He's the world's youngest self-made billionaire, raking in millions from a company you've probably never heard of.

But don't expect to see Stripe founder John Collison, 27, in the front row at Victoria's Secret or partying on Richard Branson's Necker Island.

The Irish tech giant, who founded the company in 2010 with his brother Patrick, 29, told the BBC he struggles to answer the question of how much money he really has.

"People now ask this a lot and I feel like they always want some really interesting answer - and I have nothing for them," he told the UK broadcaster.

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Stripe co-founder John Collison is the youngest self-made tech billionaire in the world, Forbes reports. He's pictured here at the 2014 Web Summit. Photo / Getty
Stripe co-founder John Collison is the youngest self-made tech billionaire in the world, Forbes reports. He's pictured here at the 2014 Web Summit. Photo / Getty

"People ask 'how has your life changed?', and they want me to have taken up some elaborate new hobby, like Faberge egg collecting or yacht racing."

Instead, the low-key coder prefers his "practical" hobby of running and thinks of being a billionaire as a "calculator exercise" rather than having stacks of cash at his disposal.

"The valuation is predicated on us continuing to execute and launch very compelling products in a highly competitive space - so good signs, but still a lot to do," he said.

The brothers are the brains behind Stripe, a software company that helps websites take online payments, store data and provide security.

Its latest funding round in November 2016 saw it valued at US$9.2 billion ($13.5 billion) with each brother having a $1.1 billion ($1.6b) stake. It makes them two of the three youngest self-made tech billionaires in the world along with Snapchat founder Evan Speigel, who is two months older than Josh.

The siblings grew up in Western Ireland's County Tipperary and went to state school before going to university in the US. The self-taught coders had already founded payments company Auctomatic which they sold after one year for US$5 million ($7.3m) and quickly moved on to working on Stripe, after a trip to South Africa in 2010.

"I remember saying to Patrick 'how hard can it be? Maybe we should give it a try?'" John said.

Now, the company is on a path to making a fortune with a payment model that sees it gain a small slice of online payments from clients. It has been backed by tech giants Elon Musk and Peter Thiel and has 750 staff based in San Francisco and Europe. The future is "indexed to the growth of the internet economy", Josh claims, making it a "very safe thing to bet on".

Snapchat co-founder Evan Spiegel, has an estimated fortune of US$2.9 billion, Forbes reports, and has recently married Australian model Miranda Kerr.

Forbes' list of the 40 richest Americans under 40 places Facebook co-founders Mark Zuckerberg and Dustin Moskowitz, both 33, at the top.

Zuckerberg has an estimated personal fortune of US$71 billion while Moskowitz, who left to found his own company Asana, has US$14.2 billion.