It's been a tough week for Warren Buffett's big investment in IBM.
Big Blue's stock plunged 8.3 percent Friday to $190 per share after the company reported disappointing first quarter revenues.
According to its most recent portfolio filing, Buffett's Berkshire Hathaway owned 68.1 million IBM shares as of the end of December.
Assuming Berkshire's stake is still around that size, Friday's slide cut the value of those shares by $1.168 billion to $12.94 billion.
That's on paper, of course. Berkshire won't actually lose any real money until it sells the shares, and given Buffett's buy-and-hold track record that won't happen until years from now, if it happens at all.
Still, it's a big number and it represents roughly half of Berkshire's paper profits on the stock it started buying in 2011.