,000+ interest. Investing 50k at 7% grows to 203,000 over 25 years — far more than the mortgage interest saved. Investing wins when expected return exceeds mortgage rate.

Run it with sensible defaults

Using lump sum of 50,000, mortgage rate of 4%, investment return of 7%, horizon of 25, the calculation works out to 138,079.82. The defaults are meant as a starting point, not a recommendation.

The levers in this calculation

The inputs — Lump Sum, Mortgage Rate, Investment Return, and Horizon — do not pull with equal force. The rate and the time horizon usually dominate — compounding means a small change in either reshapes the final figure more than a similar shift in contribution size. Test this by doubling one input at a time.

How the math works

Investment future value compared to total mortgage interest saved (approximated as compound growth at mortgage rate).

Stress-testing the plan

Run the calculation at your current rate, then run it again at a rate 2–3 percentage points higher. That's roughly what a product reset could bring at renewal, and it's a useful check on whether you can afford the mortgage in a higher-rate world, not just today's.

What this doesn't capture

The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.

Worked example

Suppose you have 100,000 in savings, a mortgage balance of 300,000 at 3.5%, and 20 years remaining on the term. You expect to earn 5.5% annually on investments.

• Path A (Pay down mortgage): Reduce the balance to 200,000. Interest saved over 20 years is approximately 70,000.
• Path B (Invest the 100,000): At 5.5% annual growth, the lump sum grows to roughly 295,000 after 20 years.

The difference between investment return (5.5%) and mortgage rate (3.5%) is 2 percentage points. Over two decades, investing produces a larger end value. The calculator models both paths and displays the spread.

When this calculation matters

This model applies when you have a lump sum and face a genuine either/or decision: redirect it toward mortgage principal, or place it into investments (stocks, bonds, savings products, or other vehicles). It's most useful when:

• Mortgage and investment rates differ significantly
• You have flexibility in timing and don't face immediate pressure to choose
• Your mortgage allows overpayments without penalty
• You have clarity on your expected investment returns
• You're comparing strategies over a multi-year horizon

What the result shows and does not show

The calculator estimates the mathematical outcome of each path — how much capital you accumulate under each strategy. It does not account for:

• Tax on investment gains or interest relief on mortgage payments
• Changes in interest rates during the period
• Liquidity: money in investments may be less accessible than equity in property
• Personal preference for debt reduction over asset growth
• Volatility or timing risk in investment markets
• Fees, spreads, or transaction costs on either strategy

The result is an illustration for educational purposes. Actual outcomes depend on real market performance, rate movements, and individual circumstances.