On April 21st, 2015, Google is rolling out a massive algorithm change to the way it ranks sites on mobile:

Call it mobile-geddon, mobile-pocalypse, the end of the world as we know it; whatever it is, it's coming April 21st and you need to be prepared.

Google knows mobile is crucial

Mobile is important. Your customers are using mobile.

According to comScore, mobile internet usage surpassed desktop usage for users in the United States in January 2014. There is now more time spent online on a mobile phone than on a desktop.

Your customers are checking you out on their smart phones on the subway, in the waiting room, at their kid's soccer game, and waiting for a table at a restaurant. Those are big chances to make an impression, and lot's of companies are missing out.

The 6 Things You Must Know About the April 21st Mobile Algorithm Update

There's been a lot written about this April 21st update, so I'll just summarize the key points:

1. The update affects mobile devices only. It does not affect desktop or tablet devices. Pages that are not mobile-friendly will be demoted in the mobile search results.

2. Mobile-friendliness is determined on a page-by-page basis, not at the site level.

3. Use Google's mobile-friendly testing tool to check if a page is mobile-friendly.

Pro tip: Be sure you setup Google Webmaster Tools. Within Webmaster Tools you'll find a section called, "Mobile Usability" where Google will list all the pages on your site with mobile usability issues. It saves you from checking every page one-by-one using the testing tool.

4. This update is bigger than Panda / Penguin

Google has said that this update will affect more searches than Panda / Penguin, which were pretty major updates themselves.

I've seen some numbers thrown around like, "you'll lose 70% of mobile traffic" or something to that affect. Google hasn't published any hard numbers like that and if I had to guess I'd say it's going to vary dramatically by industry and website.

My guess is, if your site isn't mobile-friendly, mobile search traffic will drop significantly, but what that number is I don't know.

5. It's pass / fail

There's no grey area. At least not right now. A page will either be judged mobile-friendly or not.

6. After the update: Find out if you were affected

Obviously the Mobile Usability report within webmaster tools is the first place you should check to ensure you don't have any major issues.

Since the update is rolling out over a week or so, wait 2 weeks or so until you compare numbers. Then check your mobile traffic from Google organic search to verify if you've been affected. You may have to segment your numbers by page since this update affects specific pages rather than the entire site.

Use the "site content" reports in Analytics with a "compare to" date setting to make this a bit easier. Be sure you're only looking at entrances from Google organic search.

I imagine mobile-friendly sites might actually see an increase in organic traffic from Googles imply because of all the non-mobile-friendly sites getting bumped down in the search results.

Here's the original announcement from Google.