Sal Khan's nonprofit now contains more than 3,100 free video tutorials, mostly on math and science—but the site has begun expanding its scope to other subjects, too.

Sal Khan, whose online Khan Academy serves up video tutorials to more than 6 million students worldwide each month, wants to reassure teachers that the free educational service isn’t out to take their jobs—nor is it a statement about a teacher’s ability to deliver a lesson effectively.

On the contrary, Khan said, teachers who are using the service with their students feel more empowered than ever.

“It liberates the classroom,” he told attendees of the National School Boards Association’s 72nd annual conference in Boston, “and teachers’ creativity comes out.”

Warm and engaging, with a self-deprecating sense of humor, Khan discussed his nonprofit venture during an April 22 keynote speech to a few thousand school board members, who interrupted him with frequent rounds of applause.

Khan Academy now contains more than 3,100 free video tutorials, mostly on math and science—but the site has begun expanding its scope to other subjects, too.

What started as an idea to tutor his 12-year-old cousin Nadia from a distance in 2004 has now surpassed 140 million lessons streamed online and is helping 10 times more students learn each month than the entire number of students who’ve graduated from Harvard University since 1636, Khan said.

And the reason for the website’s success is simple: Students can access the content “when and how they want it.”

When he first posted his video tutorials on YouTube in 2006, his relatives said they liked the YouTube versions better than Khan’s live tutoring, because they were more comfortable watching the videos privately on their own time. No one was looking over their shoulder, or waiting for them to indicate they understood the material before moving on.

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In fact, this ability to engage with the content in private—over and over again, if necessary—was cited as a key advantage in a video testimonial that Khan showed of a man who was able to earn a degree in electrical engineering with help from Khan Academy.

After admitting that he’d had to watch some of the website’s videos 20 or 30 times before understanding their high-level math concepts, the man noted: “There’s no [human] tutor who could sit with me and go over the same material 20 or 30 times.”

In 2009, encouraged by the reaction his video tutorials were getting on YouTube, Khan took a leap of faith, quit his day job as a hedge fund analyst, and established Khan Academy as a nonprofit organization. The site eventually caught the attention of Microsoft Chairman Bill Gates, whose Gates Foundation supported it with a $2 million grant. Google also has contributed $2 million, allowing Khan to hire staff and pursue his dream of “providing a free, world-class education to anyone, anywhere.”

Now, the site’s video tutorials are sequenced, so students can move through increasing levels of competency on the path to mastery. This is how video games work, Khan said—but until now, it’s not how schools traditionally have operated.