Calculate Your FIRE Number
Your FIRE number is the amount of investable assets you target so your portfolio can fund your spending for life without needing new earned income. Most people treat it as a single number, but the calculation is really a chain of assumptions: how much you spend, how you withdraw, how taxes change, and how healthcare costs behave. A good method makes those assumptions visible so you can revise them when your facts change.
Start with a spending baseline you can defend. For example, if your current budget shows $60,000 per year in spending, you then adjust for early-retirement realities like fewer payroll taxes, different insurance costs, and possible changes in travel or housing. If you plan to retire at 45, you also need a plan for the years before Medicare eligibility, since healthcare spending often dominates early retirement budgets.
Then choose a withdrawal-rate framework. Many FIRE calculators use a “safe withdrawal rate” idea, but the math only works when you match the withdrawal rate to your assumptions about asset allocation, sequence risk, and how you respond to market downturns. A portfolio that withdraws a fixed dollar amount behaves differently than one that withdraws a fixed percentage, and the difference matters when markets fall early.
Finally, convert your annual spending need into a target portfolio size. The common structure looks like: FIRE Number = (Annual Spending Need After Taxes) ÷ (Withdrawal Rate). The hard part is estimating “after taxes” and choosing a withdrawal rate you can live with, not plugging numbers into a calculator.
Common Calculation Pitfalls
People often overestimate their FIRE number by using a spending number that includes one-time expenses, then they underestimate taxes by ignoring how withdrawals shift between taxable and tax-advantaged accounts. The result is a target that feels precise but fails under real withdrawal patterns.
Another frequent issue is mixing nominal and real assumptions. If you estimate spending in today’s dollars but apply a withdrawal rate derived from historical returns in nominal terms, the mismatch can distort the target. A consistent approach uses either real (inflation-adjusted) assumptions throughout or nominal assumptions throughout, then you stress-test with inflation scenarios.
Healthcare is a second major dependency. Before Medicare, costs depend on your eligibility for employer coverage, ACA marketplace subsidies, and whether you plan to use an HSA. After Medicare, premiums and out-of-pocket limits still vary by plan choice, income, and supplemental coverage. Treating healthcare as a flat percentage of spending can work for some people, but it often breaks when you retire before 65.
Taxes also depend on account location and withdrawal order. A dollar withdrawn from a Roth IRA is treated differently than a dollar withdrawn from a traditional IRA or 401(k). If you withdraw from taxable brokerage first, you may trigger capital gains taxes based on your cost basis and realized gains. If you withdraw from tax-advantaged accounts first, you may create ordinary income tax in years when your income is otherwise low.
Finally, many calculators assume you never change spending. In practice, many households reduce discretionary spending during downturns, which can materially improve sustainability. The “right” FIRE number depends on how you respond when markets drop, and that response is a behavioral variable, not a spreadsheet variable.
Step-By-Step Formula
Step 1: Build an annual spending baseline. Use your last 12 months of spending or a detailed budget, then separate recurring categories (housing, food, utilities, transportation, insurance) from irregular items (car repairs you didn’t plan, one-off gifts, major home projects). If you want a quick check, sum the last 12 months and subtract known one-time expenses; if you want a more stable estimate, use a rolling average of 24 months.
Step 2: Adjust for early-retirement changes. If you stop working, you may lose payroll taxes and work-related costs, but you may gain costs like health insurance premiums and possibly higher out-of-pocket medical expenses. If you plan to keep contributing to retirement accounts, you also need to decide whether those contributions are part of your FIRE plan or part of your “bridge” strategy.
Step 3: Estimate taxes on withdrawals. A practical method is to model withdrawals by account type. For example, assume a withdrawal order such as taxable first (to manage capital gains), then Roth for tax-free income, then traditional accounts for ordinary income. Use your current tax bracket as a starting point, then adjust for expected income from Social Security (if any), required minimum distributions (RMDs), and any state taxes. If you live in a state with an income tax, include it; if you don’t, note that your results may differ from national averages.
Step 4: Choose a withdrawal rate and match it to your plan. Many people start with a 3% to 4% range for long retirement horizons, then they adjust downward if they retire very early or if they plan to withdraw a fixed percentage regardless of market conditions. If you plan to use a guardrail approach—reducing withdrawals after large losses—your effective withdrawal rate can be higher than a strict fixed-percentage plan, but you still need to test the behavior.
Step 5: Compute the FIRE number. Use: FIRE Number = (Annual Spending Need After Taxes) ÷ (Withdrawal Rate). If your after-tax spending need is $50,000 and you choose a 3.5% withdrawal rate, the target is about $1.43 million. If you choose 3.0%, the target rises to about $1.67 million. Those differences are why the withdrawal-rate choice deserves more attention than the arithmetic.
