What time does to money.

The gap between the two lines is the point.

The lower line is money you put in. The upper line is what it became. Early on they sit close together; in the second decade they separate; by the third, growth is usually the larger share. Nothing about that requires a high return. It requires time, and not interrupting it.

Three things the chart teaches

• The first decade looks disappointing. Most of the value is still your own contributions. People give up here.
• Interruptions are expensive. Withdrawing in year twelve removes not just the money but all the growth it would have earned to year twenty-five.
• Fees compound too. A 1% difference in annual cost over twenty-five years is roughly a fifth of the final pot. Ask any adviser for the total cost before you sign.

How the numbers are built, and where they fall short.

Contribution timing. The monthly saving you enter is treated as paid in equal instalments through the year rather than as one lump sum, so in the year it is paid it earns, on average, only half a year of growth; from the following year it earns a full year like the rest of the balance. If you increase your saving each year, that increase is applied once, at the start of each new year, not gradually within it.

Nominal versus real. The return you enter is treated as nominal: before inflation, and after the fees you have already deducted from it. The calculator compounds that nominal return every year regardless of the toggle. Switching "Show results in today's money" on does not change the underlying arithmetic; it only divides each year's nominal figures, including the doubling time, by cumulative inflation to date, so you see what the pot and its growth would buy at today's prices rather than the larger, inflated future number.

Volatility. The calculator compounds one smooth, unchanging annual return every year. Real portfolios do not move in a straight line: some years are sharply up, some down, and a volatile series of returns compounds to less than the same average return applied evenly, an effect sometimes called volatility drag. A steadier, lower-risk portfolio can end up ahead of a higher-return but more volatile one over the same period, which this tool cannot show.

Sequence risk. Because you are adding money throughout the period rather than investing a lump sum once, the order in which good and bad years arrive can change the final value even when the average return over the whole period is identical. A downturn that lands when your balance and contributions are largest does more damage than the same downturn early on, when there is less at stake and more time to recover. This effect is smaller during accumulation than it is for a retiree drawing an income, see our retirement calculator, but it is not zero, and it grows as your saving increases each year.