The only math trick in my book: 72 divided by your annual return equals the years it takes your money to double. At 12%, that’s 6 years. At 14%, about 5. At 10%, about 7. At 8%, it’s 9. That’s the whole calculator — it fits on an index card, and it explains most of what happened to my money.
I’m 44 and 3/4 as I write this, with about $800,000 and twelve and a quarter years to fifty-seven. At 12%, that’s two full doublings — $800,000 becomes $1.6 million becomes $3.2 million — before a single new paycheck lands. Every contribution on top is a head start on the third doubling. This is also why the early years felt so pointless and weren’t: the doublings are built in the years when the balance looks too small to matter.
Look backward and the rule is the story of the receipts. From 2008 to 2026 I put in about $328,000 — roughly $269,000 of my own pay, $53,000 of agency match, and the $6,236 rollover that seeded it. The balance is about $800,000. The other ~$472,000 isn’t money I earned at a job. That’s the Rule of 72 doing its job, one doubling at a time, through two crashes — with one response from me: nothing.
I run every projection at 8, 10, 12, and 14 percent, and the Rule of 72 is how I sanity-check the spreadsheet: each rate is just a doubling clock running at a different speed. On the full model — contributions continuing, limits stepped, catch-up spilling from 2031 — the plan ends 2038 at $2.72 million, $3.32 million, $4.05 million, and $4.93 million across the four rates. The spreadsheet lives in How a Federal Employee Retired at 57 with $5 Million; the playbook that feeds it is 401(k) Investing on One Ordinary Paycheck.
That’s the whole trick. Not timing. Not stock-picking. Not genius. Just: don’t interrupt the doubling, and let the rule do the rest.