If you’re a UK investor looking to build a diversified portfolio, you’ve probably wondered whether to invest in an S&P 500 index fund tracking American companies or stick closer to home with a FTSE 100 fund. It’s one of the most common investment dilemmas, and the answer isn’t always straightforward.
Both indices represent the largest companies in their respective markets, but they offer very different exposures, costs, and potential returns. The S&P 500 gives you a slice of America’s tech giants and global corporations, while the FTSE 100 focuses on established British companies across sectors like banking, oil, and mining.
In this comprehensive comparison, we’ll break down everything you need to know about S&P 500 vs FTSE 100 index funds. You’ll discover the key differences in performance, costs, currency risks, and tax implications to help you make an informed decision for your investment portfolio.
What Are S&P 500 and FTSE 100 Index Funds?
An S&P 500 index fund tracks the Standard & Poor’s 500 index, which includes the 500 largest publicly traded companies in the United States. Think Apple, Microsoft, Amazon, and Google – the household names that dominate global markets. When you buy an S&P 500 index fund, you’re essentially buying a tiny slice of all these companies in proportion to their market value.
The FTSE 100, on the other hand, tracks the 100 largest publicly traded companies in the UK by market capitalisation. This includes familiar British names like Shell, AstraZeneca, ASML, and Unilever. A FTSE 100 index fund gives you exposure to these companies in the same proportional way.
Both types of funds are “passive” investments, meaning they simply mirror their respective indices rather than trying to beat the market through active stock picking. This approach typically results in lower fees and more predictable performance relative to the broader market.
The key appeal of index funds lies in their simplicity and broad diversification. Instead of picking individual stocks and hoping you’ve chosen winners, you’re buying a representative slice of an entire market segment.
Performance Comparison: Historical Returns and Volatility
Over the past decade, S&P 500 index funds have significantly outperformed FTSE 100 funds, largely driven by the explosive growth of American tech companies. From 2014 to 2024, the S&P 500 has delivered annualised returns of approximately 12-14%, while the FTSE 100 has managed around 6-8% annually.
However, past performance doesn’t guarantee future results, and these numbers don’t tell the whole story for UK investors. The S&P 500’s superior returns come with higher volatility – American markets tend to experience sharper ups and downs compared to the more stable, dividend-focused FTSE 100.
Currency fluctuations add another layer of complexity. When you invest in S&P 500 funds as a UK investor, your returns depend not just on how American stocks perform, but also on the pound-to-dollar exchange rate. A strong pound can erode dollar-based gains, while a weak pound can amplify them.
The FTSE 100, being dominated by international companies that earn revenue globally, isn’t immune to currency effects either, but the impact is generally less direct. Many FTSE 100 companies actually benefit from a weaker pound, as their overseas earnings become more valuable when converted back to sterling.
It’s worth noting that both indices have experienced significant periods of underperformance. The S&P 500 struggled during the dot-com crash and 2008 financial crisis, while the FTSE 100 has faced headwinds from Brexit uncertainty and the decline of traditional industries.
Sector Allocation and Geographic Diversification
The sector composition of these indices reveals stark differences in what you’re actually buying. The S&P 500 is heavily weighted towards technology companies, with firms like Apple, Microsoft, and Alphabet making up a substantial portion of the index. This tech concentration has driven much of the recent outperformance but also creates concentration risk.
| Sector | S&P 500 Weight (Approx) | FTSE 100 Weight (Approx) |
|---|---|---|
| Technology | 28% | 2% |
| Healthcare | 13% | 8% |
| Financials | 13% | 20% |
| Consumer Services | 11% | 12% |
| Energy | 4% | 11% |
| Materials | 2% | 15% |
The FTSE 100 has a more traditional economic structure, with significant weightings in financials (banks like Barclays and Lloyds), energy (Shell, BP), and basic materials (mining companies like Rio Tinto). This makes it more sensitive to commodity prices and economic cycles, but potentially more resilient during tech downturns.
Geographic diversification is another crucial consideration. While the S&P 500 companies are US-based, many generate significant international revenue. However, you’re still exposed to US regulatory environment, tax policies, and economic conditions.
The FTSE 100 companies, despite being UK-listed, often have extensive international operations. Shell operates globally, AstraZeneca sells pharmaceuticals worldwide, and Unilever has brands in virtually every country. This can provide indirect global diversification even within a “UK” index.
Costs and Fees: What You’ll Actually Pay
Index fund fees can significantly impact your long-term returns, making cost comparison essential. Generally, both S&P 500 and FTSE 100 index funds are available at very low costs, but there are subtle differences to consider.
For UK investors, the cheapest S&P 500 index funds typically charge annual management fees (ongoing charges figures or OCF) of around 0.05% to 0.15%. Popular options include Vanguard S&P 500 UCITS ETF and iShares Core S&P 500 UCITS ETF, both offering institutional-level pricing to retail investors.
FTSE 100 index funds are similarly priced, with fees ranging from 0.05% to 0.20%. The Vanguard FTSE 100 Index Fund and iShares Core FTSE 100 UCITS ETF are among the most cost-effective options available to UK investors.
