cell population doubling - (Apr/10/2012 )

Hi guys so I am trying to do a population doubling on one of my fibroblast cell lines, and i have been using this formula t= H*ln2/ln(c2/c1), and i tried the website calculator http://www.doubling-time.com, but I noticed I keep getting negative numbers, and that just does not seem right So I am wondering what other formula I could be using instead, and if someone could help answer the reason I may be obtaining negative numbers.

Have you plotted the growth on a graph? Perhaps your cells don't fit the formulae you are using.