What is a "FI number" and where does 25× come from?

Your FI number is the portfolio size that can sustainably fund your annual spending. At a 4% withdrawal rate, you need spending ÷ 0.04 — exactly 25 times annual spending. Spend $50,000 a year and FI is $1.25 million; cut spending to $40,000 and it drops to $1 million. The 25× multiple is just the 4% rule rearranged.

Why does the savings rate matter more than income?

Because it attacks from both sides: saving more grows the portfolio faster and simultaneously proves you need less to live on, which shrinks the target. A household earning $200,000 and spending $190,000 is decades from FI; one earning $90,000 and spending $58,500 (a 35% savings rate) is on a roughly 25-year track from a standing start. The rate, not the salary, sets the timeline.

Is the 4% rule actually safe?

It comes from the Trinity study, which tested historical US market sequences and found 4% initial withdrawals (inflation-adjusted annually) survived 30-year retirements in the overwhelming majority of cases. For retirements longer than 30 years — the whole point of FIRE — many planners model 3.25–3.75% instead. The calculator lets you set any rate; lowering it raises your FI number and adds years, honestly.

Why does the calculator ask for a "real" return?

Using after-inflation returns keeps every number in today's dollars — your FI number, the chart, all of it. Historical US stock returns of roughly 10% nominal correspond to about 7% real; a diversified portfolio is often modeled at 5% real, the calculator's default. If you enter a nominal return instead, your timeline will look several years more optimistic than reality.

Does FI mean I have to retire?

No — FI is the point where work becomes optional. Many people at FI keep working by choice, switch to lower-paying work they enjoy, or go part-time ("Coast FI" and "Barista FI" describe intermediate versions). The number buys freedom of decision, not a mandatory exit.

FIRE Calculator

Calculate how many years until financial independence based on your savings rate — the single number that drives early retirement — using the 4% rule and real, after-inflation returns.

Written by Daniel Mercer, CFP® · Reviewed by Sarah Lindqvist, CFA

Last reviewed: July 15, 2026

Years to financial independence

23.04

Saving 35% of income

FI number (annual spending ÷ withdrawal rate): $1,462,500
Annual spending: $58,500
Annual savings: $31,500

Net worth until FI

Net worthFI number

What this calculator does

FIRE — financial independence, retire early — reduces to one question: when does your portfolio become big enough to pay for your life indefinitely? This calculator answers it from four inputs: take-home income, savings rate, current invested net worth, and return assumptions. It computes your FI number, your years to reach it, and charts your projected net worth climbing toward the target line.

The design choice that matters: everything runs on your savings rate, because that single percentage captures both how fast you accumulate and how much you need. It’s the steering wheel of the entire FIRE framework.

How the math works

Three steps:

Annual spending = income × (1 − savings rate). Your spending defines the goal.
FI number = spending ÷ withdrawal rate. At 4%, that’s 25× spending.
Years to FI solves the growth equation — current net worth compounding, plus annual savings accumulating:

NW₀(1+r)ⁿ + S·((1+r)ⁿ − 1)/r = FI number

solved for n with logarithms. Because both sides of the equation are in today’s dollars, r must be a real (after-inflation) return.

A worked example

Income $100,000 after tax, saving 50%, starting from zero, at 5% real returns and a 4% withdrawal rate:

Spending: $50,000 → FI number: 50,000 ÷ 0.04 = $1,250,000
Annual savings: $50,000
Years to FI: about 16.6

That “16–17 years at 50%” result is the famous one from the FIRE movement’s shockingly-simple-math argument, and it reproduces here from first principles. Move the savings-rate slider and watch the leverage: at 35% the same household needs about 25 years; at 65%, about 10.5. Notice income never changed — only the split between spending and saving.

Practical tips

Attack spending before chasing returns. Cutting $500/month of permanent spending removes $150,000 from your FI number (at 25×) and adds $6,000 a year of savings. No realistic portfolio tweak competes with that double effect.
Count only invested assets in net worth. Home equity you live in and cars don’t produce withdrawals. Use your brokerage, retirement accounts, and other investable assets for the starting number here.
Stress-test with a lower withdrawal rate. Run your plan at 4%, then at 3.5%. If the extra years feel acceptable, you have margin; if they feel devastating, your plan was leaning on optimism. Knowing which is worth thirty seconds.
Expect the last third to feel different. Early on, contributions drive growth; past roughly two-thirds of the way, market movement dominates and yearly progress becomes noisy. The chart’s smooth curve is the average of many jagged possible paths — anchor to the savings rate you control, not the year-to-year balance you don’t.

Where this fits with traditional retirement planning

This tool deliberately ignores age, Social Security, and employer matches to isolate the savings-rate math. For the fuller employer-sponsored picture — matching, raises, inflation-adjusted balances at a specific retirement age — pair it with the retirement calculator. The two answer complementary questions: how many years of work does my lifestyle cost? and what will my accounts hold at 67? Serious plans benefit from both answers agreeing with each other.

Frequently asked questions

What is a "FI number" and where does 25× come from?: Your FI number is the portfolio size that can sustainably fund your annual spending. At a 4% withdrawal rate, you need spending ÷ 0.04 — exactly 25 times annual spending. Spend $50,000 a year and FI is $1.25 million; cut spending to $40,000 and it drops to $1 million. The 25× multiple is just the 4% rule rearranged.
Why does the savings rate matter more than income?: Because it attacks from both sides: saving more grows the portfolio faster and simultaneously proves you need less to live on, which shrinks the target. A household earning $200,000 and spending $190,000 is decades from FI; one earning $90,000 and spending $58,500 (a 35% savings rate) is on a roughly 25-year track from a standing start. The rate, not the salary, sets the timeline.
Is the 4% rule actually safe?: It comes from the Trinity study, which tested historical US market sequences and found 4% initial withdrawals (inflation-adjusted annually) survived 30-year retirements in the overwhelming majority of cases. For retirements longer than 30 years — the whole point of FIRE — many planners model 3.25–3.75% instead. The calculator lets you set any rate; lowering it raises your FI number and adds years, honestly.
Why does the calculator ask for a "real" return?: Using after-inflation returns keeps every number in today's dollars — your FI number, the chart, all of it. Historical US stock returns of roughly 10% nominal correspond to about 7% real; a diversified portfolio is often modeled at 5% real, the calculator's default. If you enter a nominal return instead, your timeline will look several years more optimistic than reality.
Does FI mean I have to retire?: No — FI is the point where work becomes optional. Many people at FI keep working by choice, switch to lower-paying work they enjoy, or go part-time ("Coast FI" and "Barista FI" describe intermediate versions). The number buys freedom of decision, not a mandatory exit.

Sources

Written by

Daniel Mercer, CFP®

Daniel is a Certified Financial Planner™ with 12 years of experience helping households manage debt, savings, and retirement planning. He writes ToolGrym’s calculator guides and explains the math behind every tool.

Reviewed by

Sarah Lindqvist, CFA

Sarah is a CFA charterholder who reviews every ToolGrym calculator and article for mathematical accuracy. She has 10 years of experience in fixed-income analytics and consumer lending models.