Author Ann Wylie (excerpts from an article first published in Public Relations TACTICS, October 2013)

Here are some numbers to consider when writing copy for Web pages, blog posts and status updates on social media.

50 percent: That’s how much shorter your Web copy should be than your print copy, according to a 1997 study by usability expert Jakob Nielsen. Nielsen had study participants read short and long versions of a Web page and found that the short version was 58 percent more usable. That is, subjects were able to read it faster, understand it better and remember it longer. They also like the shorter piece better.

420 characters: The longer your Facebook post is—up to the social network’s limit of 420 characters—the more Likes and comments you’ll get, suggests 2011 research by Momentus Media. Most posts weigh in at 140 characters or less, thanks to the Twitter effect (people posting simultaneously on Twitter and Facebook), Momentus reports. So posting longer status updates may also be a way to stand out.

Facebook image120 characters: That’s the new black for tweets. At that length, your tweet will give followers room to comment when they retweet. Unless you have a URL in your tweet, in which case, 118 characters is the new black. The reason: A 2013 behind-the-scenes change in Twitter’s link wrapper means that links now take up more space.

0 percent: That’s the percentage of passive voice to go for in all of your online copy. People read active sentences faster and easier, remember them longer and prefer them to passive sentences, according to Jan H. Spyridakis, author of “Guidelines for Authoring Comprehensible Web Pages.”

Fifth: That’s the level to aim for on Facebook, according to 2010 research by Dan Zarrella, HubSpot’s viral marketing scientist. Status updates written at fifth-grade level were shared 15 percent more often than average. Those written at the 15th-grade level were shared 20 percent less often than average.

6.5: That’s the grade level to target for tweets if you’d like to go viral on Twitter, Zarrella says. Retweets, on average, scored 6.47 on the Flesh-Kincaid Index, his 2009 research shows. Don’t like your score? Improve your grade level by reducing sentence and word length.

1 or 2: That’s the average length of a word, in syllables, that’s most likely to be retweeted, according to Zarrella’s research. Retweets on average have 1.62 syllables per word.

Being down for the count

Just hitting these numbers, of course won’t transform a terrible tweet into a terrific one.

But once you’ve written a relevant, valuable, interesting message, following these guidelines will make your Web copy—heck, all of your copy—more readable.

So count your syllables, characters, words, percentage of passive voice and grade level—or else, when it comes to reading your message, your audience may count themselves out.