Traders work on the floor of the New York Stock Exchange (NYSE) in New York, U.S., January 31, 2018.

U.S. stocks lost nearly $1 trillion in this week's sell-off, according to Howard Silverblatt, senior index analyst at S&P Dow Jones Indices.

The fell 3.85 percent in its worst week since the ugly beginning to 2016, before Brexit and before President Donald Trump's election. The last week's decline erased $945 billion from the index's market value, with $511 billion lost on Friday alone, Silverblatt said.

Stocks plunged as investors worried about interest rates rising too quickly.

"While most agree (publicly) some pullback is needed to permit profit taking and to create a secure base, the main question is if this week is an adjustment or the start of a correction, or the end of the bull run (started on March 9, 2009, and now up 308%)," Silverblatt said in a note late Friday.

All 11 stock sectors fell this week, and 36.2 percent of the S&P 500 stocks fell at least 5 percent, according to Silverblatt's data. Just 42 stock issues in the index gained this week, including three that rose more than 10 percent: Dr. Pepper Snapple Group, Qorvo and Newell, the data showed.

The nearly $1 trillion decline in market value this past week followed an unprecedented month of inflows into equities in January.

EPFR Global estimated Thursday that all equity funds took in a "record-setting" $99 billion to $100 billion last month, while ETFGI said preliminary January data for U.S.-listed exchange-traded funds and exchange-traded products showed record net new assets of $78 billion. that marked an increase of $19 billion in net new assets from the prior record in Dec. 2016.