An index fund is a simple way to invest in hundreds or thousands of companies at once, without having to pick them one by one. That is why it is one of the most popular products for people who start investing for the long term.
Investing involves risk, including the loss of part of your capital. This guide explains concepts; it is not investment advice.
What an index fund is
It is an investment fund that tracks a stock market index, such as the S&P 500 or the MSCI World, by buying the securities in the index in the same proportions. It does not try to beat the market: it tries to follow it. This is called passive management.
Advantages and limits
- Diversification: with a single investment you are spread across many companies and, depending on the index, several countries.
- Low costs: because no team is needed to pick securities, the management fee is usually lower than that of actively managed funds.
- Simplicity: you do not have to decide when to buy or sell each share.
- No protection: if the index falls, your fund falls with it. An index fund does not guarantee your capital or any return.
Shares, funds and ETFs: the basic differences
| Product | What it is | Tax-free switching in Spain |
|---|---|---|
| Share | A stake in a single company | Not applicable |
| Index fund | A fund that tracks an index | Yes |
| ETF | A fund that tracks an index and is traded on the stock exchange like a share | No |
Basic concepts worth understanding
- Risk and volatility: the value of your investment goes up and down. The more equities, the bigger the swings.
- Time horizon: the stock market can fall for years, so you invest money you will not need in the short term.
- Costs: the annual fee (ongoing charges) appears in the fund’s key information document. A one-point difference every year adds up to a lot over the long term, as you will see in our guide to compound interest.
- Regular contributions: investing a fixed amount every month avoids having to guess the best moment, although it does not guarantee results.
- Past performance: the fact that an index has risen in the past does not ensure it will do so in the future.
A cost example
Suppose you invest 10,000 euros for 20 years and the market return before costs is 6% a year. In a fund that charges 0.2% a year, your money would grow at about 5.8% and end at roughly 30,883 euros. In a fund that charges 1.5%, it would grow at about 4.5% and end at roughly 24,117 euros. The difference is about 6,765 euros, only because of costs. The figures are an illustration, not a forecast.
How to start, step by step
- First, your cushion. Before investing, build an emergency fund and get your spending under control.
- Define your goal and your timeframe. Saving for a distant retirement is not the same as saving for a home deposit in two years.
- Choose the index. A broad, diversified one reduces the risk of depending on a few companies.
- Compare funds. Look at the ongoing charges, check that the fund really tracks the chosen index and that it is sold by a regulated institution.
- Open the account and invest regularly. Set up an automatic monthly contribution.
How they are taxed in Spain
While you invest, you can move money from one fund to another through transfers (“traspasos”) without paying tax, as long as the money does not pass through your bank account. That advantage does not exist for ETFs. You only pay tax when you redeem units and make a gain.
The gain is added to the savings base of your Spanish income tax (IRPF), with these combined rates in the common regime:
| Gains and savings income | Rate |
|---|---|
| Up to €6,000 | 19% |
| €6,000 to €50,000 | 21% |
| €50,000 to €200,000 | 23% |
| €200,000 to €300,000 | 27% |
| Over €300,000 | 30% |
If the fund is a distributing fund, the dividends it pays are taxed each year. If it is an accumulating fund, you only pay tax when you sell. The Basque Country and Navarre have their own regional rules, so if you live there check the rules of your regional tax authority. If you are tax resident in another country, the rules of that country apply to you.
Frequently asked questions
Are index funds safe?
They are diversified, but not risk-free: they can lose value, sometimes a lot, in a market fall. They suit money you will not need for many years.
Do I need a lot of money to start?
Many providers let you start with small regular amounts, but minimums and fees vary, so compare before you choose.
What is the difference between an index fund and an ETF?
Both can track the same index. A fund is bought and sold at its daily price through the fund provider, while an ETF trades on an exchange like a share. In Spain, the tax treatment differs, as shown above.
Sources
- Helpmycash: how investment funds are taxed in Spain
- Raisin: IRPF tax brackets 2025-2026
- Agencia Tributaria: Manual práctico de Renta
This article is for information and education only. It is not personalised financial, tax or legal advice: for an important decision, consult a qualified professional. Last reviewed: October 2026.