Character width refers to the horizontal space a character occupies when rendered on screen or in print. In East Asian text processing, the concepts of "full-width" and "half-width" remain deeply ingrained: full-width characters occupy twice the horizontal space of half-width characters. This distinction was established in the 1980s when fixed-width fonts were the standard on computer terminals.

Unicode classifies character width using the East Asian Width property. F (Fullwidth) denotes full-width characters, H (Halfwidth) denotes half-width characters, W (Wide) covers East Asian wide characters (kanji, hiragana, katakana, etc.), Na (Narrow) covers narrow characters (ASCII alphanumerics, etc.), and A (Ambiguous) covers characters whose width depends on context. The Ambiguous category is particularly troublesome: it includes some Greek and Cyrillic characters that are treated as full-width on East Asian systems but half-width on Western systems.

In proportional fonts, each character has a different width. "W" is roughly three times wider than "i," and "m" is about twice as wide as "l." This means the rendered width of proportional text cannot be predicted from the character count alone. Ten characters of "WWWWWWWWWW" and ten characters of "iiiiiiiiii" produce vastly different display widths. Typography references on Amazon explore these proportional spacing mechanics in detail.

Monospace fonts (fixed-width fonts) give every character the same width. Code editors and terminals use monospace fonts because they make alignment trivial: indentation and column alignment never break. In Japanese monospace fonts, full-width characters are designed to be exactly twice the width of half-width characters.

The relationship between character count and character width becomes critical in display layout design. For example, if a "Name" column in a database listing is allocated space for 20 characters, it can fit 10 full-width characters or 20 half-width characters. Since real data mixes both, the layout must be designed for the worst case (all full-width).

In web design, the CSS ch unit is a relative unit based on the width of the "0" (digit zero) character. Specifying width: 40ch creates a container roughly 40 characters wide. However, with proportional fonts, character widths vary, so the ch unit is only an approximation. For East Asian text, the em unit (where 1em equals one full-width character width) provides more intuitive width control.