There is a year when the pot starts earning more than you pay in
Every compound interest calculator gives you a final figure. Almost none of them split that figure into what you put in against what the growth added, and none of them name the year the second overtakes the first.
That year is the whole argument for starting early, and it lands much later than people guess. On a fund returning 5% a year, a monthly saver is contributing more than the pot earns for well over a decade. Before the crossover, you are doing the work. After it, the money is.
Which is why starting earlier beats saving harder
Somebody who begins ten years sooner does not simply get ten more years of contributions. They get ten more years on the far side of the crossover, which is exactly where the growth is largest.
That is why the gap between an early and a late starter is so much wider than the difference in what they actually paid in. On a typical set of figures, ten extra years adds several times more to the final pot than the extra contributions themselves come to. The tool shows both numbers so the ratio is visible rather than asserted.
The headline figure is not what it will buy
A pot projected at a nominal return ignores inflation completely, and over a long term that is what misleads people most about the answer.
At 2.5% inflation, money roughly halves in real terms over 28 years. So a thirty year projection quoted in today's pounds will buy about half of what the number suggests. The figure that matters is the real return, which is the nominal return less inflation, because that is what compounds your buying power rather than your balance.
This tool shows both: the nominal pot, and what it is worth in today's money. The second one is the one to plan against.
Two smaller things that are worth knowing
Contributions are added at the end of each month here. That is the conservative convention and it matches a standing order leaving after payday. Paying at the start of the month gives every contribution one extra month of growth: small individually, but it compounds over decades.
A steady annual percentage is a convenience, not a forecast. Real returns arrive unevenly, and the order matters as much as the average. A bad run early does more damage than the same run late, because it hits when there is longest left to recover but also when your contributions are large relative to the size of the pot. Treat any long projection as a shape rather than a prediction.
Working the question backwards
The other useful direction is starting from the pot you want rather than the amount you can save. Set a target and a horizon and the tool solves for the monthly contribution needed, taking any starting balance into account.
That framing is often more honest, because it turns a vague intention into a specific standing order, and because it makes the cost of a shorter horizon immediately obvious: the same target over fifteen years rather than thirty does not need twice the monthly amount, it needs considerably more than twice, precisely because there is less time on the far side of the crossover.
Common questions
When does compounding actually start doing the work?
Later than most people assume, and this tool names the exact year. On a fund returning 5%, a monthly saver is putting in more than the pot earns for well over a decade. Before that crossover you are doing the work. After it, the money is. That single year is the strongest argument for starting early rather than saving harder.
Is it better to start earlier or to save more?
Earlier, and by a wide margin over a long horizon. Somebody who starts ten years sooner does not simply get ten more years of contributions, they get ten more years on the far side of the crossover, which is where the growth is largest. The tool prices that directly: the extra pot from ten more years is usually several times the extra money paid in.
Why is the real figure so much lower than the projection?
Because a nominal projection ignores inflation entirely. At 2.5% inflation money roughly halves in real terms over 28 years, so a 30 year projection quoted in today’s pounds buys about half what the number suggests. The real return is the nominal return less inflation, and that is the rate that compounds your buying power.
Does it matter when in the month I pay in?
A little, and it favours paying early. This tool adds contributions at the end of each month, which is the conservative convention and matches a standing order leaving after payday. Paying at the start gives every contribution one extra month of growth, and that adds up over decades if you can arrange it.
Is a steady annual return realistic?
No, it is a modelling convenience. Real returns arrive unevenly, and the order matters as much as the average. A bad run early in the term does more damage than the same run late, because it hits when there is longest to recover but also when contributions are large relative to the pot. Treat the output as a shape rather than a forecast.