We don't normalize the Altmetric Attention Score, so it doesn't have a scale per se (though a score of 0 indicates that we haven't tracked any attention).
To put the score into context you should use the Scores tab, which can tell you where the current score fits in with others from the same journal or across the whole dataset.
We're working hard on ways to help you interpret the score. Of course, the most important thing you can do is actually read the posts that went in to making it up - the score is a snapshot of attention, but the underlying data is what is really important.
What's a good Altmetric Attention Score?
You can't really say that a score is 'good' as it measures attention - which could be good or bad. For example the 'arsenic life' paper famously debunked by blogger Rosie Redfield (amongst others) got a lot of attention online, but not necessarily the kind of attention you'd want as an author.
Furthermore the average score for journals varies: an article in Science or Nature will typically score much higher than one in a smaller journal, not least because more people have read it and are thus more likely to share it. A good score for one journal might be a low one somewhere else.
Bearing those two things in mind - in general if an article scores 20 or more then it's doing far better than most of its contemporaries.
Putting the Altmetric Attention Score in context
The "Attention Score in Context" tab on the Altmetric Details Pages show the score in some different contexts, to help you understand if the level of attention is typical compared to similar articles.
For example, you can see how the article's score compares to other articles from the same journal, or from the same journal and published within the same three month period.
We calculate these percentiles by looking at everything indexed in the Altmetric database, which you can browse using the Explorer. It's important to note that we don't include articles that didn't get any attention: so if the Score tab says that an article is ranked #10 out of 220 published in the same journal it means the 220 articles that have been mentioned at least once on a data source we track, rather than that the journal has published 220 articles in total.
The "Attention Score in Context" is not currently generated for Books