You've probably seen the FIRE pitch: save aggressively, invest, retire at 40. The subreddits are packed with spreadsheets and projections. But most back-of-napkin FIRE math makes the same mistakes, and they compound badly over a 20-year horizon.
Let's break down the actual math, where the popular shortcuts fail, and how to run the numbers properly.
The Core FIRE Equation
FIRE boils down to one question: how many years until my investment portfolio covers my annual spending?
The classic rule of thumb is the 4% rule (from the 1998 Trinity Study). If your annual expenses are X, you need 25X saved. That's it. Simple division.
FIRE number = Annual expenses / 0.04
Spending 30,000/year? You need 750,000.
But this formula hides three assumptions that quietly wreck projections.
Mistake #1: Ignoring Inflation During Accumulation
The 4% rule accounts for inflation-adjusted withdrawals in retirement, but most people forget inflation during the savings phase.
If you earn 60K and spend 30K today, you're saving 30K/year. But in 15 years at 3% inflation, that same lifestyle costs 46,739. Your FIRE number just jumped from 750K to 1.17M.
Future expenses = Current expenses x (1 + inflation)^years
Adjusted FIRE number = Future expenses / withdrawal_rate
This alone adds 3-7 years to most projections. Try it with real numbers
Mistake #2: Using Average Returns Instead of Sequence of Returns
Markets don't return a steady 7% per year. They crash, they spike, they go sideways.
A portfolio averaging 7% over 30 years might look like:
| Scenario | Year 1-5 avg | Year 6-30 avg | Final value (500K start) |
|---|---|---|---|
| Steady | 7% | 7% | 3.81M |
| Early crash | -8% | 9.4% | 2.94M |
| Late crash | 9.4% | 5.2% | 3.12M |
Same average. Different outcomes. The early crash scenario is the killer during retirement because you're selling low to fund expenses.
This is called sequence of returns risk, and it's why a single number like "7% growth" is dangerously misleading.
Mistake #3: Treating the Withdrawal Rate as Fixed
The 4% rule assumes a 30-year retirement. If you're retiring at 35, you need that money to last 50-60 years. The safe withdrawal rate drops.
Research from Wade Pfau and others suggests:
| Retirement length | Safe withdrawal rate |
|---|---|
| 30 years | 4.0% |
| 40 years | 3.5% |
| 50 years | 3.25% |
| 60 years | 3.0% |
At a 3% withdrawal rate, your FIRE number for 30K/year spending isn't 750K. It's 1,000,000. That's a 33% increase from a single variable change.
Putting It All Together
A proper FIRE calculation needs:
- Current savings and monthly contribution
- Expected return (use real returns, net of inflation, around 4-5% historically)
- Current annual expenses inflated to your target retirement date
- Withdrawal rate adjusted for your expected retirement length
- Tax treatment of withdrawals (ISA vs pension vs general account matters hugely in the UK)
The formula for years to FIRE, accounting for compound growth on contributions:
Years = ln((FIRE_number * r + C) / (P * r + C)) / ln(1 + r)
Where:
P = current portfolio value
C = annual contribution
r = real annual return (after inflation)
FIRE_number = annual expenses / withdrawal rate
This is a log equation, which means the relationship between savings rate and years to retirement is non-linear. Going from a 30% savings rate to 50% doesn't cut your timeline by a third. It might cut it in half.
The Savings Rate Curve
Here's what the numbers look like at a 5% real return, starting from zero:
| Savings rate | Years to FIRE (4% rule) | Years to FIRE (3% rule) |
|---|---|---|
| 20% | 37 years | 42 years |
| 30% | 28 years | 33 years |
| 40% | 22 years | 26 years |
| 50% | 17 years | 21 years |
| 60% | 12 years | 16 years |
| 70% | 9 years | 12 years |
The jump from 50% to 70% savings rate saves you 8-9 years. The math heavily rewards higher savings rates.
Run Your Own Numbers
I built a FIRE calculator that handles all of this: inflation-adjusted expenses, variable withdrawal rates, and compound growth on ongoing contributions. No signup, runs in your browser.
If you want to explore the components separately:
- Compound interest calculator to model portfolio growth scenarios
- Inflation calculator to see what your expenses will actually be in 20 years
- Savings goal calculator to reverse-engineer monthly contributions
The Honest Take
FIRE math isn't hard. It's logarithms and compound growth. The hard part is the inputs: predicting returns, inflation, tax policy, and your own spending over decades.
Anyone who gives you a single number without showing sensitivity analysis is selling you a fantasy. Run the numbers yourself, stress-test the assumptions, and plan for the version where things don't go perfectly.
The spreadsheet doesn't care about your feelings. That's what makes it useful.
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