But this rule has its flaws, especially when applied to young people. It’s generally used for those retiring in their 60s, who aren’t likely to need money for more than 30 years.

The maths doesn’t add up, says Holly Mackay, founder of consumer financial website Boring Money. “If you retire in your 30s, you could be living for 70 more years. I think there’s a bit of naivety.”

Putting a precise dollar amount on the amount someone needs to retire at 35 is difficult, she says. If someone wants £20,000 a year, they’d need 55 times that – reduced assuming there were investments. “But it’s at least half a million, and then that only gets you £20,000 a year,” she explains.

Living costs in the UK are much higher than that – the average annual spend for a family of four, according to the ONS, is £39,000 ($50,000). Using Mackay’s rule of saving 55 times that amount, someone retiring at 35 would need £2.15 million ($2.75 million).

Cutting back, not cutting off

Those calculations assume someone retiring at 35 won’t work at all once they “retire”. That’s not the case for most in the FIRE movement.

Gwen Merz is 28 and has $200,000 in assets (mostly in property, stocks and a little cash). The American quit her job in IT in March aged 27 and now hosts the Firedrill podcast. “I’m not retired,” she says. “I still have to work but I have the freedom to choose something that I really enjoy that maybe doesn’t pay so much.”

She hopes her investments will provide a living income whenever she decides to stop working. And for her, FIRE isn’t just about finances – it offers community and camaraderie. “There are more people to hang around with and don’t look at you because you have a 13-year-old car.”