Bill Gates: We've given away $100bn, but my children won't be poor when I'm gone

It's towards the end of our interview that Bill Gates reveals new numbers on how much his charitable Foundation has now spent in its efforts to combat preventable diseases and reduce poverty.

"I've given over 100 billion," he says, "but I still have more to give."

That's dollars, just to clarify, worth about £80bn.

It's roughly equivalent to the size of the Bulgarian economy or the cost of building the whole HS2 line.

But to put it in context, it's also around the same as just one year of Tesla sales. (Tesla owner Elon Musk is now the richest man on the planet, a position Gates held for many years.)

The co-founder of Microsoft and his fellow philanthropist Warren Buffett are combining their billions through the Gates Foundation he originally set up with his now ex-wife Melinda.

Gates says philanthropy was instilled in him early on. His mother regularly told him "with wealth came the responsibility to give it away".

His Foundation's 25th anniversary is in May, and Gates exclusively revealed the $100bn figure to the BBC.

He tells me, for his part, he enjoys giving his money away (and around $60 billion of his fortune has gone into the Foundation so far).

When it comes to his day-to-day lifestyle, he doesn't actually notice the difference: "I made no personal sacrifice. I didn't order less hamburgers or less movies." He can also, of course, still afford his private jet and his various huge houses.

He plans to give away "the vast majority" of his fortune, but tells me he has talked "a lot" with his three children about what might be the right amount to leave them.

Will they be poor after he's gone? I ask him. "They will not," he replies with a quick smile, adding "in absolute, they'll do well, in percentage it's not a gigantic number".

Gates is a maths guy and it shows. At Lakeside School in Seattle, in eighth grade, he competed in a four-state regional maths exam and did so well that, at 13, he was one of the best high school maths students of any age in the region.

Maths terminology comes second nature to him. But to translate, if you're worth $160bn, which Bloomberg's Billionaires Index claims he is, even leaving your children a tiny percentage of your fortune still makes them very rich.

I'm with one of only 15 people on the planet who are centibillionaires (worth more than $100bn), according to Bloomberg. We're in his childhood home in Seattle, a mid-century modern four-bedroom house set into a hill, and we're meeting because he's written a memoir, Source Code: My Beginnings, focusing on his early life.

I want to find out what shaped a challenging, obsessive child who didn't fit the norm into one of the tech pioneers of our age.

He's brought along his sisters, Kristi and Libby, and all three excitedly tour the home where they grew up. They haven't been back in some years and the current owners have refurbished (fortunately, the Gates siblings seem to approve of the changes).

But it's bringing back memories including, as they walk into the kitchen, of the now-long-gone intercom system between rooms beloved by their mother. She used it to "sing to us in the morning", Gates tells me, to get them out of their bedrooms for breakfast.

Mary Gates also set their watches and clocks eight minutes fast so the family would work to her time. Her son often rebelled at her efforts to improve him, but now tells me "the crucible of my ambition was warmed through that relationship".

He puts his competitive spirit down to his grandmother "Gami", who was often with the family in this house and who taught him to outsmart the competition early on with games of cards.

I follow him down the wooden stairs as he heads off to find his old childhood bedroom in the basement. It's a neat guest room now, but young Bill spent hours, even days, in here "thinking", as his sisters put it.

At one point, his mum was so fed up with the mess that she confiscated any item of clothing she found on the floor and charged her stubborn son 25 cents to buy it back. "I started wearing fewer clothes," he says.

By this time, he was hooked on coding and, with some tech-savvy school friends, had been given access to a local firm's one computer in return for reporting any problems. Obsessed with learning to program in those nascent days of the tech revolution, he would sneak out at night through his bedroom window without his parents knowing to get more computer time.

"Do you think you could do it now":[]}