The snowball effect: what could your investments be worth?

How compounding actually works

Suppose £10,000 earns an illustrative 5% a year and every return remains invested. After year one, the balance is £10,500. The following year’s 5% applies to £10,500 rather than the original £10,000.

No extra money was added. The first year produced £500; the tenth produces about £776 because the return is being applied to a larger balance.

Real investments do not rise in this smooth staircase. One year might be strongly positive, another sharply negative. The fixed 5% is a calculation assumption designed to expose the mechanism—not a forecast of the journey or the destination.

The famous early-starter story

Now consider Sarah and James, the two people who have appeared in countless investing presentations.

Sarah invests £200 a month from age 25 until 35, contributes £24,000 in total and then leaves the portfolio untouched until 65. James starts at 35 and invests the same £200 every month until 65, contributing £72,000—three times as much.

At an illustrative 7% annual return before fees, Sarah finishes with about £262,000. James finishes with about £235,000. The usual lesson arrives on cue: Sarah invested less but won because she started earlier.

That lesson is directionally useful. The example is also hiding something important.

Change one assumption and the winner changes

The comparison is not proof that ten early years will always beat thirty later years. It is proof that early money becomes extremely valuable if the assumed return is high enough.

In this simplified model, Sarah only moves ahead at a return of roughly 6.3% a year before fees. That is not a threshold to build a plan around. It shows how much of the impressive conclusion comes from the selected return.

Starting earlier still helps every individual pound because it has more time to participate in markets. What the parable cannot promise is that a much smaller amount invested early will always defeat much larger contributions made later.

Time matters enormously. Contributions still matter. Any example that makes one of them disappear is selling a story, not explaining the full calculation.

Try the comparison

Move the assumed return below and watch the lead change. Both paths use the same smooth annual rate and exclude tax, fees and inflation, so treat the outputs as illustrations rather than expected outcomes.

The five forces behind the final number

1. Contributions: the fuel

This is the money you add. It is less glamorous than investment performance and far more controllable. Increasing a monthly contribution from £200 to £250 is a real action; assuming a higher future return is only a more optimistic spreadsheet.

Regular contributions also reduce the need to decide afresh every month. They buy more units when prices are lower and fewer when prices are higher, although they do not guarantee a profit or protect the portfolio from loss.

2. Time: the multiplier

Early contributions receive more opportunities to grow, fall and recover. This is why delaying can increase the monthly amount required to reach the same target.

Time helps, but it does not turn a poor investment into a good one or remove market risk. A long horizon expands the number of possible outcomes as well as the potential for compounding.

3. Returns: the uncertain engine

The return assumption usually drives the most dramatic part of a projection—and is the part you cannot choose. The FCA stresses that investments can lose money and that higher potential returns normally require accepting more uncertainty.

A single percentage also hides the path. A portfolio does not earn 7% with the reliability of a clock. Lower, central and higher scenarios are more honest than one line presented to the nearest pound.

4. Fees: compounding in reverse

Platform charges, fund costs, advice fees and transaction costs leave less money invested. The percentage may look small, but it is deducted repeatedly from a balance that may be growing.

In the James example, £200 a month for 30 years grows to about £235,000 at an illustrative 7%. Reducing the net return to 6%—a simplified way to represent an additional one percentage point of annual cost—cuts the result to about £196,000. The difference is roughly £39,000.

Lower cost is not automatically better if two products provide different services, risks or value. The essential point is that costs should be visible and included in the projection.

5. Inflation: the shrinking ruler

A future balance is normally shown in future pounds. Inflation means those pounds may buy less. If £235,000 is reached in 30 years and inflation averages an illustrative 2.5%, it would have purchasing power similar to about £112,000 today.

That does not mean inflation has taken money out of the account. It means the measuring unit has changed. A useful long-term projection should show nominal value and today’s-money value, or state clearly which one it uses.

Build projections that are difficult to fool

Compound-growth calculators are most useful when they reveal sensitivity, not when they produce a seductive final total.

• Use at least three return scenarios. A lower case should be uncomfortable enough to change the conclusion.
• Deduct recurring fees rather than discussing them in a footnote.
• Show results before and after inflation, with the inflation assumption stated.
• Separate money contributed from growth. A large pot created mostly by contributions tells a different story from one dependent on high returns.
• Remember the time horizon. Money needed within a few years may not have time to recover from an investment fall.
• Do not mistake precision for knowledge. £235,213 is a formula output, not a credible forecast to the nearest pound.

The bottom line

Compounding is powerful because returns can remain invested and generate further returns. It is not a cheat code, and it does not rescue unrealistic assumptions.

Start when you reasonably can. Contribute consistently. Understand every recurring fee. Use a range of returns and translate the result into today’s spending power.

Time can do a great deal of the work—but only after you give it money to work on.