Videos are an essential aspect of online education. But how long is too long? And how short is too short?

If your video strategy for creating course videos was to tape a lecture and throw it online, it’s unlikely many of your learners are staying till the end. If they are, they’re probably pausing it mid-stream and coming back to it later.

Why? Because a forty-five-minute lecture is more than what the average learner can process in one sitting. And no, it’s not because attention spans are shrinking. It’s simply a tiring length of time to listen to someone monolog.

Of course, in a college setting where learners have to travel to lectures, forty-five minutes—or more—isn’t so much to ask. But online courses operate under different conditions, where the longer video format isn’t just unnecessary, it may even be counterproductive. So, how long should your e-learning videos be?

TED Talks and the 18-minute rule.

Of course, you could take your cue from the guidelines from the world-famous TED Talks: No presenter, no matter who they are, is allowed to deliver a talk longer than 18 minutes. The rule has some pretty sound reasoning behind it. According to TED curator Chris Anderson:

It [18 minutes] is long enough to be serious and short enough to hold people’s attention. It turns out that this length also works incredibly well online. It’s the length of a coffee break. So, you watch a great talk, and forward the link to two or three people. It can go viral, very easily. The 18-minute length also works much like the way Twitter forces people to be disciplined in what they write. By forcing speakers who are used to going on for 45 minutes to bring it down to 18, you get them to really think about what they want to say. What is the key point they want to communicate? It has a clarifying effect. It brings discipline.

This all sounds great so far as it goes, but most online instructors aren’t delivering TED talks. They’re covering dense material, some of it technical, much of it instructional.

Now, imagine you were trying to learn how to do something. Do you think you would still remember Step 1 eighteen minutes later? Almost certainly not. What if you were covering a complex historical topic, and were presented a pop quiz at the end of the video. How well do you think you’d remember information presented in the first minute?

The right video length for e-learning depends on context and application. There’s no right answer for all e-learning, but there is a right answer for you. Let’s take a quick look at some of the primary ways online educators put video content to use.

1. Capturing interest on social media. (1–3 minutes)

Social media is a fantastic medium for sharing video. As you followers, subscribers, or (in the case of paid advertising) potential learners scroll down the news feed, you have the opportunity to catch their eye with content.

But even those who do stop to watch your video probably won’t stay for more than a few minutes. After all, they came to that channel for something else. So, keep videos here on the shorter side. You can even repurpose content from other parts of your course to make them snappier and more engaging.

2. Training, trailers, and recaps. (2–5 minutes)

Ever notice how movie trailers are all about 2:30? I think there’s a reason for this. It’s just long enough to give you a taste without giving too much away. And the time commitment is low enough that there’s almost no reason not to watch them.

That said, there’s not a lot of information you can fit into short videos. You can deliver very specific training information for how to use a piece of equipment, deliver a succinct recap (or introduction) to a module, or preview a course. To get to the heart of your course, you’ll need something longer.

3. Guides, tutorials, and overviews. (6–10 minutes)

There is some research that says that six minutes is the optimal length for a video. That’s certainly a compelling case, and the bulk of your content should lie in this range. Six minutes is just long enough for learners to absorb something of value, but not so long that they can’t watch it (and complete a review quiz) during a coffee break.

From my personal experience, I can attest that six minutes is a pretty addictive length for video content. I can always justify watching just one more, while also feeling like I treated myself to something interesting and valuable.

4. Deep content. (Up to 20 minutes)

Now we’re into the TED talk zone. This is for complex topics—subjects that can’t be covered in enough detail in only six minutes. And yes, there’s plenty of content that falls into this category.

However, with longer videos, there are two things to keep in mind. First, some learners won’t stay till the end. I’m willing to accept this in exchange for incredible long-form content, but that’s the key: your videos have to be top notch and riveting all the way through if you expect to maintain interest for the duration.

Second, these are not good for mobile learners. Not only does a twenty-minute video draw on a lot of data, it’s also a bit awkward to consume in a public setting (where most mobile learners are).

Remember: videos are passive learning. Activities are engaging.

There’s a very good reason learners lose interest in educational videos after a certain point: they’re a passive learning method. After all, how long do you maintain interest in a one-way conversation? Learners crave the opportunity to put the knowledge they have just gained into action. Once they see the video, they want to start trying it out.

Which, of course, is the point. Learners who practice a scenario directly after finishing a six-minute video are primed to recall relevant information very well. They’re likely to be more successful, and that will encourage them to watch another video.

This sets a great pace for mobile content and micro learning. So, next time you’re tempted to film an entire lecture in one sitting, re-think your approach. A forty-five minute lecture broken into seven or eight videos will be more effective—and it makes for a perfect e-learning module.