Compound Interest: The Numbers Behind "Start Early"
"Start saving early" is the most repeated personal finance advice in existence. It is also among the least understood, because the word "early" usually comes without the actual numbers that make it meaningful. This article runs those numbers so you can see exactly what the difference is — not as a vague encouragement, but as concrete mathematics.
The Mechanics of Compounding
Compound interest means earning returns not just on your original deposit (the principal), but also on the returns you have already accumulated. In year one, you earn interest on your deposit. In year two, you earn interest on your deposit plus last year's interest. By year ten, your interest is being earned on a balance that includes nine years of accumulated interest.
The formula is: FV = P × (1 + r)^t, where FV is the future value, P is the starting amount, r is the annual return rate, and t is the number of years. The exponent — the power to which (1 + r) is raised — is why compounding accelerates so dramatically. Each additional year of growth is applied to a larger base than the year before.
The Cost of Waiting: Real Numbers
Assume three people each invest €5,000 per year into an account earning 7% annually. The only difference is when they start and when they stop:
| Investor | Starts | Stops | Years invested | Total contributed | Balance at 65 |
|---|---|---|---|---|---|
| Early Emma | Age 25 | Age 65 | 40 years | €200,000 | €1,068,000 |
| Middle Marcus | Age 35 | Age 65 | 30 years | €150,000 | €472,000 |
| Late Lena | Age 45 | Age 65 | 20 years | €100,000 | €196,000 |
Emma ends up with more than five times what Lena has, despite contributing only twice as much money. She ends up with more than twice what Marcus has, despite contributing only €50,000 more. The extra returns come not from saving more, but from giving the existing savings more time to compound.
The "Early Then Stop" Paradox
One of the most striking illustrations of compounding is the "early then stop" scenario:
| Investor | Invests €5,000/yr from | Total contributed | Balance at 65 (7%) |
|---|---|---|---|
| Early Stopper | Age 25–35 only (10 years) | €50,000 | €468,000 |
| Late Starter | Age 35–65 (30 years) | €150,000 | €472,000 |
The early stopper contributes for just 10 years, then never adds another cent. The late starter contributes for 30 years — three times as long, contributing three times as much money. Yet they end up with almost exactly the same balance at 65. The early stopper's 10-year head start and 30 years of doing nothing produces essentially the same result as the late starter's 30 years of consistent effort.
This is not a trick or an unusual scenario. It is simply the mathematics of compound growth, applied consistently.
What Rate of Return Should You Assume?
The calculations above use 7%, which reflects the approximate long-run historical return of a globally diversified equity index fund after inflation has been accounted for (nominal returns have historically been 9–10%, with inflation of roughly 2–3% producing real returns of 6–8%). The precise figure depends on the period, the market, and the specific portfolio.
For planning purposes, these are common benchmarks:
| Investment type | Approximate annual return | Risk level |
|---|---|---|
| Cash savings account (2026) | 3–5% | Very low |
| Government bonds | 3–5% | Low |
| Balanced fund (60/40) | 5–7% | Medium |
| Global equity index fund | 7–9% (historical) | Medium-high |
Note that these are long-run averages. In any single year, returns can be substantially higher or lower — equity markets have seen years of −40% returns alongside years of +30% returns. Long time horizons smooth out this volatility, which is another reason starting early matters: a 25-year-old who invests for 40 years can ride out market crashes that would devastate someone investing for only 5 years.
Monthly Contributions vs. Lump Sums
The examples above use annual lump-sum contributions, but for most people, monthly contributions are more realistic. The mathematics are effectively the same — monthly contributions that add up to the same annual total produce nearly identical long-run results (slightly better, in fact, because monthly contributions start compounding sooner than a year-end lump sum).
The practical implication is that a €417/month contribution — which many people can manage by cutting a few discretionary expenses — is equivalent to the €5,000/year figures in the examples above. A commitment at age 25 to automatically transfer €417 each month into an investment account, maintained until 65, would grow to over €1 million at 7% annual returns.
The Practical Takeaway
Compound interest is genuinely as powerful as the cliché suggests — but the cliché rarely comes with the numbers. Now you have them. The single most effective action most people in their 20s and early 30s can take for their long-term financial security is to start investing something — even a small amount — as soon as possible, and to do so in a diversified, low-cost investment rather than a cash savings account. The mathematics reward patience and time more than they reward size or sophistication.