What Could Your Investments
Actually Grow To?

Why can't an investment calculator predict your return?

Because real markets deliver a sequence of wildly different years, not the single "expected return" a calculator smooths into a straight line. Enter 7%, 8% or 10% and you get a reassuring upward curve for 20 or 30 years — but it's deeply misleading.

Markets don't return 8% every year; they return some years up 30%, some down 40%, most in between, and the order matters enormously. Two investors with the same plan and the same "average return" can end up vastly apart depending purely on which years they lived through.

How does pound-cost averaging help while you're building wealth?

During accumulation, falling markets aren't your enemy — your monthly contributions simply buy more shares at lower prices, which then ride the recovery. This is pound-cost averaging, one of the most powerful features of regular investing.

It's why the worst outcomes for regular monthly investors are typically much less severe than for a lump sum invested at a peak. You'll see this in the simulator: the absolute-worst window is far gentler for steady contributions than for a single ill-timed lump sum.

How do you read the simulator results?

Each grey line is one real historical investment window; the coloured lines mark the best, worst, typical and percentile outcomes — the whole distribution. Overlaid on the grey are seven reference lines:

• Absolute best (bright green, solid) — the single best start date in the dataset. The luckiest window history has on record.
• Absolute worst (bright red, solid) — the single worst start date. Even this still typically beats keeping cash.
• Typical outcome (white) — the median. Half of all investors did better, half did worse.
• Good era (orange, dashed) — better than 75% of all historical starts.
• Tough era (purple, dashed) — worse than 75% of all historical starts.
• Best 10% (teal, dashed) — the 90th percentile. Exceptional but real.
• Worst 10% (amber, dashed) — the 10th percentile. Difficult but survivable.

You can also overlay the influencer line — a smooth fixed-return projection at any annual rate — to directly compare what the "guaranteed 8%" promise looks like against the messy reality of actual markets.

Just want a single answer for one start year — "what would £X invested in year Y be worth today?" Try our simpler S&P 500 Calculator, which runs one lump sum through the same real historical returns.

How should you use these results practically?

Focus on the range and lengthen your horizon: over 30 years almost every historical period delivered a positive real return; over 5 years the variance is huge. The best outcomes cluster around investments started just before a major bull market — the early 1980s, say — while the worst involve buying at a peak into a prolonged downturn like the late-1960s stagflation era or the late-1990s dot-com bubble.

The consistent lesson is that time in the market beats timing the market. Once you've built the pot, the drawdown simulator shows how the same history treats the spending phase, and the personal investment return calculator works out the real return you've actually earned.

Do monthly contributions really beat a lump sum?

On average a lump sum wins, because markets rise more often than they fall — but drip-feeding reduces your worst-case timing risk. If you have a lump sum, deploying it gradually is insurance rather than optimisation: it trades a little expected return for a lot less regret if you'd have bought at a peak.

Why is time your most powerful tool?

Because the spread of outcomes narrows sharply as duration grows. Short periods produce wildly variable results; over 20 or 30 years the range tightens and the vast majority of historical periods produced positive real returns. Starting early and staying invested is the single most effective thing you can do.

Does the pound-dollar exchange rate change the picture for UK investors?

Yes — this simulator measures returns in US-dollar terms, but a UK investor's real return also moves with the GBP/USD rate. A weaker pound lifts your US returns in sterling, while a stronger pound trims them; most US and global funds held in ISAs are unhedged, so that currency swing is baked into your result.

It uses the S&P 500 total-return index (dividends reinvested) from March 1957, and most UK ISA and SIPP investors hold global funds rather than the S&P 500 alone, so your own returns may differ. The figures also exclude platform fees, fund charges and tax — a realistic ISA tracker costs 0.1–0.5% a year, trimming outcomes by roughly 0.3–0.7% annually over long horizons.