Calculating Curves is a book about a beautiful but forgotten paper of Thomas Hakon Gronwall. Gronwall was a Swedish mathematician and an American immigrant who published over 80 papers in pure and applied mathematics. He was a practicing civil engineer in Europe and the United States as well as a professor at Princeton University. The paper considered in the book was the first work to produce a necessary and sufficient condition for the representability problem of a graphical problem-solving construct called an alignment mogram. The book includes a biography of Gronwall, a translation of the paper with an extensive commentary and a complete bibliography of Gronwall's work.