Print Production, Bookbinding & Packaging Engineering
Paper GSM, US Basis Weight, PPI & Book Spine Formula
Convert US 500-Sheet Basis Weight (lb) across Text (25x38") vs Cover (20x26") parent sheets into ISO Metric Grammage GSM = (Lb * 1406.5) / (W_in * L_in), Pages Per Inch PPI = 2000 / Caliper_mils, and Perfect-Bound Spine Width.
Why 80 lb Cover stock (216 GSM) is nearly twice as heavy as 80 lb Text stock (118 GSM)!
Print buyers and packaging designers frequently order the wrong paper stock because North American Basis Weight (lbs) weighs 500 uncut sheets of each grade’s historical PARENT sheet size—25" x 38" (950 sq in) for Book/Text, 20" x 26" (520 sq in) for Cover cardstock, and 17" x 22" (374 sq in) for Bond! ISO 536 Metric Grammage eliminates that ambiguity by measuring grams per square meter across all grades via TAPPI conversion GSM = (US_Basis_lb * 1406.5) / (Parent_Width_in * Parent_Length_in)! For book cover mechanicals, paper bulk is governed by single-leaf Caliper in mils (thousandths of an inch): Pages Per Inch (since 1 leaf = 2 printed pages!) is PPI = 2000 / Caliper_mils, and Perfect-Bound Spine Width is Spine_in = (Page_Count / 2) * (Caliper_mils / 1000) + Cover_Wrap_Allowance_in!
Verify in a shared print-production spec sheet—US_Basis_Weight = 80 lb (B2), Parent_Width = 25 in (B3), Parent_Length = 38 in (B4 -> Text Parent Area = 950 sq in in E2), Leaf_Caliper = 5.0 mils (0.005", B5), Total_Interior_Pages = 300 pages (150 leaves, B6), and Cover_Crease_Allowance = 0.02 in (B7): Metric Grammage is (80 * 1406.5) / 950 = 118.44 GSM (E3); Pages Per Inch is 2000 / 5.0 = 400.00 PPI (E4); and Perfect-Bound Spine Width is 150 * 0.005 + 0.02 = 0.7700 in (E5)!
| Print Production Spec (80 lb Stock, 25x38" Text, 5.0 mil Leaf, 300 Pages) | Formula | Verified Output |
|---|---|---|
| Uncut Parent Sheet Area (sq in) | Parent_W_in * Parent_L_in | 950 sq in (520 sq in Cover) |
| ISO 536 Metric Grammage (GSM) | (US_Basis_lb * 1406.5) / Area_sq_in | 118.44 GSM (216.38 GSM Cover!) |
| Pages Per Inch Bulk (PPI) | 2000 / Leaf_Caliper_mils | 400.00 PPI |
| Perfect-Bound Spine Width (in) | (Pages / 2)*(Caliper / 1000) + Allow | 0.7700 in |
TAPPI US Basis Weight to ISO Grammage (GSM) & Spine Formula
Validate positive basis weight, parent dimensions, leaf caliper, even/positive page count >= 2, and non-negative cover allowance!
=LET(lb_w, B2, wd_p, B3, ln_p, B4, cl_m, B5, pg_n, B6, al_c, B7, IF(OR(lb_w <= 0, wd_p <= 0, ln_p <= 0, cl_m <= 0, pg_n < 2, al_c < 0), "Invalid Print Stock Spec", ROUND((lb_w * 1406.5) / (wd_p * ln_p), 2)))80 lb Cover Cardstock Comparison (Set Parent Size to 20" x 26")
Change B3 = 20 and B4 = 26 (Cover parent sheet = 520 sq in) while keeping 80 lb in B2: Metric Grammage E3 jumps from 118.44 GSM to 216.38 GSM!
=IF(ISTEXT(E3), E3, ROUND((B6 / 2) * (B5 / 1000) + B7, 4))Try the example
Use the original values or change them to test your understanding.