Photoshop gives a few measurement units for setting your own dimension, namely- Pixels, Inches, Centimeters, Millimeters, Points, and Picas. Here is the description of the following units of measurement scale.
The smallest unit of measurement in an image or graphic displayed on a digital device is a pixel or picture element. Pixels (px) combined together make an image, artwork, video, and text. Converting 1 pixel to millimeter, you get, 1 px = 0.352777778 mm whereas, 100 pixels equal to 1.0416666666666667 inches in 96 dots-per-inch (DPI).
An inch (in) is a unit of linear measure equal to one-twelfth of a foot. An inch is a distance that is equal to 25.4 millimeters or 2.54 centimeters. 1 inch is equal to 96.000000000001 pixels (X) in 60 dot-per-inches (DPI).
Choose centimeters (cm) if you want your document dimensions in centimeters. When converting 1 centimeter to pixels in 72 DPI you get 28 pixels. For example, if your document size is 10 cms X 20 cms in 72 DPI resolution you get 283 px X 567 px document size.
A millimeter (mm) is another unit measurement given in Photoshop to create your custom document size. It is a unit of length that is equal to one-thousandth of a meter and equivalent to 0.03937 inches. When converted into pixels, a millimeter is equal to 3.779528 pixels.
With point (pt) you can measure the height of a font. It is also capable of measuring any length, leading, or any item on a printed page. Point is the smallest measurement unit.
A point is 1/72 of an inch which is 0.353 mm. In earlier American measurements it is 1/12 of a pica. When you convert it to pixels, 1 point is equal to 1.3333333333333333 pixels in 96 DPI. It is 1 point equal to 1 pixel in 72 DPI resolution.
Picas (pc) contains 12 points and is less than 1/6 of an inch or 1/72 of a foot. You can picas use to represent fixed horizontal measurements and column width. Newspapers, ads, magazines, journals, and newsletters designing are done in picas. 1 picas are equal to 16 pixels. Picas, points, and other units are used for design.