Shoe posted a while ago that it was 28 inches per 1ml, which is not far off one drop (in fact more than 1 drop for his dropper). But i didn\'t think this was right, so I put 2ml of water in my SOE roll on, and marked on an 8\" strip on my arm. This gave me 160\" per ml, which i though was huge, and wrong. But then i did it again with 1 ml, and got the same result.