How the TSP works: the G, F, C, S, I, and L funds

An asset class is a category of investment that tends to move on its own rhythm: cash, bonds, U.S. stocks, and international stocks are the major ones. A portfolio's real risk comes from how its pieces move together, not from any single piece.

Hold only one asset class and you’re exposed to whatever sinks that one category. Hold a mix, and the others often cushion a bad year in one. This is the cheapest protection an investor has, and it’s the whole idea behind how the TSP menu is built.

How the TSP works follows directly from that idea. The plan gives you five core funds, and each one holds a single asset class. The G fund holds government securities, the F fund holds bonds, and the C, S, and I funds hold stocks.

You aren’t picking individual companies. You’re choosing how much of each broad category to hold (a bit like choosing aisles in a grocery store rather than individual cans).

The five funds are deliberately simple building blocks, so a federal employee can build a diversified portfolio from them without needing to be an investor.

Beyond the five core funds, the TSP offers the Lifecycle (L) funds. An L fund is a ready-made mix of all five core funds, set to a target retirement year, that starts stock-heavy when the target is decades away and shifts toward bonds and government securities as the date nears.

The TSP places most new federal employees in an L fund automatically. If you’ve never changed your investment choice, you’re almost certainly in one now.

The menu runs from the safe anchor (the G fund) to bonds (the F fund), then the three stock funds (C, S, and I), then the L funds that blend them. Before any of this matters, make sure you’re capturing the agency match: allocation is the second decision, not the first. Once the match is locked in, how you spread your balance across these funds is the next lever you control.

The G fund is the TSP’s safe anchor. It is invested in U.S. Treasury securities specially issued to the TSP, with both principal and interest guaranteed by the federal government.

That guarantee is the G fund's defining feature. Its share price doesn’t fall, so in any given year the G fund earns interest and never posts a loss. No other TSP fund, and almost no private-sector investment, offers that.

The trade-off for that safety is modest growth. The G fund pays an interest rate tied to medium- and long-term Treasury yields, which usually runs a bit above short-term cash rates, and over a long career it tends to grow more slowly than the stock funds.

The G fund protects your dollars from market drops, but it doesn’t protect them from inflation. If prices rise faster than the G fund earns, your money loses purchasing power even though the balance never falls.

That makes the G fund a tool, not a whole plan. It shines as the stable portion of a mix, especially for money you’ll need soon. A retiree drawing income often holds several years of withdrawals in the G fund so a market drop doesn’t force them to sell stocks at a loss. A younger employee with decades to go usually holds little G, because the bigger long-run risk is growing too slowly, not a bad year.

Think of the G fund as the floor under your TSP. It can’t fall, which makes it the calm center of a portfolio, but it also can’t do the heavy lifting of long-term growth on its own. The skill is using it for what it’s good at: protecting the part of your balance you can’t afford to see drop, while the stock funds carry the growth over the decades you’re still working.

The F fund holds bonds. It tracks the Bloomberg U.S. Aggregate Bond Index, a broad basket of U.S. government, corporate, and mortgage-backed bonds. When you own the F fund, you’re effectively lending money to those borrowers in exchange for interest.

The F fund pays more interest over time than the G fund, on average. In return, it carries a risk the G fund doesn’t: its value can fall.

The key to the F fund is the seesaw between interest rates and bond prices. When market interest rates rise, the price of existing bonds falls, because newer bonds pay more and older ones look less attractive. When rates fall, existing bond prices rise.

So the F fund can post a losing year, especially when rates climb quickly. This is the single most important thing to understand about it: the F fund is safer than stocks, but it isn’t the G fund, and it can drop.

Over long stretches, the F fund has tended to grow, with the interest it pays outweighing the price swings. Since it moves differently from stocks, it’s useful in a mix. In many market downturns, high-quality bonds hold up better than stocks, so the F fund can cushion a portfolio when the stock funds fall. That diversification benefit, not raw return, is the main reason to hold it.

Put the G and F funds side by side, and the difference is clear. The G fund never falls but grows slowly, while the F fund grows a bit faster on average but can lose value in a bad year for bonds.

Neither is "better," since they’re different tools. The G fund is a guarantee; the F fund is a diversifier that adds bond-market exposure your stock funds don’t give you.

