The thickness of 1 page is 0.12 mm . So, its simple you just have to count the number of pages and divide it by 2. And after that multiply that number by thickness of 1 page. For example: If book is 600 pages then it should be (600/2)*(0.12) = 36mm or ~1.5 inches.
Thick of 500 page book
0.08 mm
Thick of 300 page book
3/20 cm
