What Will Your Savings Really Be Worth?

This article represents a personal view and is not financial advice. Investment returns are not guaranteed, and past performance is no guide to the future. The figures below are illustrations, not predictions.

"What will it be worth when we need it?" is the question every saver is really asking, and most tools answer a different, easier one: what are you paying in. The gap between the two is where all the interesting maths lives: compounding, inflation, and above all the time horizon. Change when you'll need the money and the whole answer changes, even if the monthly contribution never moves.

Nominal numbers flatter you; real numbers tell the truth

Start with the distinction that makes every projection honest or dishonest: nominal versus real. A nominal projection tells you the number that will appear on the account statement. A real projection tells you what that number will buy. At 2.5% inflation, £100 of spending power today needs about £128 in ten years and £164 in twenty. So a projection that says "£147,000 in twenty years" is only meaningful once you ask: £147,000 of what? "Will be worth" only makes sense in today's money, which means quoting real returns, not nominal ones.

In real terms the menu looks roughly like this. Cash savings paying 4% against 2.5% inflation earn about 1.5% real, so the money is holding its ground with a little to spare. A diversified equity portfolio in a stocks and shares ISA has historically delivered something in the region of 4% a year real over long periods, with plenty of bad years along the way. A pension holds similar investments but adds tax relief and, usually, employer contributions on the way in, so the same £300 of your take-home can arrive as £375 gross or more before it's even invested.

The same £300 a month, two horizons

Take a household saving £300 a month and run it over two time horizons, all in today's money. Over five years, contributions total £18,000. In cash at 1.5% real, the pot ends at roughly £18,700. Invested at 4% real, roughly £19,900. The entire difference between the cautious choice and the ambitious one is about £1,200, and the invested version carries genuine risk of finishing below £18,000 if the five years end badly. Over a short horizon, the answer to "what will it be worth" is dominated by one thing: what you paid in. The vehicle barely matters; the direct debit does.

Now run the same £300 for twenty years. Contributions total £72,000. Cash at 1.5% real reaches about £84,000. Invested at 4% real, about £110,000, in today's spending power. Growth is now more than a third of the final pot, and the gap between cash and investing has widened from £1,200 to £26,000. Nothing about the household changed. Only the horizon did.

Your own contribution and horizon will differ, and the shape of that gap is easier to see than to describe. The compound interest calculator takes a starting amount and a monthly contribution and separates out how much of the final figure is growth rather than money you put in yourself.

The nominal illusion, quantified

That twenty-year invested pot makes the real-versus-nominal point neatly. At a 6.5% nominal return (the same 4% real plus 2.5% inflation) the statement would show around £147,000. The honest figure is £110,000 of today's purchasing power. The £37,000 difference isn't wealth; it's just inflation quietly restating the units. Any projection that doesn't make this adjustment is overstating your future by roughly a quarter over twenty years.

Translating between the two is a small piece of arithmetic worth doing rather than eyeballing. The inflation calculator converts a future sum back into today's money, and shows what a cost you face now is likely to be by the year you actually face it.

Match the vehicle to the date, not the return to the hope

• Money needed within roughly five years, such as a house deposit, a car, or next year's costs, belongs in cash-like savings. Contributions do the work; the priority is that the money is there.
• Money needed in ten to twenty years suits a stocks and shares ISA: long enough for a realistic real return to compound meaningfully, accessible if plans change.
• Money for retirement suits a pension first: tax relief and employer contributions boost every pound before growth starts, in exchange for the lock-in.
• Whatever the vehicle, judge the projection in real terms; it's the only version of the number your future self can actually spend.

Whether the long-horizon money goes into a pension or an ISA first is its own decision, and we've compared the two directly, but the horizon question comes before the wrapper question.

The practical failure mode isn't picking a slightly wrong growth rate. It's holding twenty-year money in cash out of caution, or five-year money in equities out of optimism. That's a horizon mismatch, not a forecasting error. The fix is knowing, with reasonable honesty, when each pot of money will actually be needed.

That's a modelling exercise more than a guess. In CrestCast you can set growth assumptions on each savings and investment account, project them alongside everything else in the household, and see what each pot reaches by the year you'll actually call on it, then nudge the assumptions down and check the plan still holds. A plan that only works at optimistic returns isn't a plan; a forecast makes that visible before it matters.