Compare paying extra on your mortgage versus investing the same amount

The trade-off this tool answers

You have extra cash each month. You can pour it into the mortgage principal and pay the loan off faster, or you can invest it and let a portfolio compound. Both paths build wealth. They just build it in different places — one reduces a liability, the other grows an asset. Which wins depends almost entirely on two numbers: the rate on the mortgage and the expected return on the investment.

How the math decides

At the simplest level, the comparison is a spread. If your mortgage rate is 4% and you expect 7% from a diversified portfolio long term, investing wins the raw math by about 3 percentage points per year on the extra amount. If your mortgage is 7% and you expect 5% from a conservative portfolio, paying the mortgage wins. The tool runs both paths over the horizon you choose and shows the net wealth difference at the end — extra principal payments vs. investment contributions compounded at your expected return, minus the mortgage balance remaining in each scenario.

Why it's not just about the spread

A 3-point raw spread does not mean investing automatically wins. Three things shift the answer:

Tax treatment. Mortgage interest is paid with after-tax money in most jurisdictions. Investment gains may be taxed when realised, or may be tax-sheltered if held in retirement or tax-advantaged accounts. The real after-tax spread is what matters, not the headline rates.

Volatility. The mortgage rate is fixed (or defined). The expected investment return is an average — in any given decade the actual return could be half or double that number. Paying down a mortgage is a certain, rate-matched return with no variance. Investing carries a probabilistic return that varies meaningfully year to year.

Behaviour. The path that gets executed matters more than the path that's theoretically optimal. Someone who commits to extra mortgage payments and never misses one may finish better than someone who plans to invest and actually spends the money. The tool assumes both plans get executed fully — real life rarely delivers that.

When paying the mortgage usually wins

Paying extra on the mortgage is typically the better call when the rate is high (above 6-7%), when the investor is risk-averse and would not actually tolerate equity volatility, when the mortgage is close to the end of its term (most of the payment is principal anyway), or when the household values the psychological freedom of being debt-free over the statistically-expected upside of investing. It is also the safer call for anyone who is not confident they would consistently invest the extra amount rather than spending it.

When investing usually wins

Investing the extra typically wins when the mortgage rate is low (under 4-5%), when the investor has decades of horizon to let volatility average out, when the investment vehicle is tax-sheltered, and when the household is disciplined enough to actually make the contributions. The math strongly favours investing over 20-30 year horizons at current historical equity returns versus current mortgage rates for many borrowers, though this swings with rate cycles.

What this calculator does not model

The tool runs a deterministic comparison using the single expected return you provide. It does not model return variance, sequence-of-returns risk, job loss, changing contribution levels, or tax. Use it to get a sense of the direction and rough magnitude, not as a final answer. A conservative approach is to subtract a couple of percentage points from your expected investment return before running the comparison — if investing still wins, the case is solid; if the margin is thin, the certainty of paying down the mortgage becomes more attractive.