The FIRE Gap Most Calculators Completely Ignore

I spent three weekends plugging numbers into every FIRE calculator I could find. They all gave me a retirement number — but none of them answered the question that was keeping me up at night: what do I live on between 50 and 59½?

My 401k and Roth IRA would be locked. Withdrawing early means a 10% penalty on top of ordinary income taxes. I needed a separate pool of money in a taxable brokerage — a bridge fund — and nobody was telling me how much that needed to be or how much to save each month to hit it.

So I built MyFIRE to solve exactly that. This article explains the math behind the bridge fund, with real numbers from the calculator’s simulation engine.

Most FIRE calculators make three assumptions that work fine for traditional retirement at 65 but break completely for anyone retiring before 59½:

• All savings are accessible on day one. They’re not. 401k and Roth IRA earnings carry a 10% early withdrawal penalty before 59½.
• Your spending is fixed. Many early retirees earn part-time income during the gap — barista FIRE, consulting, freelance. This dramatically changes the math.
• Social Security doesn’t factor in yet. If you claim at 62 or 67, that benefit changes your full-retirement corpus target by $500K or more.

The bridge fund target is the present value of all future withdrawals during the gap phase. I ran this through MyFIRE’s simulation engine using real S&P 500 return data from 1928–2023 (Damodaran/NYU Stern). Here’s the core formula:

When I first modeled this for myself, the bridge fund number felt overwhelming. Then I added $2,000/month in part-time income — freelance consulting a couple days a week — and the number dropped dramatically. Here’s the exact impact from MyFIRE’s simulation:

That $2,000/month could mean retiring 2–3 years earlier than a purely passive approach, or hitting your target with significantly less stress. You can model this directly in MyFIRE using the part-time income slider.

On the full retirement side, Social Security income has a dramatic effect on how much corpus you need. I ran both scenarios through MyFIRE at $8,000/month spend with 2.5% inflation and a 5% retirement return:

The bridge fund is particularly vulnerable to sequence-of-returns risk. A market crash in year one of semi-retirement — when you’re actively drawing down — is far more damaging than the same crash during accumulation. You don’t have time to recover.

I tested this in MyFIRE’s Monte Carlo simulator, which runs 1,000 scenarios using real S&P 500 annual returns from 1928–2023. Starting retirement in 1929, 1937, or 2000 produces very different outcomes than starting in 1982 or 2009 — even with the same average return assumption.

• The bridge fund gap is real and often larger than expected. Anyone retiring before 59½ needs a separate taxable brokerage to cover living expenses — not just their 401k number.
• Part-time income is the biggest lever. Even $1,500–2,000/month dramatically reduces the corpus needed and can move your FIRE date years earlier.
• Social Security is worth $500K+ in reduced corpus. Include your expected benefit — check it at ssa.gov — and plug it into your full-retirement calculation.
• Model sequence-of-returns risk. A straight-line 7% return assumption ignores the real danger of retiring into a crash. Run Monte Carlo with historical data.
• Use a calculator that models all three phases. Accumulation, bridge, and full retirement are three different problems that interact. MyFIRE handles all three for free.