Typically, paper comes in A0 size (841x1188mm) which is then cut in half X times to get the desired paper size, which is either eyeballed against a ruler, or me

Typically, paper comes in A0 size (841x1188mm) which is then cut in half X times to get the desired paper size, which is either eyeballed against a ruler, or measured by laser. If the operator is skilled, or the machine is well-calibrated most A4 paper is therefore exactly 297mm tall, and 210.25mm wide (so slightly wider than the standard)