TSP & 401(k) simulator

Retirement saving really comes down to two things you control: how much goes in each paycheck, and where it's invested. This tool lets you test both against real market history. Set a biweekly or monthly contribution, spread it across the five TSP funds (C, S, I, F, G) or a simple 401(k) ETF mix, and watch how the balance would have grown — next to an S&P 500 benchmark on the same schedule.

It's built to show the trade-offs. Lean heavy on stocks (more C and S) and you usually grow faster but have to stomach bigger drops; lean on bonds and the G Fund (F and G) and the ride smooths out, but long-run growth slows. And because the money goes in steadily, you'll notice something most people learn the hard way: just sticking with it through a bad year often matters more than the exact mix. The test leaves out employer matches, taxes, fees, and the yearly IRS limit changes, so read it as a lesson in compounding — not a retirement projection or advice. Want the mechanics? See how it works.

The Thrift Savings Plan keeps it to five core building blocks, and each has a distinct job. The C Fund tracks the S&P 500 — the large-company US market, and the engine of most long-run growth. The S Fund covers the rest of the US market: the small and mid-sized companies the S&P leaves out, historically a bit bumpier with similar long-run character. The I Fund holds international developed-market stocks, which is diversification against any single country's decade going badly — including America's. The F Fund is the broad US bond market: it usually cushions stock crashes, though 2022 showed it can fall alongside stocks when rates jump. And the G Fund is the TSP's oddity — a government security that can't lose value and pays a longer-term rate on cash-like safety. Nothing else in retail investing quite matches it, which is why our BIL stand-in here understates it slightly.

Most allocations are just different answers to one question: how much crash am I willing to hold through in exchange for growth? A stock-heavy mix (say 70% C, 20% S, 10% I) compounds fastest across most long windows in this tool but regularly spends stretches down 20–30%. A balanced mix (60% stocks, 40% F/G) gives up some endpoint in exchange for shallower dips. The right comparison isn't which mix "won" a given decade — it's which drawdown you'd realistically have contributed straight through, because quitting mid-dip costs more than any allocation choice. Run both through 2008 or 2022 above and look at the gap between the mixes at the bottom, not just at the end.

Why steady payroll contributions punch above their weight

Retirement investing has a structural advantage most backtests hide: the money arrives on a schedule, in equal slices, for decades. That schedule buys automatically through every crash — the contributions that landed in early 2009 or late 2022 were small at the time and turn out to have been the best purchases on the whole chart. It's the same steady-buying arithmetic as dollar-cost averaging, applied at career length. The corollary: in the early years, your contribution rate matters far more than your fund mix, because contributions dwarf returns when the balance is small. The mix takes over as the driver later, when the balance is large — which, not coincidentally, is when a written plan for bad years matters most. Our risk checklist covers exactly that, and the main simulator lets you run the same discipline on individual stocks and custom baskets.