Paper is measured is grams per meter square. A standard sheet of A4 size is 80 grams per metre square. Thus a standard A4 sheet weighs 5g. The 'heavier' the paper the thicker it is.

A4 is a standard paper size (1/16 square metres). The standard for printer paper is 80 gsm (grams per square metre). An 80 gsm sheet of A4 paper weighs 5 grams.

A standard A4 sheet of paper is 21.0 x 29.7 cm, for a total area of 623.7 sq cm. A US standard A-size sheet of paper is 8.5 x 11 inches, which is 21.59 x 27.94 cm, for a total area of 603.2246 sq cm.

To get a rough answer, you'd have to multiply its surface by its thickness or divide its weight by its density. There is no definite answer because there are lots of kinds of paper and also a lot of standard sheet dimensions.

A standard sheet of 'everyday' paper is likely to be measured in either millimetres or centimetres. Larger, commercial-sized sheets of paper may, however, be measured in metres.