Thrift Savings PlanThe I Fund is the Thrift Savings Plan’s international stock fund. It holds shares of companies based outside the United States, and it is the fund that has tested federal investors’ patience most — the lowest twenty-year return of the three stock funds, and the deepest single-year loss of any TSP core fund.
Then 2025 happened: the I Fund returned 32.45%, beating every other core fund that year. Both facts belong on the same page.
The I Fund returned 5.91% a year over 2006–2025 — roughly half the C Fund’s 11.01% — and lost 42.43% in 2008, the worst calendar year of any TSP core fund in the period. It is a diversifier, not an engine, and it spends long stretches lagging U.S. stocks before years like 2006 and 2025 when it leads.
What the I Fund actually holds
The I Fund is an index fund holding stocks of companies domiciled outside the United States, giving your TSP exposure to economies and currencies that do not move in lockstep with the American market. Part of its return in any year comes from currency movement rather than the underlying companies, which is a large reason its results diverge so sharply from the C and S funds.
One important caveat: the TSP has changed the I Fund’s benchmark index since this backtest period began, so the mix of countries the fund tracks today is not identical to the mix behind the earlier years in the table below. Confirm the current benchmark on tsp.gov before you draw conclusions about what the fund holds now. The formal TSP name is the International Stock Index Investment Fund; like all TSP funds it has no ticker symbol.
What the I Fund costs
TSP administrative expenses are among the lowest of any retirement plan in the country, and the TSP publishes a net expense ratio for every fund each year. We do not print a figure here, because our data source covers returns rather than the fee schedule and we only publish numbers we can point to — check the current published net expense ratio for the I Fund on tsp.gov. What matters for a decision is the ranking: TSP costs are low enough that they are almost never the reason to choose one of these funds over another. Choose on risk and role instead.
Risk profile: stock-level risk, bond-level reward
This is the uncomfortable part of the record. The I Fund carried the full volatility of a stock fund — a 42.43% loss in 2008, a 13.43% loss in 2018, six negative years in twenty — and over the full period $10,000 grew to $31,547.58. The C Fund turned the same $10,000 into $80,821.11.
What the I Fund did offer was different timing. It was the best core fund of 2006 (+26.32%) and of 2025 (+32.45%), and it fell less than the C and S funds in 2022. A holding that leads when your largest position lags is doing something useful, even when its own long-run number is unimpressive.
I Fund annual returns, 2006–2025
| Year | I Fund return |
|---|---|
| 2006 | 26.32% |
| 2007 | 11.43% |
| 2008 | -42.43% |
| 2009 | 30.04% |
| 2010 | 7.94% |
| 2011 | -11.81% |
| 2012 | 18.62% |
| 2013 | 22.13% |
| 2014 | -5.27% |
| 2015 | -0.51% |
| 2016 | 2.10% |
| 2017 | 25.42% |
| 2018 | -13.43% |
| 2019 | 22.47% |
| 2020 | 8.17% |
| 2021 | 11.45% |
| 2022 | -13.94% |
| 2023 | 18.38% |
| 2024 | 4.27% |
| 2025 | 32.45% |
Calendar-year returns for the I Fund, January 2006 through December 2025. Click a column heading to sort — sorting by return is the fastest way to see the best and worst years. Source: Portfolio Visualizer export used for the Retirement Edge TSP backtest.
2008, 2020 and 2022 — the I Fund in the three stress tests
2008: −42.43%. The deepest single-year loss of any TSP core fund over these twenty years — worse than the S Fund’s −38.32% and the C Fund’s −36.99%.
2020: +8.17%. It participated in the crash and lagged badly in the recovery, finishing well behind the S Fund’s +31.85%.
2022: −13.94%. Its best relative showing of the three tests — better than the C Fund’s −18.13% and much better than the S Fund’s −26.26%.
Who the I Fund suits — and who it does not
It suits you if you want a modest slice of international exposure so your retirement does not depend entirely on one country’s market, and you can hold it through years when it plainly underperforms.
It does not suit you if you will judge it every twelve months, or if you plan to make it a large share of your balance on the strength of one good year. It is a supporting position.
The other TSP funds
- C FundLarge-cap U.S. stocks · 11.01%
- S FundSmall and mid-cap U.S. stocks · 9.53%
- F FundU.S. investment-grade bonds · 3.41%
- G FundGovernment securities · 2.85%
- All five, side by sideThe TSP funds hub
The part a table cannot tell you
How much of the I Fund to hold, and when that should change, is the actual question. Our TSP allocation by age guide gives a decade-by-decade framework, you can pressure-test any mix in the TSP Calculator, and the TSP Edge model portfolio is a rules-based monthly update built from these same five funds — research you choose to act on, with most months saying nothing needs to change.