known quantity

noun

1. Mathematics. a quantity whose value is given: in algebra, frequently represented by a letter from the first part of the alphabet, as a, b, or c.
2. any factor, circumstance, etc., that is already accepted or familiar:

   Her honesty is a known quantity.

Word History and Origins

Origin of known quantity¹

First recorded in 1975–80

Example Sentences

But several former military and Pentagon officials said that Hegseth was not a known quantity in the Washington defence community.

From BBC

It is worth stating the obvious too – while Sir Keir and Harris have never met, she is a vastly more known quantity and far more likely to be conventional in her approach to high office than her rival.

From BBC

Donald Trump, she said, felt like a known quantity and like “a man of his word”, while Kamala Harris looked promising but still somewhat unknown.

From BBC

Harris, if she becomes president, is at least something of a known quantity to Europeans, having met many leaders and senior policymakers at high-profile events such as the Munich Security Conference, which she attended in February.

From Los Angeles Times

While she might have the opportunity to distance herself from some of the more unpopular elements of Mr Biden’s record, she also does not have the luxury of being a known quantity for voters.

From BBC