What Is an Inch (in)?
Definition
Inch (in): A unit of length used in the United States and the United Kingdom, symbol in. Since 1959 one inch has been exactly 2.54 centimeters by definition; 12 inches make a foot.
The inch (symbol in) is a unit of length used in the United States and the United Kingdom. You meet it most often in the diagonal size of TV, monitor and phone screens. Since the 1959 international yard and pound agreement, one inch has been exactly 2.54 centimeters. Twelve inches make a foot, and 36 inches make a yard.
Where it appears in the formulas
1 in = 0.0254 m = 2.54 cm. The length converter uses this exact factor (NIST SP 811, Appendix B; NIST Handbook 44, Appendix C). For example, the diagonal of a 55-inch screen is 55 × 2.54 = 139.7 cm, and 1 cm is about 0.3937 inches.