You've probably seen the simple formula: take your annual expenses, multiply by 25, and congratulations — that's your FIRE number. It's clean, satisfying, and unfortunately incomplete.
The 25x rule is a useful starting point, but it leaves out some of the biggest variables that will actually determine whether you run out of money at 58 or live comfortably until 95. Taxes. Healthcare. Inflation that compounds over 40 years. Sequence-of-returns risk in those critical first years. Social Security income that might offset withdrawals in your 60s.
If you're serious about FIRE, you need a number that accounts for all of it. This guide walks you through how to calculate your FIRE number properly — and what most people miss along the way.
The Classic Formula (And Why It's a Floor, Not a Ceiling)
The 4% rule comes from the Trinity Study, which found that a portfolio of 50–75% stocks could sustain a 4% annual withdrawal for 30 years with a high probability of success. Multiply your annual spending by 25 (which is 1 ÷ 0.04), and you have your target.
Example:
- Annual expenses: $60,000
- FIRE number: $60,000 × 25 = $1,500,000
Simple. But here's the thing: the Trinity Study was designed for 30-year retirements. If you retire at 40, you might need your money to last 55 years. The math changes meaningfully at those longer time horizons.
For 40+ year retirements, many FIRE practitioners use a 3.5% or even 3.3% withdrawal rate, which pushes the multiplier to 28–30x instead of 25x.
More conservative FIRE number (same example):
- Annual expenses: $60,000
- FIRE number at 3.5% withdrawal: $60,000 × 28.6 = $1,714,000
That's an extra $214,000 you'd need to save — which is why getting this right matters.
What the Simple Formula Ignores
Taxes Are One of Your Biggest Retirement Expenses
This is where most FIRE calculators fall short. When you withdraw from a traditional 401(k) or IRA in retirement, that money is taxed as ordinary income. If you're pulling $80,000/year out of pre-tax accounts, you're not actually spending $80,000 — you're spending $80,000 minus whatever you owe to federal and state governments.
The good news: early retirees who manage their income carefully can often pay very low effective tax rates, especially if they're doing Roth conversions in low-income years. The bad news: that requires actual tax modeling, not just "I'll figure it out later."
At minimum, you need to know:
- What type of accounts hold your savings (Roth vs. traditional vs. taxable)
- What state you plan to retire in — nine states have no income tax, and some explicitly exempt retirement income
- Whether you'll have capital gains from taxable brokerage sales
- When you plan to take Social Security, since up to 85% of benefits can be taxable
Social Security Changes the Picture (Even for Early Retirees)
If you retire at 45 but start taking Social Security at 67, you have a 22-year gap to fund entirely from savings. Once benefits kick in, your required portfolio withdrawal drops — sometimes dramatically.
A couple who each receive $2,000/month from Social Security at 67 has $48,000/year in inflation-adjusted income arriving without touching their portfolio. That offsets a huge chunk of their annual spending, and their actual portfolio drawdown becomes much smaller from that point forward.
A proper FIRE calculation should model the transition from "100% self-funded" to "partially Social Security funded" — not just assume you're pulling the same amount from your portfolio every year for 50 years.
Required Minimum Distributions (RMDs) Can Bump Your Tax Bracket
If you have significant pre-tax retirement accounts, the IRS eventually forces you to take distributions starting at age 73 — whether you need the money or not. Those distributions are taxable income, and they can push you into a higher bracket right when you thought you'd have your taxes figured out.
This is one of the strongest arguments for Roth conversions during low-income years in early retirement: reduce your future RMD burden while you can still do it cheaply.
Step-by-Step: Calculating Your True FIRE Number
Step 1: Get Your Annual Expenses Right
Don't use your current spending. Build a picture of what you'll actually spend in retirement:
- Remove work-related costs: commuting, professional wardrobe, lunches out, daycare
- Add healthcare: if you're retiring before Medicare (age 65), budget $600–$1,200/month per person for ACA marketplace coverage
- Add what you'll actually do: travel, hobbies, eating out more, helping adult kids
- Inflation-adjust for big purchases (car replacement, home repairs) rather than treating every year as identical
Most people find their FIRE spending target is 85–105% of their current spending — not the dramatic reduction some expect.