However, costs extend beyond annual fees. Currency conversion costs can add to S&P 500 investments, though these are often minimal with modern ETFs that handle currency hedging efficiently. Some investors choose currency-hedged versions of international funds to eliminate exchange rate risk, though these typically carry slightly higher fees.
Platform fees from your broker or ISA provider can also vary depending on which funds you choose. Some platforms offer zero-fee trading on certain ETFs, while others charge per transaction or impose monthly account fees.
Tax Implications for UK Investors
Tax treatment represents one of the most significant practical differences between these investments for UK investors. Both S&P 500 and FTSE 100 index funds can be held within ISAs and SIPPs, providing tax-free growth and withdrawals (subject to annual contribution limits).
Outside of tax-sheltered accounts, both investments are subject to UK capital gains tax when you sell, with the same annual exemption applying (currently £6,000 for the 2024/25 tax year according to gov.uk). However, dividend taxation differs significantly.
Dividends from FTSE 100 funds benefit from the UK dividend allowance and are taxed at preferential dividend tax rates. S&P 500 fund dividends are typically subject to US withholding tax (usually 15% for UK investors due to the double taxation treaty), plus UK income tax on the remaining amount.
This tax drag can be meaningful over time. While S&P 500 companies generally pay lower dividends than FTSE 100 companies (preferring share buybacks), the additional tax complexity and withholding can reduce your net returns.
For investors using ISAs or SIPPs, these tax differences become irrelevant, making tax-sheltered accounts even more attractive for international investing. The ability to hold both types of funds tax-free within these accounts removes one of the key disadvantages of S&P 500 investing for UK investors.
Currency Risk and Hedging Options
Currency risk represents perhaps the most underappreciated aspect of international investing. When UK investors buy S&P 500 index funds, they’re making two bets: that US stocks will rise, and that the dollar won’t weaken significantly against the pound.
Historical currency movements have been substantial. The pound-dollar exchange rate has fluctuated from below $1.20 to above $1.40 in recent years. A 10% currency move can significantly impact your returns, sometimes overwhelming the underlying stock performance.
Some fund providers offer currency-hedged versions of international index funds, which aim to eliminate exchange rate risk. These funds use financial instruments to lock in the exchange rate, so you get the pure stock performance without currency fluctuations.
However, currency hedging isn’t free and doesn’t always work perfectly. Hedged funds typically charge slightly higher fees (often 0.10-0.15% extra annually), and the hedging process can introduce small tracking errors. Additionally, currency movements sometimes work in your favour – a weakening pound can boost your dollar-based returns.
For FTSE 100 investments, currency risk is more indirect but still present. Many FTSE 100 companies earn substantial revenues in foreign currencies, so their share prices can be affected by exchange rate movements. However, this relationship is more complex and often works both ways.
Building a Balanced Portfolio: Integration Strategies
Rather than choosing between S&P 500 and FTSE 100 funds, many investors opt for both as part of a globally diversified portfolio. This approach can provide the growth potential of US markets while maintaining some home bias and reducing concentration risk.
A common allocation might be 60-70% international equities (including significant S&P 500 exposure) and 30-40% UK equities (primarily FTSE 100). This reflects the UK’s relatively small share of global market capitalisation while avoiding complete home bias.
Consider your existing exposures when deciding allocation. If your pension is heavily UK-focused, adding S&P 500 exposure through your ISA could improve diversification. Conversely, if you work for a US multinational with significant share options, you might prefer more FTSE 100 exposure to balance your portfolio.
Time horizon matters significantly. Younger investors with longer time horizons might favour higher S&P 500 allocations to capture potential growth, accepting higher volatility. Investors closer to retirement might prefer the steadier dividends and lower volatility typically associated with FTSE 100 investments.
Regular rebalancing between the two can also enhance returns by systematically selling high and buying low. If S&P 500 funds significantly outperform, trimming them to buy more FTSE 100 funds maintains your target allocation and locks in some gains.
Conclusion
The choice between S&P 500 and FTSE 100 index funds isn’t binary – both have roles in a well-constructed portfolio. S&P 500 funds offer exposure to the world’s most innovative companies and have delivered superior historical returns, but come with currency risk and higher volatility. FTSE 100 funds provide home market exposure, steadier dividends, and simpler tax treatment, but have lagged in recent performance.
For most UK investors, a combination approach works best, with allocations depending on your risk tolerance, time horizon, and existing exposures. Use tax-sheltered accounts like ISAs and SIPPs to minimise the tax disadvantages of international investing.
Focus on keeping costs low by choosing funds with sub-0.20% annual fees, and don’t overlook the impact of currency movements on your international investments. Consider currency-hedged versions if exchange rate volatility concerns you, but remember that hedging comes with additional costs.
Regular portfolio reviews and rebalancing can help you maintain your target allocations and potentially enhance returns through systematic profit-taking. Remember that past performance doesn’t predict future results, and both markets will experience periods of underperformance.
Most importantly, start investing sooner rather than later. The power of compound returns means that beginning your investment journey is more important than perfecting your initial allocation, which you can always adjust as your knowledge and circumstances evolve.
Next read: Ready to start investing? Learn the basics in our beginner’s guide to index fund investing: /index-fund-investing-beginners-guide