Roth IRA vs 401k for FIRE: Optimal Account Strategy for Americans

The four-account stack

US FIRE planners have a more complicated tax landscape than UK planners, but with proportionally more leverage. The four main accounts:

• Roth IRA: $7,000/year (2026), post-tax in, tax-free forever after. Income limits apply ($165,000 single / $246,000 MFJ for full contribution).
• Traditional 401k: $23,500/year employee + $46,500 total inc. employer, pre-tax in, taxed on withdrawal. Access from 59½ (or 55 if separated from employer).
• Roth 401k: $23,500/year, post-tax in, tax-free out. Same access rules as Traditional 401k.
• HSA: $4,300/single / $8,550/family (2026), triple-tax-advantaged if used for medical expenses.

Plus the taxable brokerage account for anything beyond the contribution limits.

The optimisation problem: which account types and how much to put into each, given your current tax bracket, future expected tax bracket, and FIRE timeline.

The Traditional vs Roth math

For Traditional (pre-tax) contributions: you save the marginal tax rate today. £100 of gross income that would have been £76 net becomes a $100 pre-tax 401k contribution. Future withdrawals are taxed at your future marginal rate.

For Roth (post-tax) contributions: you pay full tax today. $100 of gross becomes $76 net into a Roth account. Future withdrawals are tax-free.

The decision rule:

• Future tax rate higher than current rate → Roth wins
• Future tax rate lower than current rate → Traditional wins
• Future tax rate equal to current rate → mathematical wash (slight edge to Roth due to tax-free growth on the saved tax)

For a US FIRE planner, the typical situation is:

• During accumulation: high earnings, often 24–32% marginal rate
• During early retirement: low realised income, often 0–12% marginal rate

That asymmetry favours Traditional during accumulation. The 24%+ deduction today, paid back at 12%+ in retirement, is a substantial arbitrage. Most US FIRE planners should be Traditional-heavy during peak earnings years.

The Roth conversion ladder

The catch: Traditional 401k money is locked until 59½, with 10% penalty for earlier withdrawals. FIRE planners retiring in their 40s have a problem.

The solution: the Roth conversion ladder. The strategy:

1. After retiring early (low realised income years), convert chunks of Traditional 401k or IRA money to Roth IRA.
2. Pay tax on each conversion at your current low marginal rate.
3. Wait 5 years from each conversion (the Roth 5-year rule for converted principal).
4. Withdraw the converted principal tax-free and penalty-free.

The result: you've converted Traditional dollars (taxed at 24%+ when earned) to Roth dollars (taxed at 12% or less on conversion) AND made them accessible by your mid-50s.

For a typical FIRE retiree converting $30,000/year:

• Tax cost: ~$2,500/year (at 12% marginal rate, assuming standard deduction covers the rest)
• Result: $30,000 of accessible Roth principal available 5 years later
• Lifetime savings: vs the 24% marginal rate you saved on the original contribution = ~$3,600/year of net tax arbitrage

Done for 10 years, that's $300,000+ of pre-59½-accessible money built up at minimal tax cost.

The 5-year wait means you need at least 5 years of taxable brokerage savings to bridge the gap from your FIRE date to when the first converted principal becomes accessible. For more on the broader drawdown sequencing, see our drawdown order article.

The HSA trick

The HSA is the most-tax-advantaged account in the US system if you handle it right:

• Contribution: tax-deductible (federal and most states)
• Growth: tax-free
• Withdrawals for medical: tax-free
• Withdrawals for non-medical after 65: taxed as ordinary income (same as Traditional IRA)

The trick: pay medical expenses out-of-pocket during accumulation years, save receipts indefinitely, and reimburse yourself decades later when you actually need the cash. The HSA grows tax-free in the meantime, and the eventual reimbursement is also tax-free.

A 35-year-old who maxes HSA contributions and pays medical out-of-pocket for 25 years can accumulate $200,000+ in their HSA, all of which becomes tax-free spending money in retirement (whether for accumulated medical receipts or post-65 general use).

You must be enrolled in an HSA-eligible high-deductible health plan to contribute. Worth checking whether your employer offers one.

The Mega Backdoor Roth

For high earners whose 401k plan supports it: contribute after-tax dollars to the 401k beyond the $23,500 employee limit, up to the $70,000 total annual limit. Then immediately convert those after-tax dollars to Roth. The result: an extra $46,500/year of Roth contributions, far above the regular $7,000 Roth IRA limit.

Not all 401k plans support this. Check your plan's allowance for in-plan Roth conversions or in-service distributions. When available, it's the single largest tax-advantaged accumulation tool in the US system.

The pragmatic priority order

For most US FIRE planners:

1. 401k to employer match. Free money.
2. HSA to max (if eligible).
3. Roth IRA to max ($7,000). Income limits permitting; use Backdoor Roth if you're above them.
4. Traditional 401k up to limit during high-earning years for the marginal tax arbitrage.
5. Mega Backdoor Roth if your plan supports it.
6. Taxable brokerage for bridge funds and anything else.

The cleanest test: at retirement, you should have roughly 5 years of expenses in taxable accounts (bridge), 5+ years in Roth contributions (principal accessible immediately), and the bulk of remaining wealth in Traditional 401k/IRA (to drain via conversion ladder).

Test this drawdown sequence on your own plan in our simulator — set the US country toggle and model each account type separately.