The core trade-off
When you put a lump sum toward your mortgage and recast, you stop paying interest on that principal for the rest of the loan. That avoided interest is a guaranteed return equal to your mortgage rate — and because you don't pay tax on money you never spent, it's effectively tax-free. A 4% mortgage rate makes recasting the equivalent of a risk-free 4% investment.
Investing the same lump sum instead has a higher expected return — a diversified stock portfolio has historically returned around 7–8% annually over long periods — but that return is not guaranteed. Markets fall, sequences of returns matter, and gains in a taxable account are taxed. So the real question isn't "which earns more on average," it's "how much extra return am I being paid to take on risk, and is it worth it?"
Worked example: $50,000 over 20 years
Suppose you have $50,000, a 4% mortgage rate, and a 20-year horizon. Compare the two paths assuming an 8% expected market return:
| Path | Annual return | Risk | Value of the $50k after 20 years |
|---|---|---|---|
| Recast (pay down mortgage) | 4% guaranteed, tax-free | None | ~$109,556 |
| Invest the lump sum | 8% expected, before tax | Market risk | ~$233,048 |
On paper, investing comes out roughly $123,000 ahead over two decades. That gap is the opportunity cost of recasting — the return you give up for certainty and a lower monthly payment. The math here mirrors how our calculator models it: the lump sum compounds at the mortgage rate on the recast side (50,000 × 1.0420) and at the expected return on the invest side (50,000 × 1.0820).
Why the gap shrinks fast as your rate rises
That example used a 4% mortgage. Flip the rate to 7% and recasting now earns a guaranteed 7% — a very high bar that a risky portfolio may not clear after tax. The higher your mortgage rate, the smaller the expected edge from investing, and the more attractive the certainty of recasting becomes. When your mortgage rate approaches or exceeds expected after-tax market returns, recasting can be the mathematically better bet, not just the safer one.
Beyond the math: things the spreadsheet misses
- Cash flow. Recasting lowers your required monthly payment immediately, freeing up cash every month. Investing locks the money away in the market.
- Liquidity. Money invested can be sold if you need it; money paid into your mortgage is locked in home equity until you sell or borrow against it.
- Behavior and sleep. A guaranteed return and a smaller payment are easy to value. Many people rationally prefer certainty even when the expected value favors investing.
- Taxes. Investment gains are taxed; mortgage interest saved is not. If you don't itemize, you get no offsetting mortgage-interest deduction, which tilts the math toward recasting.
Run your own numbers
Plug your actual balance, rate, and lump sum into the calculator below to see the guaranteed side of the equation — your new payment and total interest saved. Then compare that guaranteed return against what you'd realistically expect from the market at your risk tolerance.