A FIRE budget is not a standard household budget with extra steps. The question it answers is different: what percentage of my portfolio does my life cost each year, and can I build a portfolio big enough to pay that forever? Everything else — the categories, the tracking, the automations — is in service of that one number.
To make it concrete: a single person in a mid-cost metro earning $90,000 take-home, spending $5,400/month ($64,800/year), needs a portfolio of roughly $1.62 million at a 4% safe withdrawal rate. That's the "full FIRE" number. The Coast FIRE number is lower — it's whatever you need today so that compounding gets you there by your target retirement age. The checklist below is how you get the $5,400/month figure honest.
Not financial advice. The benchmarks are planning heuristics. Your numbers depend on your city, your health, and your tolerance for variance. See our editorial policy.
Sort your spending into two buckets. Fixed costs are the ones that would still be due if you disappeared for a month: rent or mortgage, property tax, insurance, loan payments, subscriptions you haven't actually cancelled. Variable costs are the ones that flex with your choices: dining, entertainment, travel, discretionary shopping.
The ratio matters more than the total. A $6,000/month budget that's 80% fixed is a very different plan than a $6,000/month budget that's 40% fixed — the first one can't be compressed in a downturn, the second can. If your fixed share is above 70%, your flexibility is your biggest underappreciated asset, and cutting one fixed cost is worth three variable ones.
The 50/30/20 rule (50% needs, 30% wants, 20% savings) is a reasonable floor for a household that isn't optimizing for early retirement. A FIRE planner frames the same question differently: what is my spending as a percentage of my portfolio, and can I lower that percentage faster than I can grow the portfolio?
The practical consequence: a FIRE budget treats the "wants" line as the primary lever, not a fixed 30%. If your wants line is 40% of income and your savings rate is 15%, your target number is meaningfully higher than someone with the same income whose wants line is 20% and savings rate is 35%. The 50/30/20 rule tells you your budget is healthy. The FIRE framing tells you how far it is from your finish line.
Healthcare inflation. If you're on a plan that ends with your job, model healthcare at 5-6% annual inflation, not 3%. On a $60,000/year budget, that's the difference between $60,000 and roughly $90,000 in 15 years.
Job loss. Run your budget at 70% income for 12 months. If the fixed line still fits, you're resilient. If it doesn't, your emergency fund target should be higher than 6 months.
Housing shock. A 20% rent increase or a 30% property-tax hike is within historical range. If housing is above 35% of your budget, a single housing event eats a meaningful share of your "wants" line.
Tax changes. A bracket shift or a loss of a deduction can move your take-home by 5-10%. If your savings rate is under 25%, that's the difference between meeting and missing your target number.
Family changes. A child, an aging parent, a long-term illness — any of these can add $10,000-$30,000/year to a budget that was otherwise stable. The budget that survives is the one with a variable line big enough to absorb it.
• Fixed costs above 70% of income — you have little flexibility in a downturn.
• Savings rate under 15% — the target number is likely higher than you think, and the timeline is longer than you'd like.
• Housing above 40% of take-home — one of the most compressible line items is also the least, which is a contradiction.
• "Other" or "Misc" above 15% of spending — you don't actually know where your money is going.
• A budget you review monthly — you're either not trusting the system or you're not committing to it; both are worth fixing.
✓ Savings rate of 20%+ by age 30
✓ Coast FIRE number reached before age 45
✓ Portfolio value at least 10x annual expenses (a 4% SWR buffer)
✓ Fixed costs under 60% of total spending
CoastFIREWhen — Educational content. Not financial advice. See our editorial policy.
CoastFIREWhen — Educational content. Not financial advice.