Washington Post bias rating is Lean Left.

The Washington Post was given a lean left bias rating that AllSides is highly confident about. Originally established in 1877 as a “four-page organ of the Democratic Party,” the newspaper was eventually sold in 1889, resulting in the abandonment of its Democratic Party allegiance. In recent decades, the Washington Post has been accused of both liberal and conservative biases. A majority of over 7000 community members agreed with a lean left AllSides Bias Rating™ as of July 2016. Among those who disagreed, 66 members gave the newspaper an average bias rating of 58.8, landing it just next to the boundary between a lean left and center bias. Thiese results, in addition to a March 2013 blind survey and a 2005 UCLA study, reinfroce our rating for the Post.

More on The Washington Post 

The Washington Post is the dominant newspaper in the U.S. capital and usually counted as one of the leading daily American newspapers. Over the years, it has distinguished itself through its political reporting on the workings of the White House, Congress and the U.S. government. Among the Post’s sections are a main section covering national and international news, a metro section covering local news, a style section and a sports section. 

Sources:

Wikipedia: Washington Post

Britannica: Washington Post

Washington Post

The AllSides Bias Rating reflects the average judgment of the American people.

The AllSides Bias RatingTM reflects the average judgment of the American people. We don't use a convoluted mathematical or artificial intelligence model, but instead have regular people representing the broad spectrum of Americans blindly rate the bias of articles. That produces a fair, verifiable bias rating.

This media bias rating was determined using the following levels of bias verification.

Basis of Rating:

Blind Survey
Third Party Data
1
Community Feedback
0
Editorial Review
1
Secondary Research

Confidence Level:

High

Unless otherwise noted, this bias rating refers only to news articles on their web site, not from opinion pieces or what is broadcasted on TV or radio. The opinion writers from the same media source may have different bias ratings, so individual writers often are rated separately.