# FIRE by Age: What Your Monthly Savings Need to Be
The FIRE number is the destination. Monthly savings is the engine that gets you there. The relationship between them is a compounding problem — and the math is more sensitive to time horizon than most people intuit.
Saving $3,000/month starting at 25 and targeting retirement at 45 produces a very different result than saving $6,000/month starting at 35 toward the same goal. The 10-year head start compounds for 10 extra years — often worth more than doubling the contribution amount.
The savings rate is the biggest lever
After tax returns are set by markets and beyond your control, the primary variable you control is your savings rate. Savings rate = monthly savings ÷ gross monthly income. Research on FIRE trajectory (including the influential work by Mr. Money Mustache and later academic analyses) shows that savings rate determines retirement age almost entirely, independent of income level:
- 10% savings rate → retire in ~40 years
- 25% savings rate → retire in ~32 years
- 50% savings rate → retire in ~17 years
- 65% savings rate → retire in ~11 years
- 75% savings rate → retire in ~7 years
The dramatic compression at higher savings rates comes from two simultaneous effects: more money invested compounding forward, and lower spending meaning a smaller FIRE number target.
What the calculator does
Input your target retirement age, current age, current savings, expected spending in retirement, and expected return. The model calculates the required monthly savings to hit your FIRE number — and shows what happens if returns are better or worse than expected.
Interactive Model
FIRE Monthly Savings Planner
See what monthly savings target gets you to FIRE at your chosen age — or when your current savings rate gets you there.
FIRE number
$1,500,000
Gap to cover
$60,000/yr
Current progress
6.7%
Required monthly savings to reach FIRE at each age
Required monthly savings to retire at 45 — by return assumption
Model uses constant nominal returns. Does not adjust for inflation, taxes, or sequence risk. Use as a planning tool, not a projection. Pair with Monte Carlo simulation for probability-based planning.
The sensitivity to return assumptions
The monthly savings number is highly sensitive to assumed return rates. At 5% returns vs. 8% returns over 20 years, the required monthly savings can differ by 40–60%. This is why range-based planning (what does this look like at 5%, 7%, and 9%?) is more honest than a single point estimate.
Income is the ceiling, savings rate is the floor
Monthly savings is bounded above by income — you can only save what you earn. But the savings rate floor is determined by spending. Cutting $500/month in spending contributes $500/month in savings AND reduces your FIRE number by $150,000 ($500 × 12 months × 25 multiplier) — a double benefit that makes spending reduction more powerful than income increases for FIRE planning.
When the math says "not possible"
If the model requires more monthly savings than your income allows, the options are: extend the timeline (later retirement age), reduce the target spending (lower FIRE number), increase income, or accept a Barista/Coast FIRE approach. The model is not a judgment — it is a navigation tool.
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*Related: [FIRE number calculator](./fire-number-calculator) for the full target and progress model. [FIRE flavors](./fire-flavors-decision-tree) — if the monthly savings is out of reach for full FIRE, Barista or Coast FIRE may be achievable sooner.*