"Composite Portland Cement" (GB12958-1999) is a mandatory cement standard. Article 9.1 of its standard stipulates: Cement can be packed in bags or in bulk, and the net content of each bag of cement is 50kg, and it shall not be less than 98% of the marked quality; the total mass of 20 bags selected at random shall not be less than 1000kg. Other packaging forms are determined by both the supplier and the buyer, but the relevant bag quality requirements must comply with the above-mentioned principles and regulations.

Therefore, unless the two parties agree on the contract, it is 50KG a bag.