The following chart shows the thickness of most used paper stocks in printing industry.
To calculate the spine size of your perfect bound book you have to do the following steps:
1) Find the thickness of the paper that you are going to use for inside pages of your book in the following chart:

Un-coated / Offset Opaque Paper Stocks

Approx. Caliper / Thickness
50lb Offset Text Uncoated paper 0.0035"
60lb Offset Text Uncoated paper 0.004"
70lb Offset Text Uncoated paper 0.005"
80lb Offset Text Uncoated Paper 0.0055"
100lb Opaque Text Uncoated Paper 0.007"
65lb Opaque Cover Uncoated Paper 0.009"
80lb Opaque Cover Uncoated Paper 0.011" (11pt Thick)
100lb Opaque Cover Uncoated Paper 0.014" (14pt Thick)

Gloss Coated Paper Stocks

 Approx. Caliper / Thickness
70lb Gloss Book coated paper 0.0035"
80lb Gloss Book coated paper 0.004"
100lb Gloss Book coated paper 0.005"
80lb Gloss Cover Coated paper 0.0075" (7.5pt)
100lb Gloss Cover Coated paper 0.0092" (9pt)

Satin / Dull Coated Paper Stocks

Approx. Caliper / Thickness
70lb Satin or Dull Book coated paper 0.0038"
80lb Satin or Dull Book coated paper 0.0045"
100lb Satin or Dull Book coated paper 0.0055"
80lb Satin or Dull Cover Coated paper 0.0081" (8pt)
100lb Satin or Dull Cover Coated paper 0.011" (11pt)

 
2) Spine size = (Thickness of the inside pages paper) x ( Number of sheets of the inside pages ), Please note that we are using the number of sheets in this formula which is number of pages divided by two. For example if your are designing a 120 pages book plus cover on 100# gloss book for inside pages and 100# gloss cover for cover, the spine size of the cover of this perfect bound book is:  120 Pages / 2 = 60 sheets, 60x0.005"= 0.3".