In the first years of investing, your contributions are the whole story. Put £500 a month into a £10,000 portfolio and you add £6,000 a year, while 7% growth adds about £700. The chart goes up because you push it up.
That balance shifts quietly as the portfolio grows. The crossover is easy to estimate: expected growth matches your contributions once the portfolio reaches your annual contributions divided by your expected return. At £6,000 a year and 7%, that is roughly £86,000. Past that point, an average year of market growth adds more than a year of saving does.
Keep going and the effect becomes dramatic. On a £300,000 portfolio, a perfectly ordinary 5% market wobble moves you by £15,000 — thirty months of contributions, gone or gained in a few weeks. Your monthly £500 is now invisible on the chart. This is the moment many people quietly stop contributing, because it feels pointless.
That feeling is worth examining, because it confuses visibility with value. The £500 you invest this month is not competing with this month’s volatility — it is buying growth for the next few decades. At 7%, money roughly doubles every ten years: this month’s £500 is on course to be £1,000 in ten years and £2,000 in twenty. The noise around it changes nothing about that.
There is, however, a point where easing off becomes a rational choice rather than a mood. The Coast FIRE test asks: if you never contributed again, would the portfolio alone reach your target by your chosen date? Divide your target by (1 + real return) raised to the years remaining. If you want £600,000 in twenty years and expect 5% real growth, the coast number is about £226,000 — reach it, and compounding can finish the job without you.
Passing your coast number does not mean you must stop. It means contributions have become optional, and that is useful information: money that was earmarked for retirement can go to a nearer goal — the house, the sabbatical, working four days a week — without putting the long-term plan at risk.
Two honest caveats. First, the crossover maths uses average returns, and real markets do not deliver averages on schedule; contributing through a downturn buys cheaply exactly when it matters most. Second, projections are only as good as their assumptions — use a real (inflation-adjusted) return and revisit the numbers yearly.
A practical way to see all of this with your own numbers: run a growth projection twice, once with your current contributions and once with contributions set to zero. The gap between the two curves is what your saving is still worth. When that gap stops mattering to the outcome you want, you have found your own crossover — on evidence, not on a feeling.