A4 measures 210 × 297 millimeters

so one sheet of 90gsm should weight 0.21 x 0.297 x 90 grams i.e. 5.6133gm

42,000 of those weigh 42000 x 5.6133 = 235,759 gm or ~236kg

A 1200x1000 pallet takes ~20kg according to User Posted Link so on that basis you'd need twelve of 'em.

Posted By: Old Man, Feb 5, 11:44:16