MORTGAGE & INVESTING
What should I do with spare money while I still have a mortgage?
Explore the trade-off between reducing mortgage debt and building a separate investment asset. See what the calculator can model, which questions it can answer and what assumptions shape the result.
What this engine helps you compare
It compares two possible uses of the same recurring monthly spare cash: reduce mortgage debt or build a separate investment asset.
The two paths
Overpay first: spare cash reduces the mortgage. Once it is cleared, the freed mortgage payment is invested for the remainder of the shared comparison period. Invest from the start: the mortgage follows its normal schedule while the same spare cash is invested each month.
Inputs and outputs
Inputs are balance, interest rate, remaining term, current payment, spare monthly cash and assumed annual return. Outputs include payoff timing, months saved, interest saved, contributions, projected investment value and the modelled difference.
How the comparison remains fair
Both paths use the same monthly cash, horizon and contribution timing. The result is a transparent model, not a prediction or recommendation.
How to read a result
Mortgage-first value is the modelled value after using spare cash to overpay, then investing the released payment once the mortgage is cleared. Invest-from-the-start value keeps the mortgage on its normal schedule and invests the spare cash throughout. Interest saved is calculated from the mortgage figures entered. Projected investment value depends on the assumed return and is not guaranteed. Months saved describes timing, not which path has the larger modelled value.
These figures describe the entered scenario. They are not a recommendation, forecast or measure of affordability.
Why assumptions change the result
Rates, terms, spare cash and the assumed return change the arithmetic. Investment returns are projections and can rise or fall.
What is not included
Tax, fees, inflation, future rate changes, lump sums, early repayment charges, lender allowances and personalised advice are outside scope.
Method in plain English
The calculator runs the same monthly cash through two paths over the same period, applies the entered mortgage rate and return assumption, and compares the resulting values. It does not add tax, fees, inflation, future rate changes, lump sums or lender-specific rules.
A separate proof-reading record defines every result label and interpretation in plain English.