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CK-12

By CK-12 Foundation (nonprofit)Checked October 31, 2026No affiliate links. Programs never pay to appear. How we label

CK-12 is a nonprofit website of free digital textbooks, called FlexBooks, with adaptive practice questions, science simulations, interactive activities and study guides. It covers math from elementary grades through calculus, science from kindergarten through high school biology, chemistry and physics, and some English and social studies, plus Flexi, a free AI tutor for students. Its lesson planner tools name homeschool educators among the people they are for.

Grades it covers

  1. PK
  2. K
  3. 1
  4. 2
  5. 3
  6. 4
  7. 5
  8. 6
  9. 7
  10. 8
  11. 9
  12. 10
  13. 11
  14. 12
As the publisher states it: Kindergarten to 12th grade, plus some college and adult courses.
Subjects
Math and science
Grades
Kindergarten to 12th grade, plus some college and adult courses source ↗
Method
Traditional source ↗
Format
On screen source ↗
Faith label
Secular
Cost a year
Free. Free: the FlexBooks, practice and the Flexi tutor cost nothing; the foundation is a nonprofit that takes donations. Read on October 31, 2026; prices change. source ↗
Placement
No placement test on the publisher’s site when we checked.

What it does well

  • Everything is free, including full middle and high school math and science textbooks.
  • Science simulations and interactive activities show ideas that are hard to demonstrate at a kitchen table.
  • FlexBooks can be customised, so a parent can trim or reorder a textbook to fit the year.
  • Practice adapts to the child, with personalised questions on each topic.

Watch for

  • It is a set of resources, not a planned year: the parent picks the books and sets the pace.
  • English and social studies are thinner than math and science.
  • The work is on a screen, and the Flexi AI tutor is a chatbot a parent may want to supervise.
  • Most of the site is written for classroom teachers and schools.

It fits

A family that wants free, solid math and science textbooks for the middle and high school years and is happy to plan around them.

It doesn’t fit

A family that wants a complete, scheduled curriculum in every subject, or printed books with little screen time.

Visit CK-12 ↗

Compare CK-12

Other programs for the same subject and grades:

  • The Good and the Beautiful Preschool to 12th grade; high school has language arts and biology only
  • Khan Academy Kindergarten to 12th grade, plus early college courses
  • BrainPOP Homeschool Grades 3 to 8 on the Homeschool plan; kindergarten to 8th grade with BrainPOP Jr. added
  • Evan-Moor Pre-K to 8th grade; the homeschool resources run pre-K to 6th grade
  • Generation Genius Kindergarten to 8th grade, in K to 2nd, 3rd to 5th and 6th to 8th grade bands
  • Apologia Science Preschool to 12th grade; each elementary course is for K to 6th grade, General Science for 7th to 8th, Physical Science for 8th to 9th
  • Beautiful Feet Books Kindergarten to 12th grade, in grade spans such as K to 3rd, 4th to 6th and 5th to 8th
  • CTCMath Kindergarten to 12th grade, through Pre-Calculus and Calculus

Lists it is on

What we read, and how we label

Every fact on a card was read on the publisher's own website on the date shown, and the page it came from is linked. A fact we could not confirm there is left out. Costs are for one child and one year of the main package, as the publisher's page showed them that day; prices and sales change. Method and format are the publisher's own description where it gives one. The only thing we say about religion is one label, secular or faith-based, given by the rule above; where the publisher's pages don't settle it, the card has no label.

The publisher’s pages we read on October 31, 2026:

Something wrong or out of date? Publishers and parents can send a correction and we will fix it. Publishers can send corrections, never money. All programs compared · Find your homeschool style