CSS: Length Units

Pixel Unit

px

Pixel. Practically, 1/16 of default font size of a device.

• CSS: How Large is a CSS Pixel?

Relative Units

vw

1% of the view port width.

vh

1% of the view port height.

vmin

Smaller of vw or vh

vmax

Larger of vw or vh

%

Percent. Relative to parent element. Exact meaning depends on the element it is used. 〔see CSS: Percentage Value〕

Font Based Units

rem

Font size of HTML: the Root Element. Equivalent to 16px.

• CSS: font-size

em

Font size of parent element.

ch

Width of digit 0 of current font.

ex

Height of lowercase letter x of current font.

Physical Units

cm

Centimeter.

mm

Millimeter.

in

Inch.

pt

1/72 of a inch.

pc

1/6 of a inch.

Which unit to use

💡 TIP: I recommend use only the px unit, and the following:

• em or ch → for local font sensitive size, e.g. margin, padding, box round corner.

• vw, vh, vmin, vmax → for layout control.

Do not use

• percent %, because the meaning depends on the HTML element.

Example: CSS Length Unit

div {margin:1rem}

🛑 WARNING: There must be no space before the CSS unit, and a unit is required, except when the value is 0, else it's syntax error.

rem, Root Font Size

The length of rem is:

The computed value of font-size of the Root Element .

When used as value on font-size property of the Root Element, rem refer to the property's initial value. (in browsers, it's 16px).

💡 TIP: think of rem as the default font size, and it's basically always 16px.

em

The length of em is:

• em is equal to the computed value of the font-size property of the element on which it is used. For example, if you say max-width:50em on a paragraph, it means, 50 times the computed value of font-size property of the paragraph.
• If em occurs in the value of the font-size property itself, it refers to the computed value of font-size of the parent element.
• If used on the Root Element, 1em equals to the font-size property's initial value (which is 16px).

ex, x-height

ex is equal to current font's “x-height”, usually the height of the letter x. Typically about half of em, depending on current font.

Like em, the length of 1ex depends on parent's font-size.

〔see CSS: Meaning of Font Size〕

ch

ch is equal to the box width of the digit 0 (ZERO) character of the current font. It is about the same as half of em.

vw, vh, vmin, vmax

vw

Equal to 1% of the initial containing block (the view port). It does NOT include the scrollbar width. When browser window changes size, vw also changes.

💡 TIP: One great use of vw is to specify height as a percentage of width. You can't do this with percentage. 〔see CSS: Fixed Aspect Ratio〕 In fact, it's useful to replace almost all CSS: Percentage Value by vw, because the meaning of percentage is different for each CSS property.

vh

Equal to 1% of the height of the initial containing block.

vmin

Equal to the smaller of vw or vh.

vmax

Equal to the larger of vw or vh.

cm, mm, in, pc, pt

cm

centimeter. 10 mm → ~0.3937 inch; ~37.795 px

Physical units such as cm is useful for specifying printed style. For example:

@media print { table {font-size: 4cm} }

〔see CSS: Media Query〕

mm

millimeter. 0.1 cm → ~3.78 px

in

Inch. 6 pc. 72 pt. 96 px. → 2.54 cm

pc

1/6 inch. 12 pt. 16 px. “pc” stands for “pica”. → ~0.423 cm

pt

1/72 inch. (~0.0138 inch) ~1.33 px. “pt” stands for “point”. → 0.3527 mm

What is CSS's Default Unit?

• There is no default unit. Omitting unit is syntax error.
• You can omit unit only when the value is 0.