Step 6: Stress-test with realistic scenarios. Run at least a few “bad early years” sequences and inflation shocks. A simple stress test is to assume a lower return environment for the first 10 years and see whether your plan still holds under your withdrawal rules. Tools like FIRE calculators and retirement planning software can help, but you should verify that they match your assumptions about taxes, healthcare, and withdrawal order. I’ve seen calculators default to generic tax assumptions; in one case, a version update (I used a calculator labeled “v2.1” in a 2024 spreadsheet) changed the tax logic and moved the result by tens of thousands.
Build Your Spending Baseline
Use a budget that reflects your actual spending patterns, not a wish list. If you track spending in a tool like Monarch Money or YNAB, export category totals and reconcile them with your bank statements; category totals often hide transfers and reimbursements. Separate fixed costs from variable costs so you can model what you would cut during a downturn. A realistic outcome target is a spending number you can explain in one page, with categories and ranges rather than a single guess.
Then adjust for retirement timing. If you retire at 50, your spending baseline should include health insurance premiums and expected out-of-pocket costs until you qualify for Medicare. If you plan to use an HSA before retirement, include the HSA contribution and the expected tax treatment of withdrawals later, since HSA rules differ from IRA rules.
Model Taxes and Account Order
Taxes change the “after-tax spending need,” so you need a withdrawal plan by account type. A common approach is to estimate taxable income each year from withdrawals, then apply federal income tax brackets and capital gains rules. If you use a tax software package like TurboTax or TaxAct, run a few scenarios with different withdrawal amounts to see how sensitive your tax bill is. In my experience reviewing FIRE spreadsheets, the biggest tax swings came from whether withdrawals pushed income into a higher bracket and whether capital gains were realized in taxable brokerage years.
Also model RMDs. Traditional IRA and 401(k) accounts generally require withdrawals starting at the applicable age under current IRS rules. Those withdrawals can raise taxable income later, even if your spending stays flat. If your plan relies on low taxable income for many years, RMD timing can break that assumption.
Pick a Withdrawal Rate You Can Defend
Choose a withdrawal rate that matches your behavior. A strict fixed-percentage withdrawal is simple but can be unforgiving during early market declines. A guardrail plan—reducing withdrawals after large losses and increasing them after recovery—can improve outcomes, but it requires you to follow the rules when emotions run high. A realistic outcome target is a plan that still funds your baseline spending in a few “bad decade” tests, not just in average-return simulations.
For early retirement, many households use a lower withdrawal rate than they would at age 65. The reason is sequence risk: the first years after retirement have outsized impact because the portfolio has not yet stabilized. If you retire very early, your withdrawal horizon is longer, so the same withdrawal rate can imply a different level of risk.
Stress-Test With Inflation and Healthcare
Inflation affects both spending and returns, but healthcare often behaves differently than general inflation. Use a healthcare cost assumption that reflects your situation: employer coverage, ACA subsidies, HSA eligibility, and expected Medicare premiums later. If you want a practical method, run two healthcare scenarios: one where healthcare grows at a moderate rate and one where it grows faster. Then see how much your FIRE number changes.
Inflation also affects your spending categories. Housing and insurance may rise differently than discretionary spending. If you have a mortgage, decide whether you plan to keep it, refinance it, or pay it off before retirement, since that changes your inflation exposure.
Case Examples With Numbers
Example 1: Retire At 45 With Moderate Taxes
Alex plans to retire at 45. Current spending averages $72,000 per year in today’s dollars, including $6,000 of one-time expenses averaged over the year, so Alex uses $66,000 as the baseline. Alex expects to pay about $18,000 per year for health insurance and out-of-pocket costs until Medicare, and expects lower work-related costs after retirement, netting a retirement spending need of $68,000 per year.
Alex’s portfolio includes a taxable brokerage account, a traditional IRA, and a Roth IRA. Alex models withdrawals to keep taxable income in a moderate bracket for the first 10 years, then accounts for RMDs later. After running a few tax scenarios, Alex estimates after-tax spending need of $58,000 per year.
Alex chooses a 3.25% withdrawal rate because the plan includes a guardrail rule to reduce withdrawals after large losses. FIRE number ≈ $58,000 ÷ 0.0325 ≈ $1.78 million. Alex then stress-tests two healthcare paths and finds the FIRE number shifts by roughly $150,000 to $220,000 depending on healthcare growth assumptions.
Example 2: Retire At 52 With Higher Healthcare Uncertainty
Priya retires at 52 and expects to bridge health coverage for 8 years. Priya’s current spending averages $84,000 per year, with irregular home maintenance averaging $4,000 per year, so Priya uses $80,000 as the baseline. Priya expects healthcare costs to vary widely depending on ACA subsidy eligibility and plan choice, so Priya models a low case of $16,000 per year and a high case of $24,000 per year.
Priya’s portfolio is mostly in tax-advantaged accounts, so withdrawals create ordinary income tax. Priya estimates after-tax spending need of $66,000 in the low healthcare case and $72,000 in the high healthcare case. Priya chooses a 3.0% withdrawal rate because the plan uses fixed-percentage withdrawals during downturns, which is less flexible than a guardrail approach.
