Nov 22 2009
CSS Measurement Units
0 commentsCSS has two forms of measurement, absolute and relative.
| Absolute | Relative | ||
|---|---|---|---|
| CSS Unit | CSS Abbreviation | CSS Unit | CSS Abbreviation |
| Pixels | px | Percent | % |
| Points | pt | Em | em |
| Inches | in | Ex | ex |
| Centimeters | cm | Root em | rem |
| Picas | pc | Viewport width | vw |
| Viewport height | vh | ||
| Viewport minimum | vm | ||
| character | ch | ||
| Grid | gd | ||
rem, vh, vw are CSS3 measurement units, supported by modern browsers.
1em is the width of the m character in the default font set for that browser. (ex is the height of the x character in a font). rem allows you to set a root font size for size for the entire page, with sizes for elements relative to this measurement:
html { font-size: 62.5%; }body { font-size: 1.4rem; }h1 { font-size: 2.4rem; }
(The measurements above are used because they map exactly to pixels: with a base font-size of 62.5% for a page, the body size maps to 14 pixels, and h1 to 24 pixels.)
The viewport is the browser window: vh, vw and vm can scale elements to its current size. This could be used to restrict the upper scaled size of images, for example:
img { max-width: 90%vw; }
ch is the equivalent of em, but applied to the width 0 numeral of the chosen font.
gd relates to the CSS3 grid; unfortunately, no browser yet supports the grid module.
You never need to specify units when declaring a value of 0: zero centimeters is the same as 0 pixels. In all other cases, units of measurement should be declared 2em, 2px, etc, not 2.
With the exception of pixels, all systems of measurement can take floating point values: 2.25em, 5.3cm, etc.
Generally speaking, CSS written for screen display devices should use relative units (percent, em and ex), with few exceptions (exceptions include bitmapped images and elements that refer to bitmapped images).
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