Ask HN: is there a Twitter hashtag reach vanity metric?

Doing the math roughly is actually easy. Every tweet that doesn't come from search endpoints has the user details inline. The user details include the number of followers.

So just consume the filter streaming api with your chosen hashtag. You will get all the tweets, or at least some and a measure of how many you missed (limit notices).

For every new user in said stream, add the number of her followers.

It's so easy you could write the code in the length of this comment. Of course, if you want to avoid counting duplicate followers, you'd have to do an average of 1-2 additional calls per user.

Well, total reach might be tough to pull off, but it would be definitely feasible to poll the Twitter API every hour and count the tweets with that hashtag that appeared in that time span.

To justify cost to brands it might be easier/more 'sellable' to insist on actual, measurable engagement (links clicked, tweets retweeted, tweets favorited etc.) around their own tweets. I'd worry about unrelated tweets (i.e. sharing the same keyword but about something else altogether), or either bad publicity being accounted for.

We currently don't do that, but its in the pipe for us at SWIX ( http://www.swixhq.com ), those types of metrics and many more.

http://tweetreach.com will do this. The basic/free version only looks at the 50 most recent tweets, but they gasp charge for more. Amazing...a Twitter app that charges! Novel! ;)