tv in tunnelLet’s go over an important truth of video engagement we’re going to call the “goldfish rule.”

THE GOLDFISH RULE: Your viewers are less attentive than goldfish.

Oh, and the goldfish rule has an addendum, too.

ADDENDUM: Viewer attention spans will only continue to decrease.

Don’t believe us? Associated Press did a fascinating study that measured the average American attention span from 2000-2012. In 2000, attention spans were 12 seconds long. By 2012, 4 of those seconds had vanished (most certainly replaced by grumpy cat videos). By now, we wouldn’t be surprised if the average American attention span was lower than 8 seconds. For reference, a goldfish’s attention span is 9 seconds.

Not only is this embarrassing for the average American, myself included, it’s an important reference for 4 “benchmarks” of video length.

NOTE: All “engagement” data taken from Visible Measures.

1. 6 seconds

6 seconds was determined by Vine to be the optimal length of a short video between 5 and 10 seconds. How, exactly, did they arrive at that number? Well, if a video is going to be under 10 seconds it better also be under 8 seconds (the average American attention span). But according to Vine cofounder Don Hofmann, 5 seconds is too short.

And Youtube agrees — 6 seconds is the amount of time you spend watching a 60-second ad by the time you can actually click “Skip Ad.”

Engagement: 6-second videos enjoy close to 100% viewer completion rate.

2. 15 seconds

Sometimes, 6 seconds just isn’t enough. Maybe you need to squeeze in a few more lines or another motion graphic. Or maybe you just can’t tolerate a video without a decent intro and outro. Whatever the case, keep in mind that 15-second videos violate the goldfish rule and suffer from viewer dropoff.

At the same time, 15 seconds is Instagram’s answer to Vine, and 16-second advertisements are standard for Youtube pre-roll, which means drop-off rates are still acceptable.

Engagement: 25% of viewers will stop watching a video after 16 seconds.

3. 30 seconds

Half a minute is generally accepted as a sufficient amount of time to sell something. Business Network International (BNI), for example, gives each chapter member 30 seconds to make an elevator pitch to potential clients.

But it’s easier to keep someone’s attention in person and harder through video. In 30 seconds, your viewers could either watch your video or check Facebook, Twitter, and LinkedIn for updates. Tough sell.

Engagement: 33% of viewers will stop watching a video after 30 seconds.

4. Anything longer than 30 seconds

For videos longer than 30 seconds I’m going to bring up another truth of video engagement: diminishing returns.

THE DR RULE: Viewers watch more when given less and less when given more.

Wistia did an excellent experiment with a 30-second control video and 90-second video that illustrates diminishing returns in viewer engagement. Both videos shared the same first 30 seconds, yet drop-off rates for the 90-second video were higher than drop-off rates for the 30-second video at the same time marks. Translation — viewers expect to be bored by longer videos.

In short, shorter videos really are more engaging.