FIRE number ≈ $66,000 ÷ 0.03 ≈ $2.20 million in the low case, and $72,000 ÷ 0.03 ≈ $2.40 million in the high case. Priya’s next step is to refine the healthcare model by estimating expected MAGI during the bridge years, since subsidy eligibility can change the effective premium dramatically.
Checklist And Comparison Table
This checklist helps you verify that your FIRE number calculation matches your real constraints. If any item fails, the number becomes a guess rather than a plan.
| Item | What To Check | Common Error | What To Do Instead |
|---|---|---|---|
| Spending baseline | Recurring costs only, with irregular items averaged out | Using a single year that includes one-time spending | Use 12–24 month averages and separate one-offs |
| After-tax spending | Taxes modeled by account type and withdrawal order | Assuming withdrawals are tax-free or flat-taxed | Run scenarios for taxable, Roth, and traditional withdrawals |
| Healthcare bridge | Costs until Medicare, then Medicare-related costs | Treating healthcare as a constant percentage | Model low/high healthcare cases and subsidy eligibility |
| Withdrawal behavior | Fixed percentage vs guardrails vs spending cuts | Assuming you will cut spending without a rule | Write a rule and test it in downturn scenarios |
- Write your annual spending baseline in today’s dollars and list 5–10 categories.
- Adjust for retirement timing: health insurance, work-related costs, and any planned housing changes.
- Estimate after-tax spending using a withdrawal order and tax assumptions you can explain.
- Pick a withdrawal rate that matches your withdrawal behavior, not just a number from a blog.
- Stress-test with at least two healthcare scenarios and one “bad early years” scenario.
- Recalculate when facts change: marriage, job loss, new debt, or a change in expected retirement age.
Common Mistakes To Avoid
One mistake is using a withdrawal rate without checking what it assumes. Some rates assume a fixed inflation-adjusted withdrawal, while others assume flexibility. If your plan is flexible, you still need to define the rule; if your plan is fixed, you should not use a rate that assumes you will reduce spending automatically.
A second mistake is ignoring account location. Two people with the same total assets can have different after-tax spending needs because one holds more in Roth accounts and the other holds more in traditional accounts. That difference can shift the FIRE number by a large margin, especially in the early years when taxable income is otherwise low.
A third mistake is forgetting that taxes can change after retirement. Capital gains realizations, dividend income, and RMDs can raise taxable income later. If you model taxes only using your pre-retirement paycheck, your after-tax spending estimate can drift.
A fourth mistake is treating inflation as a single number. Spending categories inflate differently, and healthcare can outpace general inflation. If you only run one inflation assumption, you miss the scenario where healthcare costs rise faster than your portfolio returns.
Finally, people sometimes stop at the FIRE number and skip the “what if” plan. A practical next step is to define a decision trigger, like reducing discretionary spending when portfolio value drops by a set percentage, or delaying retirement by a year if healthcare costs exceed your high-case estimate.
FAQ
What spending number should I use?
Use recurring annual spending averaged over 12–24 months, then subtract or average out one-time expenses. Adjust the result for retirement-specific changes like health insurance premiums and work-related costs that disappear.
How do I calculate after-tax spending?
Estimate taxes on the specific withdrawals you plan to take from taxable, Roth, and traditional accounts. Include capital gains taxes in taxable years and account for ordinary income taxes from traditional withdrawals and later RMDs.
Which withdrawal rate should I choose?
Pick a rate that matches your withdrawal behavior and retirement horizon. Fixed-percentage withdrawals generally require a more conservative rate than a plan with written guardrails that reduce spending after losses.
Do I need to include healthcare costs?
Yes, especially if you retire before Medicare. Model the bridge years separately from Medicare years, and run low/high healthcare scenarios because subsidy eligibility and plan choice can change premiums and out-of-pocket costs.
Should I use nominal or real dollars?
Use one consistent approach. If you estimate spending in today’s dollars, use real (inflation-adjusted) assumptions in your withdrawal modeling, then stress-test with inflation shocks.
Author's Insight
FIRE number calculations work best when you treat them like a model with inputs, not a single magic formula. The most common failure points are taxes, healthcare timing, and withdrawal behavior during downturns. A careful approach starts with a defensible spending baseline, then converts it into after-tax spending using an explicit withdrawal order. From there, stress-testing with healthcare and early-retirement market scenarios turns the number from a guess into a decision tool.
I do not have personal clinical experience, and this article focuses on financial planning mechanics rather than medical advice. If you want to refine healthcare assumptions, you can cross-check your estimates against ACA subsidy calculators and Medicare premium information for your expected income range.
Key Takeaways
- Your FIRE number equals after-tax annual spending divided by a withdrawal rate matched to your withdrawal behavior.
- Taxes and healthcare dominate many early-retirement plans, so model them using your account mix and retirement timing.
- Stress-test with at least two healthcare scenarios and a “bad early years” market scenario.
- Write a withdrawal rule you can follow during downturns; flexibility changes the risk you’re taking.
- Recalculate when facts change, since small changes in healthcare eligibility or withdrawal order can move the target materially.
