An author recently asked if I could tell him the spine size of his book ahead of time. His cover design is already done (before book interior layout has begun) and he—being the clever man and marketing guru that he is—wants to get it made into 3D graphics that look like the final printed books and start using them in marketing.
He asked, “Can you estimate how many pages you think my book will be if we use the same layout, font, line spacing and justification as that book that I like? What measurements do I give to my cover designer for the hardback cover and paperback cover?”
In order to calculate the spine size, you need to know the number of pages in the book and the thickness of the paper being printed on. The printer will tell you the paper thickness, or help you calculate, but the book interior designer must tell you the final page count.
I wished I could just send him a magical number, but since I have not seen the final manuscript or even started to set up the interior files, it is impossible to give him exact specifications. Typesetting involves a lot of variables, even if there are standard body text and spacing sizes (more or less). The page count depends on how wide or narrow the font is, what size the margins are, how big the images and graphics are, how many pages of front and back matter there are, and even how far down the page each chapter starts, etc.
But I understand that the author in question wants an estimated page count and spine size for mock-up graphics. I gave my client a number, based on my experience with similar books, and asked him to give that to his printer and get a spine size / cover layout templates from the printer. He is using IngramSpark, and can get those cover templates generated on their website if he feeds them all the right info (ISBN, ink type, paper type, binding style, etc.)
In his case, the book will be printing in both hard and soft cover, so his cover designer needs to generate two separate templates and set up the cover two different times using the specs given to her by the printer. From there she will have approximate spine sizes for her 3D cover mock-ups.
The actual spine size calculation for printing the covers always has to wait until the typesetting is done to be finalized. Once a print-ready interior file is prepared, the cover designer needs to revisit the cover templates and adjust accordingly. This can be a bit of a tedious process, so I do not recommend having your cover designer set up the book cover templates using the estimated page count unless you really need it, such as for an almost-perfectly sized 3D mockup.
How and when can you know the spine size of your book? Really, you can only know it when the book is fully typeset and approved for printing, so you are sure no more pages will be added or deleted. In the meantime, you’ll have to content yourself with a rough page count estimate provided by your book interior designer.