He is accruing value faster than he is spending it. He was worth 50 billion in 2006, and giving away half would have meant giving away 25 billion. But last year

He is accruing value faster than he is spending it. He was worth 50 billion in 2006, and giving away half would have meant giving away 25 billion. But last year he was apparently worth 100 billion, and he has already given away 45 billion worth of assets. It's a bit hard to quantify exactly how much he should give away to make good on his promise, because the dollar amount is a moving target. I think maybe he means half of his holdings in Berkshire Hathaway. I haven't looked into what sort of assets he has, but judging from his public writings I wouldn't be surprised if the shares are the only thing of significant value he owns.