Step 2: Model Your Income Sources
List every income stream you'll have and when it turns on:
- Rental income (if applicable)
- Part-time work or consulting (many FIREd folks work some in early years)
- Social Security — use the SSA's own estimator at ssa.gov, then pick your claim age
- Any pension or annuity income
Step 3: Identify Your Account Mix
The tax efficiency of your withdrawal strategy depends entirely on which buckets you're pulling from:
| Account Type | Tax Treatment on Withdrawal |
|---|---|
| Traditional 401(k)/IRA | Ordinary income |
| Roth 401(k)/IRA | Tax-free |
| Taxable brokerage | Capital gains (usually lower rate) |
| HSA (for qualified medical) | Tax-free |
Knowing your mix lets you optimize which accounts to draw from in which years — a strategy called tax bracket management that can save tens of thousands of dollars over a long retirement.
Step 4: Pick Your Withdrawal Rate Thoughtfully
Rather than a fixed percentage, consider a dynamic withdrawal strategy:
- Guardrails method: adjust spending up or down based on portfolio performance
- Flexible spending: identify a "core" spending floor and a "preferred" spending level — cut back in bad market years, spend more in good ones
- Floor-and-upside: cover fixed expenses with guaranteed income (Social Security, maybe an annuity), fund discretionary from portfolio
Dynamic strategies can allow a higher initial withdrawal rate because you're building in adaptability rather than planning for worst-case every year.
Step 5: Run the Numbers Over Time
The only way to really know if your FIRE plan works is to model year-by-year cash flows: income in, expenses out, taxes owed, portfolio withdrawals, growth, inflation. A spreadsheet can do this if you're willing to build it, but it takes hours and breaks easily when variables change.
This is exactly the kind of planning that used to require a financial planner — or a very patient weekend with Excel.
The State Tax Variable Almost Nobody Optimizes
Where you retire has a huge impact on how far your portfolio stretches. This is one of the most underappreciated levers in FIRE planning.
Some examples for a retiree with $70,000/year in income:
- Nevada or Florida: $0 state income tax
- California: potentially $3,000–$5,000+ in state taxes on the same income
- New York: similar to California depending on county
Over a 40-year retirement, the difference between retiring in a high-tax vs. no-tax state can exceed $100,000 in real dollars — enough to justify serious geographic flexibility if you have it.
Putting It All Together
A real FIRE number isn't a single figure — it's a model. The inputs include your spending, your account mix, your tax situation, your planned Social Security claim age, your state of residence, and your risk tolerance around sequence-of-returns.
When you run all of those variables properly, you might find:
- Your 25x number was plenty (because Roth conversions keep your taxes near zero)
- Your 25x number was not enough (because healthcare costs and RMDs push you into a higher bracket at 75)
- You could retire two years earlier than you thought (because Social Security offsets more than you realized)
The point isn't to be pessimistic — it's to be precise.
How Tally Approaches This
Tally's lifetime financial forecast runs exactly this kind of model. You connect your accounts, set your spending assumptions, and it projects 30+ years of cash flows including federal and state income taxes (all 50 states), Social Security timing, RMDs, capital gains, and inflation. The "what-if" scenario tool lets you test retiring at 50 vs. 55 vs. 60 and see how each plays out — not just for your portfolio balance, but for your actual after-tax income each year.
If you've been running your FIRE planning on a spreadsheet, it's worth seeing what a properly modeled forecast looks like. The free trial is 14 days, no credit card required — you can connect your accounts and have a full 30-year forecast in about 15 minutes.
Start your free trial at thetally.io
The information in this post is for educational purposes only and does not constitute financial advice. Everyone's tax situation is different — consider working with a CPA for personalized guidance.