The C, S, and I funds are the TSP's stock funds, and they’re where most long-term growth comes from. Each one tracks a different slice of the stock market.

The C fund tracks the S&P 500, the 500 largest U.S. companies. The S fund tracks the Dow Jones U.S. Completion Total Stock Market Index, the small and mid-sized U.S. companies left out of the S&P 500. The I fund tracks an international index of companies outside the United States.

Together, the C and S funds cover the entire U.S. stock market. The C fund is the large, household-name companies, and the S fund is everything smaller. To mirror the whole U.S. market, you hold them roughly by size, which means mostly C with a smaller slice of S.

The I fund then adds companies based overseas, so a portfolio with all three isn’t betting on any single country. That global spread is itself a form of diversification.

Stocks carry the most year-to-year risk on the TSP menu, and the most long-term growth potential. A stock fund can fall sharply in a bad year, though over decades stocks have outgrown bonds and cash by a wide margin.

That’s the core trade: more short-term swing in exchange for more long-run growth. For an employee with a long time horizon, the swings matter less than the growth, which is why younger savers usually hold mostly stock funds.

The three stock funds also differ. Small companies (the S fund) tend to swing more than large ones (the C fund). International stocks (the I fund) move with foreign economies and currencies, so they don’t always rise and fall in step with U.S. stocks.

Holding all three smooths the ride compared with betting on one. None is "best," and each adds a different piece of the global stock market to your mix.

A Lifecycle (L) fund is a ready-made mix of all five core funds, built around a target retirement year. Instead of choosing your own blend of G, F, C, S, and I, you pick the L fund closest to when you expect to retire, and the TSP manages the mix for you.

There’s an L Income fund for those already drawing down, plus a series of dated funds out past 2070. You hold one fund and own a slice of all five underneath.

The defining feature of an L fund is the glide path. When the target date is far away, the fund holds mostly stocks (C, S, and I) for growth. As the date nears, the TSP automatically shifts the mix every quarter toward bonds and government securities (F and G).

The stock share falls and the safe share rises, gradually, on its own. Between those quarterly shifts, the fund is rebalanced daily to stay on target (rebalancing just means trimming back to the planned mix). You do nothing; the fund grows more conservative as you age.

When an L fund reaches its target year, it merges into the L Income fund, which holds the most conservative mix and doesn’t shift further. This is the same logic behind risk and time horizon: take more stock risk when you have decades to recover, and dial it down as the money gets closer to being spent. The L fund automates a discipline that many investors struggle to follow by hand.

L funds are why most new federal employees are diversified from day one without knowing it. New hires are auto-enrolled into the L fund matched to their age, so their very first contribution buys a spread of all five core funds.

The L fund is a sensible default, not a trap, and you can always move to your own mix later if you want more control. For many feds, leaving the money in the age-appropriate L fund is a perfectly reasonable choice.

The whole TSP menu reduces to one decision: how much of each asset class to hold. The G fund is your guarantee, the F fund adds bonds, and the C, S, and I funds carry stock-market growth. An L fund makes that decision for you on a glide path; building your own mix of core funds puts your hand on the dial. Either way, the goal is the same: a spread across asset classes that fits how long until you need the money.

That timeline is the single biggest factor. With decades to go, a stock-heavy mix makes sense, because there’s time to recover from drops and the bigger risk is slow growth. As retirement nears, shifting toward the G and F funds protects the balance you’ve built.

This is exactly what the L funds automate, and it is the same logic whether a fund does it for you or you do it by hand. The mix that is right early is rarely the mix that is right late.

Diversification across the funds is the protection you get for free. Because the funds hold different asset classes, they don’t all fall at once. A weak year for stocks may coincide with a steady G fund and a resilient F fund.

No mix removes risk entirely, but spreading across asset classes removes the risk of betting everything on one and being wrong. That is the asset-class principle in action, and it is why the TSP menu looks the way it does.

To see or change your own allocation, log in at tsp.gov and review your investment choices. First, confirm that you’re capturing the full agency match, since that matters more than any fund pick. From there, your mix is yours to set.

A later Mid Career lesson on TSP allocation by age goes deeper on how to shift the mix as you approach retirement. The next lesson, on Traditional versus Roth TSP, covers a different choice: not which funds you hold, but how those contributions are taxed.