The Boomtown Rats frontman is pleased 'Do They Know It's Christmas Time?' single - which is raising money to fight Ebola in West Africa - has topped the charts in 61 countries after selling over 300,000 digital copies in a week, but is concerned the falling price of music means it won't make as much money as the 1984 original, which made £8m for famine relief thanks to sales of 3.7 million.
He said: "In 1984 a single was £3.50, today it's 99p...That worries me. Even if you already have it, delete, download again."
"We need to sell 300% more than we did then [in 1984] to even begin to make up the cash figure."
A physical CD version will be released on December 8 and the organiser admitted this was a "bright spark" to raise more money.
Bob said he was nervous about the response the song would get.
He explained to BBC Radio 1: "Did I expect it? Yes I expected it but did i expect it in such vast, willing numbers? No I didn't.
"You just can't assume anything... You don't know if the audience is interested in this.
"Certainly they like the artists but are they interested in seeing the artist do just one or two tracks?"