Thrift Savings PlanThe S Fund is the Thrift Savings Plan’s small- and mid-cap U.S. stock fund. It holds the part of the American stock market the C Fund leaves out — the small and medium-sized companies outside the S&P 500 — and it is the most volatile of the five core funds.
It is often described as the aggressive growth choice. Over the last twenty years it delivered a bumpier ride than the C Fund and a lower return.
The S Fund returned 9.53% a year over 2006–2025 versus the C Fund’s 11.01% — while losing 38.32% in 2008 and 26.26% in 2022, the worst of the five funds in both. More risk did not mean more reward over this particular twenty years. It did have the best single year of any core fund in 2020, at +31.85%.
What the S Fund actually holds
The S Fund is an index fund tracking the Dow Jones U.S. Completion Total Stock Market Index — effectively the entire U.S. stock market except the large companies already in the C Fund. That is thousands of small and mid-sized companies. Holding the C and S funds together gives you close to the whole U.S. market; holding the S Fund alone is a concentrated bet on the smaller end of it.
The formal TSP name is the Small Capitalization Stock Index Investment Fund. Like every TSP fund it has no ticker symbol and cannot be bought outside the plan.
What the S 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 S 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: the widest swings in the plan
The S Fund produced the largest single-year gain in the twenty-year record for a core fund (+38.35% in 2013, and +34.85% in 2009) and the second-deepest single-year loss (−38.32% in 2008). It also had the worst 2022 of any TSP fund at −26.26%. Over the full period $10,000 became $61,713.40 — well behind the C Fund’s $80,821.11.
That is worth sitting with, because the usual assumption is that small caps pay you for the extra volatility. Across these specific twenty years, they did not. That does not make the S Fund a mistake — it diversifies the C Fund and it led the 2020 rebound — but over this period it did not reward being the largest holding.
S Fund annual returns, 2006–2025
| Year | S Fund return |
|---|---|
| 2006 | 15.30% |
| 2007 | 5.49% |
| 2008 | -38.32% |
| 2009 | 34.85% |
| 2010 | 29.06% |
| 2011 | -3.38% |
| 2012 | 18.57% |
| 2013 | 38.35% |
| 2014 | 7.80% |
| 2015 | -2.92% |
| 2016 | 16.35% |
| 2017 | 18.22% |
| 2018 | -9.26% |
| 2019 | 27.97% |
| 2020 | 31.85% |
| 2021 | 12.45% |
| 2022 | -26.26% |
| 2023 | 25.30% |
| 2024 | 16.93% |
| 2025 | 11.38% |
Calendar-year returns for the S 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 S Fund in the three stress tests
2008: −38.32%. Deeper than the C Fund’s −36.99%. Smaller companies were hit hardest by the credit freeze.
2020: +31.85%. The best calendar-year result of any core TSP fund that year. Small caps fell furthest in March and rebounded hardest into December — which only helped the people who stayed put.
2022: −26.26%. The worst of the five funds. When rates rose, the smaller and more leveraged end of the market took the most damage.
Who the S Fund suits — and who it does not
It suits you if you are years from retirement and want broader U.S. coverage than the S&P 500 alone, held as a supporting position alongside the C Fund.
It does not suit you if you are near retirement, or if you are reaching for it because it looks like the fast lane. The twenty-year record says it was the rougher road, not the quicker one.
The other TSP funds
- C FundLarge-cap U.S. stocks · 11.01%
- I FundInternational stocks · 5.91%
- 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 S 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.