A FIRE Calculator for People Receiving an Inheritance: What to Track

An inheritance feels like it should change the FIRE math more than it actually does, and missing that gap is what trips up most heirs. The Federal Reserve's 2022 Survey of Consumer Finances puts the median U.S. household inheritance — held outside a trust — at about $69,000, with the average across all households around $46,200.^[] Plug the median into a 25× FIRE target on a median-income household, and the timeline compresses by roughly four years — meaningful, but a long way from the "I can quit tomorrow" reaction many heirs initially have.

A FIRE calculator for people receiving an inheritance has to do two specific things that a generic 4% rule explainer doesn't. It has to model the inheritance as a one-time deposit into the invested-assets line, and it has to handle the awkward fact that retirement-account inheritances now come with a ten-year drawdown clock attached.^[]

What the math actually says

A household with $80,000 of income, a 20% savings rate, and a 5% real return needs roughly 36.7 years to reach a $1.6M FIRE number from a standing start. Drop a $69,000 lump sum into invested assets on day one and the same household crosses the line at about 32.7 years — the calculation rendered below works through the formula. That's about four years off the timeline, not a different decade.

The trust-held median tells a different story. Federal Reserve data puts the median trust inheritance near $285,000.^[] Routed straight into the same plan, the FIRE timeline compresses to roughly 23.7 years — a 13-year acceleration. The takeaway isn't that trusts are magic; it's that the FIRE-relevant variable is the size of the deposit, and trust beneficiaries, on average, receive much larger ones.

The inherited-IRA wrinkle most planners get wrong

If part of the inheritance is a 401(k) or traditional IRA, the SECURE Act has likely already changed your plan, whether you've noticed or not. Most non-spouse beneficiaries who inherit a retirement account from someone who died after 2019 must empty it within ten years.^[] The IRS finalized the regulations in 2024, and missed required distributions trigger a 25% excise tax — reduced to 10% if corrected within two years.^[]

For FIRE planning that's a structural problem, not a bookkeeping one. A $200,000 inherited 401(k) is not a $200,000 deposit into your "invested assets" line — it's a stream of forced ordinary-income distributions over the next ten years, taxed at your marginal rate. If you were planning to coast on it through year ten and then withdraw at 4%, the tax code has decided otherwise.

Step-up in basis is the asset class FIRE plans should celebrate

Inherited brokerage accounts are the opposite story. Under IRC §1014 the cost basis of inherited taxable assets resets to fair market value on the date of death, which is why the Joint Committee on Taxation projects this single rule will forgo $72.5 billion in federal revenue in 2026 alone.^[] If you inherit $200,000 of stock that originally cost the decedent $50,000, you can sell tomorrow with effectively no capital-gains liability and re-deploy the cash into your FIRE allocation.

That makes a $200,000 inherited brokerage portfolio meaningfully more valuable for FIRE timing than a $200,000 inherited IRA, even though they look identical on paper. Most heirs miss this because nothing on a 1099 statement labels it.

For a FIRE plan that already has to account for variable income and quarterly tax timing, the right move is to map every inherited dollar to its tax bucket before it lands in any spreadsheet — taxable, tax-deferred, or Roth — and run the FIRE math separately on each. The combined FIRE number then becomes the sum of three sub-portfolios with three different effective withdrawal rates, which is messier but honest.

What I'd update in the FIRE plan after an inheritance

A few concrete adjustments make the math honest:

• Re-base the starting balance rather than padding the savings rate. A $69,000 deposit recorded as "this year I saved $85,000" overstates the structural change.
• Discount inherited IRAs by your marginal tax rate before adding them to the FIRE asset line. A 24%-bracket heir should treat a $100,000 inherited IRA as roughly $76,000 of post-tax capacity.
• Use the actual SWR your plan was already using. The IRS doesn't care that you got an inheritance, and neither does the Trinity Study math — 4% is still 4%, and 3.5% is still 3.5%.^[]

The pitfall: confusing windfall with retirement readiness

The most expensive mistake heirs make isn't spending the money — it's recategorizing the money. A $100,000 inheritance routed to "retirement" and then quietly used to upgrade the lifestyle leaves the FIRE plan unchanged on paper but worse in practice. The original-data calculation below assumes the inheritance is fully invested, undiluted, on day one. Every dollar of lifestyle creep that the windfall enables erases roughly $25 of FIRE-number progress at a 4% SWR.^[]

The cleanest way to track this is by re-running the projection at one-year and three-year marks, comparing the actual balance to the projected post-inheritance trajectory. If you're more than a year off plan three years in, the inheritance probably leaked into spending. That's the metric most worth watching.^[]