About the TSP Calculator
The Thrift Savings Plan is the retirement account for federal employees and uniformed service members — functionally a 401(k) with famously low costs. Its single most valuable feature is the FERS agency match: contribute 5% of salary and your agency contributes another 5%. That's an instant, guaranteed 100% return on those dollars before any market growth, and it's the first number this calculator gets right.
The projection math is standard compound growth, but the inputs deserve honesty: the often-cited ~10% figure is the C Fund's long-run historical average, not a promise, and most planners model at 6–8% to stay conservative. Run the calculator at two return rates and treat the gap as your uncertainty band.
Want to isolate pure compounding without the match rules? Use our Compound Interest Calculator.
The FERS Agency Match, Percentage by Percentage
What the agency adds at each contribution level — the jump from 0% to 5% is the highest-yield decision in federal employment:
| You contribute | Agency adds | Total going in |
|---|---|---|
| 0% | 1% (automatic) | 1% |
| 1% | 2% (1% auto + 1% match) | 3% |
| 3% | 4% (1% auto + 3% match) | 7% |
| 5% | 5% (1% auto + 3% + 2×0.5%) | 10% |
| 10% | 5% (match caps at 5%) | 15% |
Read the last two rows carefully: the match maxes out at 5% — so the leap from contributing 3% to 5% doubles your total inflow from 7% to 10% of salary, while every percent beyond 5% adds only itself. Contributing less than 5% is declining free money; the calculator shows exactly how much.
How the Projection Works
Three moving parts, compounded monthly:
- Your current balance grows at the expected return for the full period.
- Monthly contributions (your % + agency % of salary ÷ 12) are added and each compounds from its own start date.
- The agency portion is computed from the official FERS formula automatically — you only enter your own percentage.
What the model deliberately leaves out: salary raises, the IRS elective deferral limit, and Roth-vs-traditional tax treatment. All three matter at the margins — but return rate and years invested dominate the outcome by an order of magnitude.
Where TSP Money Actually Grows: The Funds
TSP offers five core index funds — G (government securities, never loses nominal value), F (bonds), C (S&P 500), S (small/mid caps), I (international) — plus Lifecycle (L) funds that glide between them by target date. Your expected-return input should reflect your mix: an all-G portfolio has historically returned far less than a C-heavy one, with far less volatility.
The quiet TSP superpower is cost: expense ratios run a few hundredths of a percent, among the lowest anywhere. On a $500,000 balance, the difference between TSP-level fees and a typical 1%-fee managed account is roughly $5,000 per year — compounding in your favor for decades.