Historical investing tools, reproducible studies, and practical reading guides
Historical investing tools, reproducible studies, and practical reading guides
Retirement
Backtest a TSP (C/S/I/F/G) or 401(k) ETF allocation with steady biweekly or monthly payroll contributions, against an S&P 500 benchmark.
Educational backtest of historical monthly data — not investment or retirement advice. TSP funds use liquid ETF proxies (C=SPY, S=VXF, I=EFA, F=AGG, G=BIL); the real G Fund is a special government security and BIL is only a cash-like stand-in. Monthly contributions invest at month-end closes; biweekly contributions invest on a real two-week trading grid (~26/year). No employer/agency match, taxes, fees, catch-up contributions, or IRS-limit changes by year. Past performance does not indicate future results.
How to use it
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.
Background
The C Fund represents large US companies; S represents US companies outside the S&P 500; F provides broad US bond exposure. The current I Fund covers developed and emerging markets excluding the United States, China, and Hong Kong, as described in the official I Fund publication. This tool uses EFA as an I Fund proxy, so it does not reproduce that current benchmark. BIL is likewise a Treasury-bill ETF proxy, not the actual G Fund; do not assume a fixed direction or size of tracking difference.
Compare two explicitly labeled hypothetical allocations using the same deposits and dates. Examine the ending balance and the decline between sampled observations. No particular mix is established here as the fastest-growing or safest across all windows. Fund proxies, costs, matching rules, and contribution limits can make an actual account differ. See model limitations.
Payroll deposits and market growth both contribute to the ending balance. Run one scenario with the same contribution schedule and another with a changed amount to separate those effects. A purchase that looks attractive after a recovery was not guaranteed to recover when made. The cash-deployment study distinguishes money already available from money arriving later as income.