This is one of the simplest yet most useful measurement tools for Twitter I’ve yet seen: TweetReach, discovered via a tweet by David Terrar earlier today.

What TweetReach does is calculate how many people have seen something you, or anyone else, has tweeted. It connects to Twitter via an API and searches what’s there to produce its results.

It works on hashtags, URLs, key words, anything in a tweet. Here’s an example of the results from searching on a URL, a post I wrote on The Next Web the other day:

Note in the screenshot that the URL is a short one, not the underlying original URL. I found out that TweetReach doesn’t work on the original if what you tweeted was a short URL. In other words, it searches only what was actually tweeted, which makes complete sense.

How does it all work? Here’s what TweetReach says:

[…] Basically we search Twitter and find all the tweets we can. Then we look at everyone who tweeted and who follows them to make a set of calculations.

The reach is the total number of different people who would have seen tweets on this topic in their Twitter stream. This takes into account people who follow the same people.

Here’s a reach example… Let’s say you have two loyal followers: User A and User B (don’t worry, you’ll get more popular). You tweet a link and they retweet it. They both have a loyal follower in common: User C. User B also has a second follower: User D. In this case, the reach comes out to 4 because Users A-D have all seen a tweet on this topic at least once.

If you’re paying attention, you’ll notice in the above example that User C would have actually seen two tweets about this topic, which brings me to exposure… Exposure is how many times someone saw a tweet about a particular topic. If 5 people I follow tweet the same link, I’ll have been exposed to it 5 times. Each time someone sees a tweet, we call that an impression

Useful (and free).

One thing I haven’t quite figured out is why the difference between the results TweetReach reports (in this case, the reach from 23 tweets as the screenshot shows) and the number of tweets reported on the website via the Tweetmeme button (in this case, 123 as I look at the post right now).

Still, it’s definitely another tool to add to your social media temperature testing toolbox to give you some helpful stats and pretty graphs.