This means that if you must have £25,000-a-year ($50,000) to live on you need to work towards a nest egg of £625,000 ($1.25m) made up of savings and returns from investments or buy-to-let properties.
If it is £50,000 ($100,000) then that amount grows to £1.25million ($2.5m).
The other battle line is to eliminate a mortgage using the half of the salary people don't save.
The Holy Grail for anyone who wants to leave work before 65 is saving - because if you want a scatter-cash lifestyle then you will pay for it in many more years at work.
Many take their inspiration from the 5:2 diet, meaning for full five days of the week they do not spend a penny only allowing themselves to have any outlays on the other two days.
London accountant Barney Whiter, a married father-of-three from Farnham, Surrey, walked away from work at 43 by saving half his annual salary after tax.
He then invested all of it in low-risk stock market funds and shares, bringing in up to 12 per cent return every year for 19 years while also paying down his mortgage.
A big house, eating out, expensive holidays, new cars, cable TV and non-essential shopping were all banned so the Whiter family could stick to their £24,000-a-year ($48,000) budget for all spending.
Frugal Mr Whiter made sure he built up a net worth of 25 times his annual spend - £625,000 ($1.25m) - in savings and investments.
And the result was retirement around 20 years before his colleagues, which he said 'is the best thing since slice bread'.
He told The Times: "If you can save 50 per cent of your take-home pay, it will take 19 years to go from broke to never needing to work again. If you can save 75 per cent, it will take seven to eight years.
"You need to have the mentality of a marathon runner or triathlete and be able to delay gratification. For most people money is leaking out of their life like a bucket shot full of holes".
There is an army of super-savers in the UK, many gaining inspiration from U.S. and British websites that advise on how to become mortgage-free early.
More than 100,000 people are said to be using blogs produced by FIRE proponents including Mr Whiter, who calls himself "The Escape Artist".
Earlier this year more than 900 people tried to get into a London pub to hear a talk about the formula, spearheaded by Canadian Peter Adeney, who retired at 30.
Mr Adeney's blog Mr Money Moustache gives people a step-by-step guide to retiring in a decade or less.
Mr Whiter threw all his energy behind his plan to retire in his forties.
He drove a battered second hand Skoda for years and cut spending to the bone while his children, now teenagers, grew up.
But he insists it was all worth it.
He said: "Tasting freedom is the most intoxicating thing and I wouldn't ever go back to full-time work. I'd rather cut my lifestyle back. My highest value is freedom and self-determination and being able to do what I want to do".
