Inches to centimetres
Multiply inches by 2.54 to get centimetres. The factor is exact by definition, not a measurement. A 55-inch television is 139.7 cm on the diagonal, a 12-inch ruler is 30.48 cm, and a sheet of US Letter paper at 8.5 by 11 inches is 21.59 by 27.94 cm.
How to convert inches to centimetres
Nominal sizes are the trap. A "two by four" is not two inches by four — it is planed down to 1.5 by 3.5 inches, so 3.81 by 8.89 cm. Pipe and screw sizes carry the same kind of legacy naming, where the label stopped describing the measurement a century ago.
Questions
Convert it to a decimal first: a half is 0.5, a quarter 0.25, an eighth 0.125 and a sixteenth 0.0625. So 3⅜ inches is 3.375.
139.7 cm diagonally. At the usual 16:9 shape that is about 121.8 cm wide and 68.5 cm tall, before the bezel.
No. A4 is 21.0 by 29.7 cm; Letter is 21.59 by 27.94 cm. Letter is wider and shorter, which is why printing one on the other clips the edges.
One thousandth of an inch, 0.00254 cm, used for film and coating thickness. It is not a millimetre, and confusing the two is a factor-of-forty mistake.