CSS unit conversion is the process of converting between various length units (px, em, rem, %, vw, vh, etc.) used in web design.
Absolute and Relative Units
CSS units are broadly divided into absolute and relative units. Absolute units (px, pt, cm, etc.) have fixed sizes, while relative units (em, rem, %, vw, vh, etc.) are expressed as ratios to a base value.
px (Pixel)
The most common unit, representing one dot on the screen. Easy to handle with fixed size, but has accessibility challenges as it ignores user font size settings.
em (Em)
A relative unit based on parent element's font size. 1em = parent's font-size. Calculated compoundly in nested structures, which can lead to unexpected sizes in deep hierarchies.
rem (Root Em)
A relative unit based on root element's (html) font size. 1rem = html's font-size (usually 16px). Unlike em, not affected by nesting, recommended in modern CSS.
% (Percent)
Represents proportion to parent element's size. For font-size, based on parent's font-size; for width, based on parent's width. Note that the base differs by property.
vw/vh (Viewport Units)
Units based on viewport (display area) size. 1vw = 1% of viewport width, 1vh = 1% of viewport height. Enables flexible layouts responsive to screen size.
pt/pc (Print Units)
Units used in the printing industry. 1pt = 1/72 inch, 1pc = 12pt. Used in print CSS and PDF generation, but px or rem are typically used for screen display.
Physical Units (cm/mm/in)
Units representing actual physical length. 1in = 2.54cm = 25.4mm = 96px (CSS definition). Used when consistency with printed materials is needed, but physical screen size is not guaranteed.
Advantages of Real-time Conversion
This tool supports real-time conversion, updating results instantly when base values change. Enables efficient CSS design by quickly verifying appearance with different screen sizes and font settings.