Short answer: Your FIRE number is yearly spending × 25 (the 4% rule): $50,000 a year means $1.25M. Your Coast FIRE number is that target discounted back to today at a real return: about $117,000 invested at 30 is enough to reach $1.25M by 65 at 7%, with no more contributions. Hypothetical, not guaranteed.
Two ideas explain almost every retirement number you’ll see online. Here they are, with the math you can check yourself.
The 4% rule
Research on US market history (the “Trinity study”) found that a retiree who withdrew 4% of their portfolio in year one, then adjusted that amount for inflation each year, almost never ran out of money over 30 years. Flip it around and you get your FIRE number: yearly spending × 25. Spend $50,000 a year → you need $1,250,000. Spend $40,000 → $1,000,000.
It is a planning rule, not a law. Longer retirements or cautious people use 3.5% (× 28.6). Social Security, pensions or part-time income reduce the amount the portfolio has to cover.
Coast FIRE
You don’t have to save the whole FIRE number yourself. Save part of it early, and compounding does the rest while you “coast”. The coast number at a given age is the FIRE number discounted back by your expected real return:
Coast number = FIRE number ÷ (1 + real return)years to retirement
At a 7% real return, $1.25M at 65 needs only about $117,000 invested at 30, or $230,000 at 40, or $450,000 at 50. Hit that, and you could stop contributing to retirement entirely and still arrive on time. Most people keep contributing anyway — Coast FIRE is a safety number, and a reason to front-load.
Why “real” return?
Using a return after inflation (about 7% for stocks historically, versus 10% nominal) keeps everything in today’s dollars, so the spending number you start with still means what you think it means.
Try it
The Coast FIRE calculator gives your FIRE number, your coast number at every age, and the month you’d cross it at your current savings rate. Pair it with the millionaire calculator if your target is a round number.
Educational content only. Not financial advice. Historical withdrawal research describes the past, not a guarantee.
Questions people ask
What is a Coast FIRE number?
The amount you need invested today so that growth alone reaches your retirement number by retirement age, with no more contributions. Example: a $1.25 million target 35 years away at 7% needs about $117,000 invested now.
Is the 4% rule still safe?
It comes from historical US data for 30-year retirements. Many planners use 3.5% for longer horizons; the Coast FIRE calculator lets you change the withdrawal rate.
What is the 4% rule?
A guideline from historical US market data (the Trinity study): withdraw 4% of your portfolio in the first year of retirement, adjust that amount for inflation each year, and the money has very rarely run out within 30 years. It implies a target of 25 times your yearly spending.
How do I calculate my FIRE number?
Multiply your expected yearly spending in retirement by 25 (or by about 28.6 for a more cautious 3.5% withdrawal rate). $40,000 a year gives $1,000,000.
Run the numbers yourself
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