How to choose an index fund: the 6 criteria that actually matter

Searching for "the best index fund" gives a different list on every site. The truth is there's no single winner: there are well-built, cheap funds, and others tracking the same thing for triple the fee. Choosing well means applying six objective criteria, not trusting a "best of" ranking that never explains why.

1. Which index it tracks

This is the most important decision, far above the brand. An index fund is only as good as the benchmark index it follows: an MSCI World (developed world) is not the same as an S&P 500 (US only) or an MSCI Emerging Markets. Decide first what you want exposure to, then find the fund. Browse the catalog by index type (global, US, Europe, emerging…).

2. The total cost (TER)

The TER is the annual fee taken from the fund. Between two funds tracking the same index, it's the real, measurable difference: over the long run, 0.20% vs 0.60% eats a meaningful chunk of your return through compound interest. Sort the catalog by cost in the cheapest funds and ETFs ranking.

3. Physical or synthetic replication

Physical replication actually buys the index's shares; synthetic mimics it with derivatives (a swap), adding counterparty risk in exchange for sometimes lower cost or better tracking in certain markets. For most investors, physical replication is the easiest to understand.

4. Accumulating or distributing

An accumulating fund reinvests dividends inside the fund; a distributing one pays them to you. While building wealth, the accumulating class is usually more tax-efficient in Spain because it defers tax — we cover it in distributing or accumulating.

5. Domicile and currency

The domicile (Ireland, Luxembourg…) affects the withholding tax the fund bears on the dividends it collects; Irish funds are popular thanks to their US treaty. The fund's currency doesn't change your real exposure (a euro-denominated MSCI World is still exposed to the dollar inside), but note that currency hedging is a separate decision with its own cost.

6. Index fund or ETF

They track the same thing, but the traditional mutual fund allows tax-deferred switching in Spain and the ETF doesn't; in exchange, ETFs tend to have very low costs and trade on an exchange. The full comparison is in ETF vs index fund.

Putting it into practice

Define the index first, filter by low TER, and confirm replication, share class and domicile on the official factsheet. Each factsheet in our catalog gathers that data and links to official docs; and with the comparator you can pit two same-index funds against each other to see, all else equal, which costs less. Remember: this is educational content, not an investment recommendation.