Marcus had been at the Navy shipyard for eleven years when a coworker looked over his shoulder at his TSP statement and said, "You're 100% C Fund? You're missing a fifth of the market."

He wasn't offended. He was curious. He had assumed the S&P 500 was the market. It's what the news quotes every evening, and it's what the C Fund tracks. But the coworker was onto something real, and it's worth understanding before you move a dollar.

What the two funds actually hold

The C Fund tracks the S&P 500, which is roughly 500 of the largest U.S. companies. It covers most of the U.S. stock market by value, but not all of it.

The S Fund tracks the Dow Jones U.S. Completion Total Stock Market Index. The name sounds like jargon, but the idea is simple: it holds every U.S. stock that is not in the S&P 500. Mid-sized companies, smaller ones, and some you've never heard of.

Put the two together and you have, in effect, the entire U.S. stock market. The TSP has no single "total market" fund, so this pairing is how you build one yourself.

Where 80/20 comes from

The S&P 500 makes up the large majority of the total U.S. market's value, and the completion index makes up the rest. In recent years that split has hovered around 80 to 85 percent large companies and 15 to 20 percent everything else. The exact figure drifts as prices move, and it has tilted toward the large end as the biggest companies have grown.

So 80/20 is a round, easy-to-remember approximation. It isn't a law of nature. If you want to check the current weights, the index providers publish them, and tsp.gov has fund fact sheets with the basics.

The logic is market-cap weighting. If small and mid-sized companies are about a fifth of the market, a portfolio that mirrors the market holds about a fifth in them. You aren't betting that small companies will beat large ones. You're simply declining to leave them out.

A worked example

Say Marcus has $200,000 in his traditional TSP, all in the C Fund, and he contributes $600 every pay period.

To move to 80/20, he does an interfund transfer so that $160,000 sits in the C Fund and $40,000 in the S Fund. Then he changes his contribution allocation so $480 of each paycheck goes to C and $120 to S. That's it. No tax bill, because moves inside the TSP aren't taxable events. The same holds inside a traditional or Roth IRA.

The two-step part matters. Many people change the transfer but forget the contribution allocation, and within a year or two they've drifted back to nearly all C.

Drift and rebalancing

Now suppose a year goes by. The C Fund gains 8 percent and the S Fund gains 20 percent. Here is what happens:

  • C Fund: $160,000 grows to $172,800
  • S Fund: $40,000 grows to $48,000
  • Total: $220,800, with the S Fund now at about 21.7 percent

That's a small drift after a big gap in returns. It takes a while to knock an 80/20 split meaningfully off course. Checking once a year is plenty, and some people do it only when they get a statement that looks lopsided.

To rebalance, you move a bit from the winner back to the laggard, or you simply aim new contributions at the underweight fund. The TSP's rules on interfund transfers have changed over the years, so confirm the current ones at tsp.gov before you plan anything fancy.

Don't fuss over 1 or 2 percentage points. Nobody retires poorer because their split was 79/21.

Where the approximation falls short

A few honest caveats.

It isn't a promise of better returns. Over some stretches the S Fund has beaten the C Fund, and over others, including much of the last decade or so, it has lagged. Adding it makes your portfolio look like the whole market. It does not make it outperform the S&P 500. If someone sells you 80/20 as a way to beat the C Fund, they're guessing.

The S Fund is more volatile. Smaller and mid-sized companies tend to swing harder, especially in downturns. At 20 percent of your stock allocation, that's usually tolerable. At 50 percent, it's a different portfolio.

The overlap with the S&P 500 isn't perfectly clean. Index membership changes. Companies get promoted into the S&P 500 and leave the completion index when that happens. The S Fund is built to handle this, but it means the 80/20 ratio you see today is a snapshot.

It only covers U.S. stocks. This is a point people tend to skip. An 80/20 C/S split leaves out international stocks (the I Fund), bonds (the F Fund), and the G Fund. Whether those belong in your plan depends on your age, your retirement date, your pension, and how you'd react to losing a third of your balance on paper. The 80/20 split describes the inside of your U.S. stock slice, not your whole portfolio.

Who this suits, and who can skip it

If you're in the L Funds, you're already covered. They hold the C and S Funds, along with the I, F, and G Funds, in a mix that shifts as the target date approaches. You don't need to build 80/20 yourself, and adding it on top would muddle the allocation you picked.

The DIY version makes sense if you prefer to set your own mix of stocks and bonds, you like the idea of owning the full U.S. market, and you're willing to check in once a year. It's a modest change with a clear rationale.

It's less compelling if you'd lose sleep over a fund that trails the S&P 500 for several years. That happens, and having a reason for the choice doesn't make it hurt less. If a plain C Fund holding lets you stay the course, staying the course is worth a lot more than a 20 percent slice.

A quick way to decide

Ask yourself two things. First, do I want my U.S. stock exposure to match the whole market, or am I fine with large companies only? Second, will I actually keep the split in place when the S Fund has a bad stretch?

If both answers are yes, set it up, write down the date, and look again in twelve months. For current fund details and transfer rules, go to