Compounding, and why time matters more than returns

Compounding is the only concept in investing that is simultaneously simple, undisputed and consistently underestimated.

What it does

Returns are earned on the original amount and on the returns already earned, which means growth accelerates over time rather than proceeding in a straight line.

Over short periods the effect is barely visible.

Over decades it dominates everything else, including the choice of investment.

Which is why the single most consequential variable in most people's investing outcome is when they started rather than what they chose.

The rough arithmetic

A useful approximation.

Dividing seventy-two by an annual growth rate gives roughly the number of years for a sum to double, which is accurate enough at moderate rates to be useful mentally.

At seven per cent, money roughly doubles every decade.

Which means a sum invested at thirty has considerably more doubling periods ahead of it than the same sum invested at fifty, and the difference is not proportional to the extra years — it is exponential.

This is the entire argument for starting with whatever amount is available rather than waiting until a larger amount can be assembled.

The same arithmetic working against you

Two applications.

Debt compounds identically, which is why credit card balances carried for years cost multiples of the original amount and why minimum payments extend repayment enormously.

Charges compound too: a difference of a fraction of a percentage point in annual costs, deducted every year and therefore also removing the returns that portion would have earned, produces a substantial difference in the final outcome over decades.

Which is why costs receive the attention they do in evidence-based investing — they are the one variable that can be controlled with certainty.

Contributions versus returns

Where the balance shifts.

In the early years of investing, the amount contributed dominates the outcome, since returns on a small balance are small in absolute terms.

In later years, returns dominate, since they apply to a much larger balance.

The practical implication: for someone starting out, the contribution rate matters far more than the investment selection, and effort spent optimising the portfolio is misdirected relative to effort spent increasing contributions.

For someone with a large accumulated balance, the reverse becomes more true.

Regular contributions

Which is how most people actually invest.

Contributing monthly rather than in a lump sum means each contribution has a different length of time to compound, which is why regular investing over decades produces outcomes dominated by the earliest contributions.

Increasing contributions with income — particularly directing a portion of any pay rise before it is absorbed by spending — is one of the most effective single actions available, because it captures the increase at the point when the money has not yet been missed.

Automating contributions removes the monthly decision, which behavioural research consistently finds improves outcomes.

Inflation

Which compounds in the other direction.

Purchasing power falls over time, which means cash held at a rate below inflation loses value in real terms even though the balance does not fall.

Over decades this is substantial.

Which is the argument for investing money with a long horizon rather than saving it, and equally for keeping short-horizon money in cash where the certainty of the amount matters more than the erosion.

Real returns — after inflation — are what matter for planning, and nominal figures overstate progress.

What compounding does not do

Worth being clear.

It does not make returns predictable, since market returns are volatile and the long-run averages quoted conceal enormous variation between periods.

It does not eliminate the risk of a poor sequence of returns, particularly when withdrawing rather than accumulating.

It does not apply to money you have to withdraw during a downturn.

And it does not rescue a plan with inadequate contributions, since compounding a small amount produces a compounded small amount.

The practical conclusions

Which follow directly.

Start with whatever you can rather than waiting.

Automate contributions and increase them with income.

Minimise costs, since they compound against you.

Use tax-advantaged accounts, since tax also compounds against you.

Leave it alone, since interrupting compounding is what most damages long-run outcomes.

And accept that the process is undramatic for the first decade, which is when most people conclude it is not working.

General information only, not investment advice. Investments can fall in value and past performance does not indicate future returns. Consult a regulated financial adviser.