Kerning Adjustment Calculator

Kerning is a per-pair correction to the advance width between two specific glyphs, expressed in thousandths of an em so the value scales with type size: minus 50 units at 48px moves the second character 2.4px closer. It exists because uniform sidebearings produce uneven optical spacing when letter shapes are incompatible. Two adjacent diagonals as in AV or VA, an open capital followed by a low lowercase as in To or Ya, and a capital T or L beside a round or diagonal form all leave a visual hole that the eye reads as a word break. Typical corrections sit between minus 20 and minus 80 units, with diagonal-on-diagonal combinations at the strong end and pairs such as TR or wy needing only slight adjustment. What the numbers cannot capture is that kerning is optical, not arithmetic. Required values differ by typeface - a serif face with generous sidebearings needs different treatment from a tightly fitted geometric sans - and a serif family typically ships several hundred pair adjustments while a simpler sans-serif may carry fewer. Size matters too: at text sizes the built-in kerning table plus normal rendering is generally enough and manual intervention is wasted, whereas at display sizes above roughly 36px individual pairs become visible and worth correcting by hand. The practical caveat on the web is that CSS letter-spacing cannot express a pair-specific adjustment at all; it applies uniformly, so replicating a single kern requires wrapping the character in its own element, and doing so also disables the font's own kerning across that boundary.

The word kern comes from the printing trade term for the part of a cast letter that overhangs its own body and rests on the shoulder of the neighbouring type. Italic f and j were the classic kerned sorts, and because the projecting metal was fragile these characters broke easily under press impression. Fitting in metal composition was therefore mostly fixed at the foundry, built into the body width of each sort, and a compositor could refine spacing only by inserting thin spaces - never by moving two letters closer together. Phototypesetting removed the physical body and made negative spacing possible for the first time, and the tight, closely fitted display typography of 1960s and 1970s advertising is a direct consequence. Digital formats then made kerning data part of the font file itself. TrueType, developed by Apple and adopted by Microsoft in the early 1990s, carried a kern table of pair values; OpenType, announced by Adobe and Microsoft in 1996, moved the function into the GPOS layout table, where class-based pair positioning lets a designer kern whole groups of similar glyphs at once instead of enumerating every combination, which is how modern fonts cover thousands of pairs without an unmanageable table. Browsers gained control over this data through the CSS font-kerning property and the OpenType kern feature, so web text now applies a font's own pair adjustments by default at most sizes.