Guaranteed vs assumed returns

When you compare putting spare money toward a mortgage against investing it, you're comparing two very different kinds of return. One is guaranteed; the other is an assumption. Understanding that difference is the whole point of the comparison. This is general education, not advice.

A mortgage overpayment is guaranteed

Overpaying your mortgage reduces the balance that interest is charged on, so it saves you interest at your mortgage rate — for certain. If your rate is 4.5%, every pound overpaid earns you a guaranteed 4.5% saving. There's no market involved and no chance it turns out lower. That certainty is its defining feature.

An investment return is assumed

Investing the same money instead might do better or worse. Any figure you put on it is an assumption, not a promise — real returns vary year to year, and capital at risk means the value can go down as well as up, so you could get back less than you put in. A 5% assumed return is a reasonable illustration, not a rate you're guaranteed to receive.

Why assumptions drive the answer

Because one side is fixed and the other is assumed, the comparison swings entirely on the assumption you choose. A slightly higher assumed return can flip which side comes out ahead, and small changes compound over long horizons. That's why a good calculator lets you change the assumption and watch the answer move, rather than presenting one number as the truth.

Why Blooom labels projections illustrative

Any projected value that relies on an assumed return is labelled illustrative for exactly this reason: it's a picture of what could happen on those assumptions, not a forecast of what will. The guaranteed side (the mortgage saving) is knowable; the investing side isn't. Being honest about which is which is what keeps a projection useful rather than misleading.

This explains the difference between the two kinds of return, not which to choose — that depends on your rate, your goals, how much certainty you want, and things a money-only comparison leaves out, like early-repayment charges and keeping cash accessible. It's general information, not advice. The mortgage-overpayment and compound-interest calculators let you explore both sides on assumptions you set.