The CK-12 Foundation is a California-based non-profit organization which aims to increase access to low-cost K-12 education in the United States and abroad. CK-12 provides free and customizable K-12 open educational resources aligned to state curriculum standards. As of August 2026, the organization's website reported that its platform had served more than 453 million users, customized 371K FlexBooks, and answered more than 2.88 billion questions.