pt (point):It is a physical length unit, equal to 1/72 of an inch.

px (pixel):It is a virtual length unit, a digital image length unit in computer systems. To convert px to physical length, you need to specify DPI (Dots Per Inch, pixels per inch), which is usually available as an option when scanning and printing. Windows defaults to 96dpi, and Apple defaults to 72dpi.

em (relative length unit, relative to the font size of the text within the current object):It is a relative length unit, originally referring to the width of the letter M, hence the name em. Now it refers to a multiple of the character width, used similarly to percentages, e.g., 0.8em, 1.2em, 2em, etc. Usually 1em = 16px.

Font size:It is a unique unit in Chinese font libraries, representing specific pt values with Chinese designations for easy memory and expression.

The conversion formula between pt and px can be derived from the definition of pt:

pt = 1/72(英寸), px = 1/dpi(英寸)

Therefore pt = px * dpi / 72

Calculated with 96dpi under Windows, 1 pt = px * 96/72 = px * 4/3